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The Bias Against Tall Plants

November 11th, 2025 by Alex Zorach

In our October 2024 post we noted that gardening and landscaping practices have ecological consequences, especially as an increasing portion of our land consists of managed landscapes. Gardening and landscaping practices are subject to cultural norms, which can include long-standing beliefs as well as fads and trends. These norms also influence the management of wild lands, especially at edges and other interfaces between wild and managed lands such as are common in parks.

Currently, the concept of native plants is trending, in part due to the popularity of the books by Doug Tallamy. But even though the concept of native plants has broken into the mainstream, numerous ecological concepts are going under-appreciated, and numerous biases, many of which have negative ecological consequences, are persisting.

This post tackles a particular bias: the bias against "tall" plants, and its counterpart, the bias in favor of "short" plants. This bias goes hand-in-hand with lawn culture, and has consequences diverse areas, including:

• proliferation of invasive plants
• weakening of native plants by breeding cultivars to be shorter
• failure of ecological restoration projects because of selecting shorter plants on sites suited to taller plants
• degradation of edge habitats by selective removal of herbaceous plants that are "too tall"

This post outlines each of these forms of potential harm, but it starts with the groundwork of explaining why some herbaceous plants grow so tall in the first place.

The Ecology of Plant Height

Plants' physical attributes, including height, are a function of their ecology, reflecting adaptation to their habitats. Habitat includes both the environmental conditions (such as moisture and light availability, and soil texture and chemistry) and other organisms present in the ecosystem, including both animals and competing plants.

In order to grow taller, herbaceous plants need resources, including water, nutrients, and light. However, growing taller is still costly, so it is only advantageous if it offers something that a short habit does not. The main advantage of height is pushing up through competing vegetation to reach more light. Competition not only increases the reward of height, but also facilitates it, since plants can lean on each other for structure. Many tall plants cannot support their own weight alone, staying upright and pushing up through other vegetation only by leaning against each other. As such, most tall plants occur in communities of other tall plants.

Besides the increased light capture, there are secondary benefits of height, including visibility of flowers to pollinators, and in wind-pollinated plants, increased reach of pollen and ability to capture pollen from distant plants. Wind-dispersed seeds can also spread over longer distances. These wind-facilitated benefits though come with a price: the higher wind speeds at greater heights worsens drought stress, thus increasing water demands more than would be expected by light exposure and leaf surface area alone.

For these reasons, most tall herbaceous plants occur in habitats where there is abundant water, light, and nutrients, and heavy ground-level competition. These habitats include tallgrass prairie, wet meadows and openings in floodplain forests, rich, moist forest edge habitats (including streambanks and shores of lakes, ponds, and open wetlands, as well as anthropogenic forest edges), swamps, and locally-disturbed sites in bottomland forests or rich mesic upland forests. In drier habitats and on sites with poorer soil, the drought stress, lack of competition, and scarcity of nutrients combine to favor shorter plants. On shadier sites such as a closed-canopy forest, low-light conditions favor either woody plants like trees or shrubs, or low-growing herbaceous plants.

Much of North America's population is found in areas favoring tall herbaceous plants.

The human population in North America concentrates along the humid coasts where forests were dominant, and into the eastern portion of the Great Plains, where tallgrass prairies were dominant. Furthermore, in forested regions, humans tend to settle near sources of water and in areas with richer soil, as these areas support agriculture capable of feeding larger populations.

Humans often increase the habitat for tall herbaceous plants.

Without the influence of humans, much of the Eastern Temperate Forests would support closed-canopy forests in which there are fewer tall herbaceous plants. In many of these regions, historically, Native Americans practiced controlled burns which favored savannas, which supported more tall herbaceous plants, including both grasses and broadleaf plants. European colonization brought the clearing of forests and conversion of land to agriculture, and tall plants proliferated on cropland margins, fallow fields, and in the early-successional growth on sites where agriculture was abandoned and forest was allowed to grow up again. In the present day, an abundance of roads and paths and the fragmenting of forests has created more edge habitat, where higher levels of light reach the ground underneath trees, and tall herbaceous plants thrive.

