Top Worst Weeds in Canada and throughout the World


Stems & Roots:
The Dodders are parasitic plants with leafless, thread-like, orange or reddish stems twisting and twining around other plants, often forming dense stringy masses. Being totally without chlorophyll, they must obtain all of their growth requirements (water, minerals, carbohydrates) from other living green plants around which they wind and become attached.
Dodder seed germinates in the soil and sends up a slender, thread-like, orange or reddish stem without any cotyledons (seed leaves). This slender stem sways or rotates slowly until it touches the stem or leaf of another plant and begins to wind around it.
If this plant is susceptible to attack by Dodder (and many are not) the Dodder stem immediately begins to form haustoria (tiny sucker-like roots). These penetrate the tissues of the host plant and extract all of the Dodder's subsequent growth requirements from it.
If the seedling is unable to contact a susceptible plant, it soon withers and dies. Once attached to a susceptible host, the lower end withers and breaks its connection with the ground, while the upper end of the stem grows rapidly, branching and rebranching.

Flowers & Fruit:
Numerous small clusters of tiny whitish flowers form along these twining stems from July to September. Each flower is followed by a small, globular seedpod with up to 8 seeds.

Habitat: Several species of Dodder are native to Ontario, but they occur chiefly in moist or swampy areas where they parasitize native plants.
Field dodder: is the most important of these as a parasite on agricultural crops, attacking carrots, petunias, tomatoes, etc.

Similar Species:
It is distinguished by its thread-like, yellowish or orange stems that occasionally form masses of tangled threads as they twine and sprawl over other plants. On susceptible plants they curl around the individual stems and produce root-like haustoria that penetrate inside.
It is also distinguished by its small, dense clusters of white flowers followed by small, round seedpods.
It spreads by seed, so when you set about removing it from your own plants, lay plastic around the bottom of the Dodder Weed to prevent the seeds from getting back into the soil. I cut the plant just above the root before I start pulling it off my trees and plants. Then I tackle the root separately by digging it right out or pouring vinegar and hot water on it, let it sit for an hour, then try to pull it out by the root.


Garlic mustard, a biennial herb in the Mustard Family, is native to Europe and Asia. It is believed that settlers brought this plant to North America for use in cooking and medicine.
Six infestations are known in B.C.; four in the North Okanagan at old homesteads, and two in the Lower Mainland. In Ontario, garlic mustard infestations can be quite dense and advance quickly. Sites within B.C. have not yet become as aggressive. Garlic mustard is shade tolerant and predominantly found in rich, moist forests and wooded streambanks.
Garlic mustard is a biennial with dark green, kidney-shaped leaves which remains a rosette the first year. Second year plants grow a stem 0.3 to 1.2 metres high with triangular, alternate, sharply toothed leaves. Small, white, four-petal flowers develop in May in clusters at the end of the stem and occasionally in leaf axils. Seeds are black and form a single row inside slender, erect, pale brown pods (siliques) 2.5 to 6.5 cm long.

Reproduction is by seed, which are produced only by second year plants. Each plant produces hundreds of seeds, which may be scattered long distances by wildlife and human activity. Garlic mustard flowers are either cross-pollinated by insects or can self-fertilize, depending on conditions. Self-fertilized seeds produce identical plants that may enhance its ability to colonize an area.

Garlic mustard is most commonly found in shaded areas, even dense shade, but occasionally grows in locations receiving full sun. It occurs in river floodplains, forests, trail and forest edges, and along roadsides. Garlic mustard prefers moist areas but does not tolerate acidic soils. Disturbed sites are most susceptible to invasion and dominance.

Garlic mustard, an early spring competitor, invades forest communities where it monopolizes light, moisture and soil nutrients, resulting in aggressive domination of the ground layer. It is a severe threat to many natural areas where it occurs because of its ability to exclude many native herbaceous species.

As with most weeds, the sooner you catch it, the better. Cut the flower stalks in the spring as soon as you see them, or if any are left over from the previous year. Also if they’re not too close to your other plants, you can also try hand-pulling them. Get rid of everything you cut down and pull to keep it from popping back up.


