Canada thistle
is a tall, erect, spiny, perennial, herbaceous plant that grows
to 4 ft. (1.2 m) tall. It has an extensive creeping rootstock. The leaves are
lance-shaped, irregularly lobed, 2-6 in. (5-15 cm) long with prickly margins.
The stems are ridged and hairy. The flowers are purple to white and can be up to
0.5 in. (1.8 cm) in diameter. Flowering occurs in late June to August. The small
fruit, called achenes, are 1 to 1.5 in. (2.5-3.8 cm) long and have a feathery
pappus which allows them to be dispersed further by wind. Numerous species of
thistle occur in North America, and while some are invasive, many are
native. Often the species are difficult to distinguish. Canada thistle can
invade a variety of open habitats including prairies, savannas, fields,
pastures, wet meadows and open forests. It forms dense stands which can
shade out and displace native vegetation. Once established it spreads
rapidly and is difficult to remove. Canada thistle is native to Europe and
Asia and was accidentally introduced to North America in the 1600s.
Leafy spurge is an erect, perennial, herbaceous plant that grows from 2 to 3.5 ft. (0.6-1.1 m) tall. Leaves are oval-shaped, smooth and 1-4 in. (2.5-10.2 cm) long. The plant produces a milky sap if stem is broken or a leaf is removed. The stem is smooth and bluish-green. Flowering occurs in the late spring (and sometimes the late summer), when yellow flowers develop in clusters at the apex of the plant. Fruits are 3 lobed capsules that explode when mature, spreading seeds up to 15 ft. (4.6 m). Large infestations give the landscape a yellowish tinge due to the yellow bracts. Leafy spurge invades prairies, pastures and other open areas. It is a major pest of national parks and nature preserves in the western United States. It can completely overtake large areas of land and displace native vegetation. Leafy spurge is native to Europe and was introduced accidentally into North America in the early 1800s as a seed contaminate
Purple loosestrife is a tall,
multi-stemmed (30-50 per plant), perennial forb that can grow up to 10 ft. (3 m)
in height. The opposite or whorled leaves are dark-green, lance-shaped, sessile,
1.5-4 in. (3.8-10.2 cm) long and round or heart-shaped at the base. Flowering
occurs in July to October, when pink to purplish flowers develop in 4-16 in.
(10.2-40.6 cm) long spikes at the tops of the stems. Flowers have 5-7 petals and
twice as many stamens as petals. Fruits are capsules that are enclosed in the
hairy sepals. Purple loosestrife is a serious invader of many types of wetlands,
including wet meadows, prairie potholes, river and stream banks, lake shores,
tidal and non-tidal marshes, and ditches. It can quickly form dense stands that
displace native vegetation. Purple loosestrife can spread very rapidly due to
its prolific seed production; one plant can produce as many as 2 million seeds
per year. Purple loosestrife is native to Europe and Asia. It was first
introduced into North America in the early 1800s for ornamental and medicinal
purposes. It has also been used as a nectar plant for bee-keeping.
Puncturevine is an annual, low-spreading forb native to the Mediterranean. Leaves are opposite, hairy, 1-3 in. (2.5-7.6 cm) long, and divided into 8-18 oblong leaflets. Flowering occurs from midsummer to frost, when 5-petaled, yellow, ½ in. (1.27 cm) wide flowers develop singly in the axils of the leaves. Fruit are circular, spiny burs that split into 5 sections. Burs can cause injury to bicycle tires, bare feet, and even small truck tires. Plants invade roadsides, pastures, fields and other disturbed areas. Plants are toxic to sheep and other grazers.
Common tansy is an aromatic, perennial herb native to Eurasia. Plants can reach to 5 ft. (1.5 m) in height and can be toxic to people and other animals if ingested in large quantities. Leaves are alternate, sessile or short-stalked, highly divided and glandular dotted. Flowering occurs from August to October, when yellow, button-like flowers develop in flat-topped clusters at the apex of the plant. Fruit are 5-angled achenes with small glands. Plants invade disturbed areas, ditch banks and riparian areas.