Resisting a natural equilibrium has a cost.

I hope I've convinced you that, at least some of the time and on some sites, the conditions favor tall plants. Attempting to resist such a natural equilibrium comes with its costs, which the rest of this post will explore.

Leaving The Tall Plant Niches Vulnerable to Invasion

If you remove a certain species from an ecosystem, its niche will usually be colonized by some other plant. Removing a tall native plant will usually lead its niche to be colonized by a plant of similar height. However, if over time, tall native plants are repeatedly and systematically removed from ecosystems, their niches will be colonized either by shorter native plants, or by non-native plants. Because tall plants tend to be more competitive than short plants on sites rich enough to support tall plants, systematic removal of tall native plants can lead a site to be more vulnerable to invasive plants.

Many of the invasive plants that fill these niches are themselves tall. But the bias against tall plants acts on them as well. People are more likely to notice tall invasives and target them for removal, and much of the management of land is not done with respect to a plant's native status, so the bias against tall plants acts equally on tall invasives. When all tall plants are excluded, they become replaced by shorter vegetation. And as I explain below, there are some systemic reasons why this shorter vegetation is more likely to be dominated by introduced plants.

Proliferation of Invasive Plants Selected for Shortness

One pattern I have noticed about invasive plants in North America is that a surprisingly large portion of the worst invasive species have a low-growing habit. This pattern is the opposite of what one would expect based on competition, as tall plants often out-compete shorter plants. Although there are more examples of such low-growing invasive plants in the humid east, this problem is not restricted to that region. The southwest has crystalline ice plant (Mesembryanthemum crystallinum) and slenderleaf iceplant (Mesembryanthemum nodiflorum), and fire-prone areas of the interior west and western great plains have cheatgrass (Bromus tectorum), both of which are lower-growing than many of the plants in the ecosystems they tend to invade.

The low height of these invasive plants tends to create other ecological problems, beyond the typical collapse of the food web caused by non-native species being eaten by fewer insect specialists. The food web problem is not directly related to height. However, another serious problem, erosion, is directly related. Although there are some exceptions, on average, shorter or smaller plants tend to have shallower and less-extensive root systems than tall plants. And, as-such, the proliferation of short or low-growing invasive plants tends to worsen erosion and increase the risk of larger-scale mass wasting such as slumping of soil.

Shorter plants also tend to have less biomass and thus, when they dominate ecosystems and out-compete taller plants, they reduce the total extent of the ecosystem, which reduces not only food available to animals, but also other ecosystem services plants provide, including cover for animals, solar capture, reduction of wind speed, and evapotranspiration, which in turn provides both temperature regulation and runoff reduction. The replacement of tall native vegetation by short invasives causes a cascade of harm extending beyond the ecosystem itself, leading to a reduction in climate regulation in the surroundings, harm to wildlife that spends only a portion of time in the ecosystem, and harm to waterways downstream.

The reason there are so many low-growing invasive plants is that humans have imported these plants to the continent disproportionately. One of the main ways short plants get introduced is through lawns, so our first examples of invasive plants will be forms of turfgrass.

Turfgrass

A large portion of the introduced grasses in North America were brought here as turfgrass. Although states and other governments are reluctant to label these plants as invasive, the reasons for doing so are more economic and cultural than ecological. One of the most widely-planted turfgrasses in the US is tall fescue (Lolium arundinaceum), which also happens to be one of the most aggressive in the wild. The USDA's FEIS acknowledges "Tall fescue can be invasive in native vegetation," giving the example of the Clymer Meadow Preserve in Texas, where it is encroaching on native vegetation, and notes "Tall fescue has devastated many other prairie remnants in Texas and to the north."