Bindweed is a perennial viney weed that spreads from an extensive rootstock as well as from seed. Stems may be several feet long and trail along the ground or climb on upright plants such as shrubs.
Trumpet-shaped white to purplish white flowers close each afternoon and reopen the following day. It is a Eurasian native that has thoroughly naturalized itself in North America and was first documented in California in 1884 when collected in San Diego.
It probably arrived in the US as a seed contaminant, but may have also been planted as an ornamental.

Other names for bindweed are perennial morning glory, creeping jenny, bellbine, sheep-bine, and corn-bind.

Mature field bindweed looks very similar to the annual morning glory, with arrow-shaped leaves between 1/2- to 2-inches long growing alternately on a viney stem and trumpet shaped flowers ranging from white to pink.
In contrast to field bindweed, the ornamental annual morning glory has a larger, more showy flower that may be white to blue or purple in color, a thicker stem that is sometimes hairy, and heart-shaped leaves.

The twisted stems of bindweed may grow to 6-feet, forming dense mats when there are no other plants or structures upon which to climb. With warm, moist conditions, bindweed leaves are larger and vines more robust than under drought conditions.

Bindweed is a hardy plant found that typically grows below 5,000 feet elevation, reproduces both from seed and creeping roots and survives in extremely diverse environmental conditions.
Seeds can remain viable in soil for 50 years or more and the plant itself is very drought tolerant. Field bindweed dies back each year, but with a deep, extensive root system and fragments as short as 2-inches forming new plants, bindweed easily overwinters without foliage.

Field bindweed, once established, is almost impossible to control with herbicides and precludes planting of certain production crops such as onions, melons, and tomatoes.
Black polyethylene mulch can be an effective control if no light is allowed to reach the soil and the plant, but any holes will allow plant growth and it may take more than three years before the bindweed is killed. Additionally, once the plastic is removed, new plants may germinate from seed
The Bindweed suckers have long roots, which makes them almost impossible to hand-pull. Be patient in your quest to free your garden from bindweed: it will take a few attempts to completely eliminate it from your gardens.
If the bindweed isn’t too close to your preferred plants, pour boiling water on it to kill it. If it is too close to other plants, keep cutting the Bindweed down until it is gone.
Some of this information on bindweed was culled from UC Davis' Integrated Pest Management site

Crab Grass

General Description:
Before heading out, both Smooth crab grass and Large crab grass are distinguished by their tapered leaf blades, their split sheaths with hairless margins, their more or less hairy lower leaf sheaths and their either hairy or smooth blades.

Stems & Roots:
Smooth hairless leaf blades, and upper leaf sheaths usually smooth and hairless, but the lower ones may be womewhat hairy; ligule also membranous but usually a bit longer (2-3mm, 1/12 - 1/8in.), and there is a tuft of long hair on either side of the leaf-base of the lower leaves.

Flowers & Fruit:
Almost identical to that of Large crab grass but usually with only a single whorl of spikes at the end of the stem. Flowers from August to September.

Both kinds of crab grass are common in southern Ontario but they also occur sporadically in nothern and northwestern Ontario. Both occur in row crops and other fields, waste places, gardens and lawns.

The best way to control most weeds is to catch them early and pull them out by the roots or simply pour boiling water directly onto the crabgrass in order to kill it at the root. I always try to catch the weeds early in the spring, the soil is wetter, the roots are not as established yet, all which makes them easier to pull.

Spotted Spurge –
This dense, low growing weed isn’t easy to get rid of, so your best offense is to constantly keep an eye out for it in the early spring before it becomes too well established and when it is easier to pull. Pull it by the roots immediately before it has a chance to grow and multiply.
Spotted spurge grows close to the ground, often forming a dense mat with dark green leaves,and frequently a red spot will mark the leaf halfway down its center vein.
Flowers, fruit, stems, and leaves are hairy. The short stems little scalelike appendage at their base, but they are hard to see. Broken stems and branches secrete a milky, poisonous sap. Although spotted spurge sap is being studied as a cure for various skin cancers, in general, the sap of all members of this genus is an eye and skin irritant.

