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Canada Lettuce (Lactuca canadensis L.)

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Summary
Range
Description
Similar Plants
Habitat
Life Cycle
Fauna
Uses
Links
Photos

Photo © Blake Bringhurst, Public Domain.

↑Summary

A tall biennial native to eastern North America, having expanded its range westward as well.

↑Range - Expand

Legend	Color
Native
Expanded
Native or Not Present
Native or Expanded
Expanded or Not Present
Native or Expanded or Not Present

This tentative map is based on our own research. It may have limited data on Canada and/or Mexico, and there is some subjectivity in our assignment of plants as introduced vs. expanded. Read more in this blog post.

Although this plant occurs somewhere in each of these regions, it may only occur in a small part of some or all of them.

This is a weedy species with wind-dispersed seeds; it thrives in disturbed and degraded habitats. We marked the new western portions of its range expanded, even though they consist of isolated populations separated by large distances, because this plant is expanding on its own and is not intentionally planted, and because these new populations have established relatively consistently across a large region.

↑Description & Identification

Canada lettuce is highly variable in height and leaf shape, which can confound identification. Second-year plants have long stems that are usually unbranched until the inflorescence.

↑Similar Plants

Prickly Lettuce (Lactuca serriola)
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Tall Blue Lettuce (Lactuca biennis)
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↑Habitat

Canada lettuce is a common and widely-distributed generalist species that is found in a variety of sunny to partly-sunny habitats that have experienced some disturbance. Natural habitats include prairies, savannas, open woodlands, and edges, gaps, or clearings in richer deciduous forests. Anthropogenic habitats include roadsides, railroads, and powerline clearances.

Canada lettuce tolerates a range of moisture conditions from moist to slightly dry, tending to be both more common and more vigorous in slightly moister habitats. It also tolerates a wide range of soil fertility and pH. It usually occurs in loamy to clay-loam soils, but tolerates soil with a significant portion of gravel or rock. In the southeasternmost portions of its range, especially in coastal areas, it also occurs on sandy soils. It tolerates some degree of salinity, and can occur along brackish marshes and in coastal areas, as well as in areas exposed to road salt in winter.

Relative to the native tall blue lettuce (Lactuca biennis) and woodland lettuce (Lactuca floridana), Canada lettuce is less tolerant of shade but more tolerant of acidic soils. Relative to hairy lettuce (Lactuca hirsuta), Canada lettuce is much more common, but less tolerant of sandy soil. Relative to the introduced prickly lettuce (Lactuca serriola), Canada lettuce is more shade-tolerant and prefers moister habitats, is less tolerant of very dry habitats and very nutrient-poor soils, and, at least in the East, is more likely to occur in high-quality natural habitats and less restricted to disturbed and anthropogenic habitats.

↑Life Cycle

Canada lettuce usually grows as a biennial, but can grow as an annual under favorable conditions.

Plants initially establish as a basal rosette and usually persist only as a rosette through their first growing season. Plants quickly establish a deep taproot, which sometimes has considerable horizontal branches. Basal leaves can be evergreen in mild winters.

Second-year plants rapidly grow upright in spring and flower in summer. The bloom period for a single plant typically lasts 3-4 weeks; individual flowers are short-lived and the plant tends to bloom incermentally, with some seed maturing and blowing away as other flowerbuds are still developing. The bloom period is variable as a function of weather and site; plants on a particular site usually tend to bloom together, but in a broader region, and across many years, the bloom time may be much more variable. If the flowering stems of a plant are top-killed, plants may also bloom later. Although bloom time is variable, this species tends to bloom earlier than other Lactuca species with which it overlaps in range.

Seeds are distributed by wind, have no required dormancy, and often germinate quickly, within a couple weeks of falling.

↑Faunal Associations

This species is browsed heavily by mammalian herbivores, including rabbits, groundhog, deer, and livestock, including horses, cows, and sheep, all of which eat this species in spite of its bitter latex sap. Rabbits are more likely to browse first-year plants, whereas deer are more likely to eat the tips off developing stems. Groundhogs often knock the stems over to eat the developing stems. Herbivore browsing tends to greatly reduce the size and productivity of this plant, with lower productivity not only reflected in lower seed count but also greatly reduced seed size.

The flowers are primarily bee-pollinated, and attract a variety of bees including the broad-handed leafcutter (Megachile latimanus). and Lasioglossum lustrans.

The foliage is eaten by the larvae of several moth species, including the chain-dotted geometer (Cingilia catenaria), the moth Kodiosoma fulvum, and four moths of the Noctuidae family. The plant is also particularly good at supporting aphids, with a number of different aphids feeding on this plant, often in large numbers.

The seeds are occasionally eaten by goldfinches and are a preferred food of the house finch.

↑Uses

The sap, along with that of other Lactuca species, can be used to produce Lactucarium, a substance with opium-like properties that has seen some medicinal use. However this species is not the primary one used to produce Lactuarium; normally bitter lettuce (Lactuca virosa) is used.

This species is rarely planted intentionally and is often, unfortunately, viewed as a weed because it does not fit conventional Western aesthetics with its short-lived, relatively inconspicuous flowers.

However, within its native range, it has high value to the food web and can be used in ecological restoration projects, where it often competes favorably against invasive plants. Because it is common and weedy, it usually shows up in suitable habitat on its own, and needs little encouragment, so the most important thing to do to utilize this plant is to educate people on how to identify it and on its ecological value, so that they do not remove it.