Some other turfgrasses that can become invasive include bermudagrass (Cynodon dactylon), Kentucky bluegrass (Poa pratensis) (which, contrary to its name, is not from Kentucky), rough bluegrass (Poa trivialis), perennial ryegrass (Lolium perenne), italian rye-grass (Lolium multiflorum), hard fescue (Festuca trachyphylla), creeping bentgrass (Agrostis stolonifera), bahiagrass (Paspalum notatum), centipede grass (Eremochloa ophiuroides), and Korean lawngrass (Zoysia japonica). This list is hardly exhaustive.

So many turfgrasses are invasive because they been bred over many generations both to be vigorous in North America's climates and soils, to be competitive against "weeds" (i.e. any other plant that might compete with them), and to resist any insect "pests" that might eat them. All three factors increase the likelihood of a species becoming invasive.

Groundcovers

Groundcovers are low-growing plants, typically either vines or herbaceous plants that reproduce by rhizomes or stolons, and that stay relatively low to the ground without needing to be mowed. Some grow taller than a typical lawn, but most are shorter than about 18 inches (1.5 feet) and some are much shorter. Many of them are evergreen. Some are succulents. Some have showy flowers. It is important to understand that groundcover is a horticultural term, not an ecological one: it refers to a plant's use in the garden, not its natural growth habit in the wild.

Some of the worst invasive plants in North America were introduced by being widely planted as groundcovers. Such examples include English ivy (Hedera helix), lesser celandine (Ranunculus ficaria), common iceplant (Mesembryanthemum crystallinum), winter creeper (Euonymus fortunei), Japanese pachysandra (Pachysandra terminalis), creeping jenny (Lysimachia nummularia), common periwinkle (Vinca minor), bigleaf periwinkle (Vinca major), stringy stonecrop (Sedum sarmentosum), and goldmoss stonecrop (Sedum acre).

Other groundcovers that have established in the wild but are not quite as aggressive include spotted henbit (Lamium maculatum), green hellebore (Helleborus viridis), bugleweed (Ajuga reptans), common houseleek (Sempervivum tectorum), evergreen candytuft (Iberis sempervirens), and several more stonecrops including orpin aizoon (Phedimus aizoon), white stonecrop (Sedum album), spanish stonecrop (Sedum hispanicum), European stonecrop (Sedum ochroleucum), and tasteless stonecrop (Sedum sexangulare). These plants can still be aggressive in some contexts, and any of them has the potential risk of becoming invasive at some point in the future.

There are likely many other plants with potential to become invasive if widely planted as groundcovers, but that have not yet become so simply because they have not been widely planted and/or bred. An example of a plant that is currently in the process of becoming more invasive is creeping liriope (Liriope spicata), which was widely planted as a replacement for English ivy, a decision that was both short-sighted and lacking in ecological awareness.

As with turfgrasses, groundcovers are also bred for traits that increase their likelihood of becoming invasive: tolerance of North America's climate and soil conditions, resistance to invasion by "weeds", and insect resistance.

Weedy low-growing plants that survive in lawns

Not all the introduced low-growing plants ended up here intentionally. A large portion of them propagate and spread as lawn weeds. Lawns provide an ideal habitat for the spread of such plants, not only because the mowing removes any taller competition, but also because the lawn equipment itself spreads plant seeds around, both locally and to new sites when the seeds ride on the equipment.

Introduced weedy plants that grow in lawns include unwanted grasses, such as annual bluegrass (Poa annua), hairy crabgrass (Digitaria sanguinalis), smooth crabgrass (Digitaria ischaemum), quackgrass (Elymus repens), awnless brome (Bromus inermis), plantain signalgrass (Brachiaria plantaginea), and indian goosegrass (Eleusine indica). Some of the turfgrasses discussed above, which may be intentionally planted on some sites, can also be unwanted weeds in other lawns. Such sometimes-unwanted grasses include tall fescue, creeping bentgrass, and rough bluegrass.