Houndstongue is a biennial plant that is 8-48 in. (20-122 cm) tall. First years growth is strictly vegetative and flowering occurs the second year. The first year the basal leaves are petiolate, elliptical, dark green and 6-8 in. (15-20 cm) long. The leaves on the flowering stem are alternate, sessile or clasping and narrower towards the top of the plant. In the early summer, flowers appear in racemes from the axils of the upper leaves. Flowers are reddish-purple with a five-lobed corolla. Houndstongue is native to Eurasia and occurs in riparian zones, coastlands, grasslands and agricultural areas.
Hoary cress, or whitetop, is a perennial forb in the mustard family that can grow up to 2 ft. (0.6 m) tall. The leaves are soft, gray-green, 1.5-3 in. (3.7-7.6 cm) long with fine hairs and heart-shaped bases. The lower leaves tend to have more hairs than the upper leaves. The upper leaves clasp to the stem of the plant. Flowering occurs in early spring to early summer, when white, four-petaled flowers develop in clusters at the apex of the stem. The fruit are heart-shaped seed pods. Hoary cress invades rangelands, pastures, streambanks, and open forests primarily in the western United States, although it does occur in the East. It can form large infestations that can displace native species and reduce grazing quality. Hoary cress is native to Central Europe and Western Asia and was first introduced into the United States in the early 20th century.
Perennial sowthistle is a perennial herb native to Eurasia. Stems can reach 4.9 ft. (1.5 m) in height and contain milky sap. Leaves are sessile, alternate, clasping, up to 12 in. (30 cm) long and decrease in size further up the stem, margins undulate with sharp spines. Flowering occurs in June to October, when yellow, tight, dandelion-like flower heads develop at the tips of the branches. Flowers open 2-3 hours after sunrise and close about noon. Fruits are reddish, ribbed and have a tuft of hair (pappus) at the top. Perennial sowthistle occurs along roadsides, railroads and other disturbed habitat.
Russian knapweed is a perennial forb that can grow to 3 ft. (1 m) in height. Stems originate from a basal rosette of leaves. The rosette leaves can be unlobed to very lobed and are 2-4 in. (5-10 cm) long. Stem leaves are oblong, pinnately lobed to entire and up to 6 in. (15.2 cm) in length at the base of the stem and become smaller and less lobed toward the top. In summer to fall Russian knapweed produces flowers that are pink to purple and turn straw colored as they mature. The flower heads are approximately ½ in (1.2 cm) in diameter and urn shaped. Fruits (achenes) are ivory-colored with a tuft of hair that falls off at maturity. Russian knapweed is native to Eurasia and was introduced into the United States in the early 1900’s. This species can cause chewing disease in horses.
Spotted knapweed is an herbaceous biennial or perennial plant that invades open areas throughout most of the United States. Its name is derived from the black margins of the flower bract tips which give the flower heads a spotted look. A basal rosette of deeply lobed leaves is produced the first year. Rosette leaves are deeply lobed, petiolate and approximately 8 in. (20 cm) long. Flowering stems are 2/3- 4 ft. (0.2-1.2 m) tall and branched. Stem leaves are alternate and may be slightly lobed or linear. Leaves become smaller and less lobed toward the apex. Flowering occurs in the early summer. Flowers are purple to pink in color and occur on small flower heads. Spotted knapweed invades a wide variety of habitats including pastures, open forests, prairies, meadows, old fields, and disturbed areas. It displaces native vegetation and reduces the forage potential for wildlife and livestock. Spotted knapweed is native to Europe and western Asia. It was accidentally introduced into North America in contaminated alfalfa and clover seed in the late 1800s.
Salt cedar is deciduous shrub that can grow up to 15 feet in height. Leaves are small, scale-like, gray-green in color, and overlap along the stem. The bark is smooth and reddish on younger plants, turning brown and furrowed with age. Several species are considered invasive in the United States and distinguishing the species can often be difficult. Salt cedar invades streambanks, sandbars, lake margins, wetlands, moist rangelands, and saline environments. It can crowd out native riparian species, diminish early successional habitat, and reduce water tables and interferes with hydrologic process. Salt cedar is native to Eurasia and Africa and was introduced into the western United States as an ornamental in the early 1800s. It occurs throughout the western and central United States, but is most problematic in the Southwest.