Habitat –
Spotted spurge is a major spurge weed in around the United States and is very common in the southwestern portion of southern Ontario, occurring in cultivated fields, gardens, waste areas and roadsides. It can also be found in the counties bordering the north shore of Lake Ontario.
All spurges
have milky sap, which can be toxic to some animals. Ground spurge and creeping spurge grow prostrate like spotted spurge but have no markings on their leaves.
All spurges reproduce by seed, and creeping spurge also can produce roots along the stem, creating new plants vegetatively. Petty spurge is a cool season annual found in shady, moist areas, particularly in flower beds.
Native to Europe, it grows upright and is much less invasive than spotted and creeping spurge species. Garden, nodding, and thyme-leafed spurges cause fewer problems.

Most weedy spurges are summer annuals that don't like competition and depend on their prolific seed production for survival. A single plant can produce several thousand seeds, which are small and can remain dormant in the soil until conditions are suitable for germination (sprouting).
Seeds produced in summer germinate immediately while those produced in late fall mostly will lie dormant and won't germinate until spring.
Spotted spurge germinates best when temperatures are 75° to 85°F, but germination can occur at temperatures as low as 60°F and as high as 100°F.
When moisture is available, germination can occur from February through September in most areas of California. Light also is a requirement for maximum germination; seeds buried deeper than 1/2 inch won't germinate well.
Plants that germinate early in spring in cool conditions can remain as small seedlings until temperatures are more desirable for growth. Once the seed germinates, a small rosette of leaves develops. As growth continues, the leaves form a dense mat that can grow up to 3 feet in diameter. Reproductive growth is rapid, and the plant can produce seeds as soon as 5 weeks after germination.

Spotted spurge can establish itself in horticultural, agricultural, and noncrop sites. It overgrows sparse turf areas and low-growing ground covers, invades open areas in gardens and landscapes, and can grow in sidewalk cracks.
In addition to reducing the growth of desirable plants, spotted spurge reduces uniformity and quality of turf, provides a habitat for undesirable insects in citrus groves, serves as an intermediate host for fungal diseases of cultivated crops, and attracts ants with its seed.
Spotted spurge is poisonous and can kill sheep grazing in pastures where it is the predominant weed. Sheep that consumed as little as 0.62% of their body weight of this plant have died within a few hours.

Canada Thistle

Canada thistle is a perennial broadleaved weed with creeping roots that extend up to 17 feet (5 m) horizontally and 20 feet (6 m) deep.

Seedling Description
The hypocotyl (stem below the seed leaves) of Canada thistle is pale green. Seed leaves are dull green, elliptical, thick, and ¼ to 3/8 inch (0.6 to 1 cm) long. They come together at the base to form a shallow cup.
The first true leaves—from seed or established root systems—are thick and covered with short, bristly hairs. Margins are wavy and irregularly lobed. Each lobe ends in a sharp prickle.