The list of introduced broadleaf weedy plants that thrive in lawns is even longer. Some of the most common ones include lesser celandine, mock strawberry (Duchesnea indica), European cinquefoil (Potentilla reptans), birdeye speedwell (Veronica persica), common plantain (Plantago major), narrowleaf plantain (Plantago lanceolata), common dandelion (Taraxacum officinale), scarlet pimpernel (Lysimachia arvensis), white clover (Trifolium repens), japanese clover (Kummerowia striata), korean clover (Kummerowia stipulacea), suckling clover (Trifolium dubium), black medick (Medicago lupulina), bird's-foot trefoil (Lotus corniculatus), ground ivy (Glechoma hederacea), common chickweed (Stellaria media), henbit deadnettle (Lamium amplexicaule), purple deadnettle (Lamium purpureum), and low smartweed (Persicaria longiseta). Again, this list is far from comprehensive; a full list for North America would be pages long.

Although people are not actively breeding these weeds to be more invasive, lawn culture unintentionally has this effect. In nature, many of the habitats that these plants would colonize are ephemeral, persisting only temporarily in response to a disturbance. But the regular mowing of lawns creates a new, artificial habitat in which these plants can produce generation after generation, persisting in this habitat consistently and long-term, often in large numbers.

So, instead of having a small number of plants producing seed and then dying out after a year or two, only to have suitable habitat crop up, potentially farther away, potentially years later, we now have a scenario in which the plants are breeding in large numbers every year. The massive amount of lawn further increases the population of these plants, and the fact that lawn equipment can move seeds from site to site, increases the speed with which genetic material spreads over large regions. All of these factors make the populations more vigorous and allow them to adapt to the conditions on this continent, increasing the likelihood that these plants become invasive.

Weakening of native plants by breeding short cultivars

Modern horticulture, landscaping, and gardening practices don't just introduce new, potentially invasive plants, but also alter our native plants. The horticulture industry breeds cultivars to have "desirable" traits in gardens. Modified traits may include flower color and structure, foliage color, and growth habit. One of the most common modifications made with such breeding is to decrease the height of plants.

Above we explained how a plant's height is a key aspect of its fitness and adaptation to its habitat. Modifying a plant's height by selective breeding makes it less fit in the habitats to which it is otherwise adapted. Shortened plants tend to be less competitive with other vegetation, and are usually less shade tolerant as the upper leaves tend to capture the most light. Shortened plants may also be less able to quickly and successfully cross-pollinate, whether the plant is wind- or insect-pollinated, and less able to distribute its seed larger distances particularly if its seed is wind-distributed. Although there are also potential benefits of shorter height (such as greater tolerance of drought and low-nutrient conditions), altering a plant from its equilibrium under natural selection tends to decrease its fitness. If it were beneficial to be shorter, it would have already become shorter through natural selection.

Plants also have some morphological plasticity, meaning that, even plants of identical genetics adjust their characteristics (including height) dynamically based on conditions. Cultivars are not just bred for specific traits, they are also typically bred to have less variation in those traits on different sites, so they are more "predictable" in the garden. In other words, their morphological plasticity is reduced, which also reduces their fitness in the wild.

One such cultivar is 'Little Joe' cultivar of coastal plain joe pye weed (Eutrochium dubium). Although E. dubium is naturally the shortest of the five Eutrochium species, this cultivar has been bred to make its maximum height even shorter: it grows only to about 4 feet, contrasting with the wild forms of the plant, which commonly grow to about 5 feet. This cultivar is also a classic example of the decrease in variability: both the cultivar and wild forms range in height, and can be as short as 3 feet at maturity. But the cultivar ranges from 3-4 feet in height, whereas the wild form ranges from 3-5 feet. The difference of that missing last foot of height tends to occur on plants grown in the conditions with greater moisture and nutrients, conditions where the plant is most likely to face competition from other plants.

The genetics encoding for height of these cultivars get passed on to future generations. Although the plants do tend to revert back to a more wild-type plant eventually, the shortened height does tends to persist, somewhat unpredictably, through at least one or two generations. I have done a lot of ecological restoration work in New Castle County, Delaware, a region where E. dubium is both common in the wild, and where the 'Little Joe' cultivar is widely planted in gardens. E. dubium is a particularly good example of the effect of these cultivars on wild plants, because the species self-seeds prolifically both in wild ecosystems and in suburban gardens and weedy marginal areas in suburbs. And I have noticed a clear pattern of volunteer E. dubium near source populations of these short cultivars, themselves being shorter at maturity than volunteer plants of the same species found in more intact wild areas, such as along the lower Christina river where it is separated from a large chunk of forest in Lewden Greene Park, or adjacent to larger wetlands in Phillips Park, Newark.

