What Are the Tall, Skinny, Green Weeds With Thorns That Grow Under Trees?

Thorny Smilax

• The greenbrier family (Smilax spp.) is large, containing over 200 species, some of which are thorny weeds that can tolerate shade. In the eastern U.S., catbrier (Smilax rotundifolia), a slender, climbing plant, flourishes in U.S. Department of Agriculture plant hardiness zones 4 through 9. Sprouting from a tuber, it has glossy heart-shaped leaves and sturdy prickles on the stems. It can be found under trees and will often climb up into those trees. Its relative, California greenbrier (Smilax californica), hardy in USDA zones 7b to 9b, resembles its eastern relative, with the same tough prickles.

Prickly Nightshades

• A number of nightshades (Solanum spp.) are prickly weeds that may grow under trees. One is horse nettle (Solanum carolinense), found in USDA zones 4 through 10. The plant grows up to 3 feet tall, with prickly stems and leaves. Its five-petaled flowers are purplish white and the round fruits are yellow. All the plant parts are poisonous. An equally spiny relative is Western horse nettle (Solanum dimidiatum), hardy in USDA zones 5 through 10, and equally poisonous. Both the leaves and branched stems are prickly on plants that grow to 3 feet tall, with purple, five-petaled flowers and round yellow fruits.

True Nettles

• Nettles (Urtica spp.) are shade-tolerant, weedy plants equipped with fine hairs that "sting" or produce skin irritation. The most common variety is stinging nettle (Urtica dioica var. procera), hardy in USDA 3 through 10. Growing 2 to 6 feet tall, the slender stems support elongated oval-shaped leaves. Both leaves and stems bear the bristly, stinging hairs. Armed with even more bristles, the closely related European stinging nettle (Urtica dioica var dioica), also hardy in USDA zones 3 through 10, grows under many of the same conditions as its North American relative.

Other Prickly Types

• There are other thorn-bearing species that may also appear under trees. Young honeylocust seedlings (Gleditsia triacanthos), hardy in USDA zones 4 through 9, are slender and bear long pointed thorns. The leaves are medium green and fern-like. These should be removed promptly, as they will grow quickly into thorny saplings. Young wild raspberry (Rubus occidentalis), hardy in USDA zones 3 through 8, also produces slender, thorny canes, with leaves in groups of three. Since birds play a large role in dispersing the seeds, wild raspberries often pop up under trees.