Canada thistle is a perennial broadleaved weed with creeping roots that extend up to 17 feet (5 m) horizontally and 20 feet (6 m) deep. Plants grow 2 to 5 feet (0.6 to 1.5 m) high. The stem is slightly hairy when young and grows hairier with age. Leaves are alternate and oblong. They have irregularly lobed margins with spiny crinkled edges terminating in a spine. The upper side of the leaf is dark green; the lower side is light green and slightly hairy. Some plants have leaves that are smooth on both sides. Canada thistle leaves are stalkless. The base of each leaf surrounds the stem, giving the impression that the stem is also spiny..
The flower heads of Canada thistle are flask-shaped, measure ½ to ¾ inches (1.3 to 1.9 cm) in diameter, and contain many small tubular flowers. Spineless bracts surround each flower head. Male and female flowers are found in separate heads and on different plants (dioecious habit). Flowering occurs from June through October. Flowers are pollinated primarily by wind, but also by insects attracted to the weed's strong honey scent. Flower color varies from white to pale blue to purple. Most flowers are rose-purple.
Each flower head produces about fifty seeds and an average stem bears twelve to fourteen flowers. Seeds are brown, smooth, and slightly tapered at the end. They measure about 3/16 inch (0.5 cm) long. The seed is attached to a white to brownish tuft of hairs called a pappus, which aids in seed distribution by wind. Seeds germinate from late spring through autumn and germinate best at 86° F (30° C).
The age of the seed influences its response to light. Young seeds germinate well in bright daylight, and old seeds respond best to weak light. Germination studies show that seeds stored in wet sand during the first winter germinate better than those stored dry. Seeds may be viable after twenty years in the soil and can germinate after four years of storage in water. Canada thistle also propagates vegetatively. Fragmentation of roots and stems can produce dense stands, and new seedlings can reproduce by runner roots seven to nine weeks after emergence. Roots penetrate as deep as 20 feet (6 m), but most settle in the top 15 inches (0.4 m) of soil.
Canada thistle grows well where summer temperatures are moderate and rainfall is not too heavy. It can survive in many types of soil and under a wide range of moisture conditions. It can also tolerate high salt content but does not thrive on very light, dry soils. Canada thistle is most competitive on deep, productive, well-aerated soils that do not become too warm. It grows in plantings of many grain crops—including barley, millet, rye, sorghum, wheat, corn, and oats—and is a serious problem in pastures and in vegetable crops such as beans and peas. Canada thistle also grows in noncultivated areas, such as in waste places, along roadsides, and in railroad yards.

Similar Species
Canada thistle differs from similar species in several respects. One difference is life cycle: Canada thistle is a perennial, whereas most other thistles (e.g. , bull and Russian) are biennials. Canada thistle is also distinguished by its almost spineless flower heads and by its green, wingless, slender, spineless stems. In addition, the male and female flowers of Canada thistle are found on separate plants, while those of other thistles are on the same plant.

Natural History
A native of Europe, Canada thistle now grows in about thirty-seven countries around the world and is widespread throughout the United States and Canada. The fine bristles of this plant can irritate the skin, while the pappus can get in the eyes and hair of humans and animals.
Canada thistle is a good pollen plant for the honey industry. It is the food plant for the caterpillars of the beautiful Painted Lady butterfly. In the United States, researchers are investigating control of this weed by insects.
Canada thistle is known by several other common names, including creeping thistle, small-flowered thistle, perennial thistle, and green thistle.

There are two kinds of control methods used for Canada thistle, cultural and chemical, depending on the nature of the infestation. The first step in any control program is to prevent infestation.
A combination of methods is most effective when crop infestation does occur. Seed production and vegetative root propagation should be prevented by depleting the energy reserves in the long, creeping roots. Cutting, plowing, cultivating, and applying herbicides are common practices for depleting the energy reserves of thistle roots.
In pastures and other noncultivated areas, repeated mowing reduces the infestation of Canada thistle by weakening the plants. This practice prevents seed production and destroys the current year's growth. Mowing for several years depletes the underground root reserves.
Tillage is a more effective control measure than mowing. Repeated cultivation exposes thistle roots to drying or freezing and, more important, prevents the buildup of food reserves in the root system. Soil should be tilled 3 to 4 inches (8 to 10 cm) deep. Fields should be disked or cultivated when thistle plants are 2 to 3 inches (5 to 7.5 cm) tall or when the seedling has emerged 4 to 6 inches (10 to 15 cm) in the spring. To eradicate Canada thistle, top growth must be destroyed by cultivating every three weeks for the rest of the season. In many areas, this practice eliminates almost all of the plants; tillage can destroy the remaining plants the following spring.
Perennial forage crops and winter-annual cereal crops compete very effectively with Canada thistle and, therefore, inhibit its emergence.
Prepared by W. Thomas Lanini, Extension weed specialist. For the College of Agricultural Sciences My notes: When chemicals are necessary should use organic methods.