Locally-sourced wild plants grown in gardens can protect the populations of those plants in nearby wild ecosystems, by seeding out into wild areas. The offspring of these garden plants compete more favorably against invasive plants in the wild because the wild populations have genetics that were the product of generations of natural selection for fitness in the wild. Such garden plants also serve as a reservoir for the genetics of the local wild populations, and can help preserve these genetics if a local wild population is temporarily extirpated. The importance of these reservoirs cannot be over-emphasized in a world where we have reduced wild land to a tiny proportion of its original extent, wild habitats are highly fragmented, and the local genetics of wild plant populations are at risk of being lost entirely.

When gardeners instead plant horticultural cultivars, it is not clear what the long-term effects on the local populations of these species will be. I have seen a lot of evidence to suggest that concern is warranted, and that these cultivars often produce seedlings that are less fit for survival in the wild. And excepting in the rare cases where cultivars are planted in the region from which their ancestors were sourced, they do not preserve the local genetics. There is also the possibility that a "worst case scenario" is also happening, in which the weakened cultivars are interbreeding with wild populations in ways that reduces their fitness, leading to decline of the populations of certain species, at least in some areas.

I have not yet seen any specific evidence that this is happening, but I also have not seen conclusive evidence that it is not. Concerningly, the horticulture industry is not studying any of these potential effects; they continue to breed and market and distribute plants primarily for their performance in gardens. What little consideration of ecology is being done has focused on the value of plants to insect populations (such as is studied at Mt. Cuba Center's trial garden.) But I have yet to see any research at all on the effect of cultivars on the genetics or fitness of local wild populations of plants.

Removal of Native Plants that Are "Too Tall"

The bias against tall plants affects property management both in home gardens and semi-wild spaces in managed parkland.

A lot of people grow "native plant gardens" but manage them intensely, not allowing plants to reproduce freely. People who want to grow only shorter plants end up only growing a subset of plants adapted to drier, more nutrient-poor conditions. People sometimes remove even showy, visually-appealing plants such as cutleaf coneflower (Rudbeckia laciniata), tall goldenrod (Solidago altissima), and late boneset (Eupatorium serotinum), specifically because of their height.

The absence of tall native plants in suburban gardens is directly related to the ways in which rich, moist habitats, the ones where tall plants tend to naturally be dominant, are so overrun with invasives. An example of such habitats are wet meadows in floodplains, which are often common in suburban areas in land too low and flood-prone to build on. Many of these meadows are overrun with invasive plants, some of which colonize from seeds washed downstream from the suburban yards themselves. If people were less averse to planting tall plants, these meadows might have more intact ecosystems, populated by plants like cutleaf coneflower, wingstem (Verbesina alternifolia), and giant goldenrod (Solidago gigantea). The reduced height of vegetation from removing tall plants can create additional problems, increasing water speed during floods, which can increase flood severity downstream, and also increase erosion which causes soil loss upstream, and nutrient pollution, resulting in eutrophication, downstream.

Bias against tall plants in managed parkland can lead park staff to expend resources removing or killing native plants, driven either by complaints from the public, or just the discretion of the staff themselves. For example, in Newark, Delaware, I have seen park workers spray and kill American pokeweed (Phytolacca americana), Canada lettuce (Lactuca canadensis), and tall blue lettuce (Lactuca biennis), but leaving in place many shorter plants which were non-native such as prickly lettuce (Lactuca serriola) and the various sow thistles (Sonchus sp.) So the management was unintentionally favoring introduced vegetation. Changing these practices, however, is relatively easy and can have benefits.

During one year I went out with park staff along the James Hall Trail in Newark, DE and showed them how to distinguish the native lettuces from the invasive prickly lettuce. That year, the staff sprayed only the prickly lettuce, and a few of the native lettuces growing too close to the path, but left native lettuces that were not posing any risk of flopping into the path. In the next year, I saw a dramatic shift in the relative abundances of lettuce species along the trail, with the two native species becoming dominant, whereas in all prior years, the invasive prickly lettuce had been dominant. This change then produced other benefits. The native lettuces supported more aphids, which in turn supported other animals higher up on the food chain such as mantises and migrating warblers.

Failure of Ecological Restoration Projects

Projects can fail due to selection of short plants on sites better-suited to tall plants.

When I first started working in the area of ecological restoration, I was not prepared for just how much plant selection would be influenced by horticultural thinking and aesthetics. Pure ecological projects are rare. A large portion of ecological restoration projects are not conducted on truly wild land, they are only semi-wild landscapes in parks, historic preserves of old estates, or on grounds owned by schools, universities, cemeteries, or other organizations. They don't necessarily want a truly wild meadow, they want something that looks a certain way, and this "look" is plagued by the bias against tall plants.

Many times I have consulted with organizations looking to restore a site, come up with a plant list, and have some of the plants I or others recommended as "ecological powerhouses" be removed in the final list, often replaced by plants that are shorter and less vigorous. A common substitution is that people want the lower-growing butterfly milkweed (Asclepias tuberosa) in place of the taller, more aggressive common milkweed (Asclepias syriaca). Similarly, people often want to substitute shorter goldenrods such as gray goldenrod (Solidago nemoralis) or early goldenrod (Solidago juncea) for tall goldenrod (Solidago altissima), canada goldenrod (Solidago canadensis), or giant goldenrod (Solidago gigantea). Although these choices might work in a managed garden, they do not work in even a semi-wild landscape where plants freely seed in from the wind or bird droppings, and "duke it out", with the winners chosen by survival of the fittest.

Shorter plants only win the battle on sites with adverse conditions, like drought-prone coarse sandy soils, thin soil over a rock outcropping, or exposed mineral soil that is devoid of organic matter. But as I observed above, humans tend to live mostly on or near rich sites where the ecology favors tall plants, so restoration projects where the short plants are favored by conditions are the exception rather than the norm.

Predictably, the restored site becomes invaded by unwanted tall plants, often rhizomatous invasives such as common mugwort (Artemisia vulgaris), creeping thistle (Cirsium arvense), johnsongrass (Sorghum halepense), Japanese knotweed (Reynoutria japonica), or in the south, giant reed (Arundo donax). Sometimes unwanted native plants move in, such as common ragweed (Ambrosia artemisiifolia) or giant ragweed (Ambrosia trifida), both major allergens.

Degradation of Edge Habitats

The management of edge habitats poses an even more widespread problem than the half-hearted ecological restoration projects discussed above. Edge habitats include forests adjacent to parks or lawns on residential, commercial, industrial, or other institutional properties, as well as edges of roads and bike/pedestrian pathways that cut through forests. Many edge habitats are managed without any consideration of ecology whatsoever, and when it is considered at all, it is usually little more than an afterthought. Once again, the bias against tall plants rears its ugly head.

Edge habitats are often managed by workers whose instructions or intentions are little more than to keep an edge "maintained" or "neat". As such, they have a great deal of discretion. They may also respond to complaints from the general public (especially in municipal parks) or the whims of management.

Tall plants also pose a practical management concern, in that they tend to flop into paths or roads. Forward-looking trail managers might want to act proactively, removing tall plants early, before they flop into a path. This removal can happen by mowing or bushwhacking farther back from the path than the standard mow line, by spot-treating plants behind the mow line with herbicide, or by weed-whacking or cutting isolated plants. Plants I have seen targeted in this way include lettuces, especially the native canada lettuce (Lactuca canadensis) and tall blue lettuce (Lactuca biennis) (which tend to grow taller than the invasive prickly lettuce (Lactuca serriola)), American pokeweed (Phytolacca americana), the taller species of goldenrod, common milkweed, and American burnweed (Erechtites hieraciifolius) (which is often mistaken for a thistle). Non-target plants are often killed in these control efforts, creating localized dead zones.

When tall plants are selected against in management of edge habitats, it can cause cascading problems in the interior of the habitat. Relative to open habitats, forest interiors have lower light levels, greater leaf litter, and a more moderate microclimate, with lower wind speeds and more regular temperatures. A natural forest edge produces a buffer of vegetation blocking both light and wind. This buffer consists both of vegetation from woody plants, such as trees and shrubs on the edge, and ground-level herbaceous plants. Edges tend to favor taller vegetation than forest interiors because of the higher light availability, and there is a bit of a positive feedback loop because taller ground-level vegetation captures more leaf litter around the base of plants, leading to high nutrient availability.

Removing the tall herbaceous plants right at the edge causes changes to the interior, farther back than the actively managed zone. Wind speeds increase, light increases, and temperature swings become more extreme. The removal of vegetation and increase in wind speed can cause leaf litter near the edge to blow away, exposing soil to erosion. These changes degrade the habitat for native plants and animals alike. Invasive shrubs and woody vines can colonize the zone set back from the edge in response to the higher light levels and disturbed soil. In the mid-Atlantic, such shrubs include Amur honeysuckle (Lonicera maackii) and burning bush (Euonymus alatus), and vines include Japanese honeysuckle (Lonicera japonica) and oriental bittersweet (Celastrus orbiculatus). The vines, and shrubs like Amur honeysuckle with a wide-arching habit, can root in a pocket of fertile soil and then extend their photosynthetic branches over areas with degraded soil. Once they establish in place of the native vegetation, they then begin the process of capturing leaf litter and nutrients, but creating a new structure that remains stable in the face of management practices, because they are rooted farther back, outside the zone of active management.

Without management that removes tall plants, there would still be invasive plants but they would find it more difficult to penetrate as far into the forest interior. Tall herbaceous plants tend to outcompete vines like Japanese honeysuckle, because the plant cannot reach a high permanent height without climbing woody vegetation, and herbaceous plants grow rapidly and shade out the honeysuckle's leaves. Removing the tall plants tends to make the honeysuckle more vigorous, as more light reaches the carpet of honeysuckle leaves growing on the ground.

Another problem with such removal is the creation of temporary dead zones, small patches which disturbance-loving plants can colonize. Depending on the timing of the control, there are a number of opportunistic invasive plants that can take advantage of these patches. Japanese stiltgrass (Microstegium vimineum) tends to colonize areas disturbed in spring to early summer, and short-lived plants like common groundsel (Senecio vulgaris) can colonize at just about any time of year. Edge habitats, which in our society are so prevalent that they form an interconnected network extending throughout whole regions, become corridors along which these invasive plants can spread.

The degradation of managed edge habitats, because it is such a widespread phenomenon, can spread to unmanaged edge habitats, such as shores of lakes and ponds, or borders between forests and open wetlands or forests and natural clearings or barrens such as rock outcroppings. Although management pressures change the nature of edge habitats, there is some overlap in the species that can occur in both. The widespread dominance of degraded anthropogenic edge habitats dominated by invasive species creates a reservoir of invasive plants such that there are more seed sources for these plants, relative to the native species that would colonize natural edge habitats.

How you can help: tips and tricks

The most important thing you can do to combat the bias against tall plants is to become aware of it, reflect on it, and move beyond it on your own. There are also some tricks to add to your toolbox that can lead to a "best of both worlds" for preserving populations of tall plants on sites where their height poses practical problems.

Utilize topography

One of the best ways to utilize tall plants is in small depressions on an otherwise dry, upland site or site with poorer soils. These depressions accumulate both moisture and litter, forming islands of richer, moister soil in drier, nutrient-poor surroundings. Their natural equilibrium is thus to support taller plants. You can utilize this gradient of conditions in a flower bed, planting tall plants that will stay put in the central dip, and shorter, more drought-tolerant plants around the outside of the bed. The lower height in the dip will also level out the plant heights somewhat.

Use stakes, trellises or other support

Although these practices are more labor-intensive and not practical on a large scale, gardeners who wish to grow tall plants in a small space can use their own structures such as a trellis or stakes to support tall plants. These supports mimic the natural support these plants would have from other vegetation, and they can keep the plants out of paths or roads.

Growing tall plants but keeping them short: the "Chelsea chop"

If you want to grow species of plants that naturally would grow tall, on a site rich enough to support such height, but you want to keep them shorter for practical reasons, a useful trick is the "Chelsea chop". The Chelsea chop refers to the practice of cutting perennials or biennials fullly or partway to the ground mid-growing-season, typically in late spring. It is named for the Chelsea Flower Show in London, which is typically held in late May; the name originated in reminding people who were aware of the dates of the show, when to cut back their plants, but it has persisted in usage even in North America where few people are aware of the dates of the actual show. Some people remember the timing as being between mother's and father's day, although this timing allows for it to be a few weeks later.

Most perennials and biennials are resilient to top-kill, and this is especially true of plants that naturally grow taller, as the lush environments where these plants thrive tend to attract heavy herbivore browsing, such as White-tailed deer in the gaps and edges in the Eastern Temperate Forests, and, historically, bison in the Great Plains. If you cut off these plants, they will simply regrow. Their maximum height will be reduced, but they will usually still flower.

The timing of the chop is important: it must be late enough to remove growth from a significant portion of the time plants were extending their height, but early enough not to affect flowering. Early bloomers, such as foxglove beardtongue (Penstemon digitalis) in the east and many other Penstemon species, will have their flowering negatively affected by the chop if too much is removed. But most taller plants bloom later, either in mid-summer at the earliest, or in late summer to fall. A full top-kill in late May will give these plants ample time to regrow, flower profusely, and produce seeds.

You can use the Chelsea chop to grow otherwise tall plants in areas such as next to paths and roads where flopping plants are a liability. You can also use it selectively on only a few isolated tall plants if you want to allow other, shorter plants to remain competitive with them, to keep a balance that can increase biodiversity. And you can use it for purely aesthetic reasons.

Park and land managers and road crews can also use the same general principle, mowing or bushwhacking a narrow strip next to paths and roads once a season, around the same time, so as to reduce vegetation flopping into the pavement, while allowing some taller plants to still grow up during the peak of the growing season. In much of North America, where sudden downpours from thunderstorms are more common in summer, this mow timing also protects soil by keeping vegetation lush during the the time when erosion risk is highest.

The optimal timing and cut length vary by vegetation type, site, and region. But the optimal timing usually falls somewhere in late spring, and plants can be cut back either to the ground, only a small distance, or anywhere in between.

In Summary

The bias against tall plants has a myriad of negative effects on North America's ecosystems. Its effects include the introduction, breeding, and spread of invasive plants originating as turfgrass, groundcovers, and lawn weeds. The breeding of shortened cultivars of native plants may weaken populations of those species, and at a bare minimum, the direct offspring of these plants are less competitive in the wild, reducing the benefit of growing them in gardens. The exclusion of tall plants from ecological restoration projects can lead to the failure of those projects and their becoming overrun by invasives. And lastly, the targeting of tall plants in the management of edge habitats can degrade these habitats and lead to the establishment of invasive plants both in the interiors behind the managed spaces, and in the disturbed areas along the edge itself. These edges then become networks through which the invasive plants spread throughout whole regions.

You can help address these issues first by becoming aware of them, and then by talking about them with others. If you are in a decisionmaking position in any organization or project where these issues are relevant, use your knowledge to make more informed choices. There are also numerous techniques that can help you to use tall plants even on sites where their height may pose problems. Utilizing topography and planting taller plants in dips, using support, and cutting plants back to the ground in late spring, a.k.a. the "Chelsea chop", are three techniques to incorporate tall plants into managed landscapers where flopping plants are a liability.