Lawn, Garden and Roadside Weeds
By Jeannette Adams, Marilyn Dudek and Teresa Lopata
Introduction: A definition of a weed is any plant growing in abundant quantities where it is not wanted. It interferes negatively with the growth of desirable plants, is persistent and difficult to eradicate.
Weeds are divided into perennial, annual, winter annual and biennial categories. Depending on the level of infestation, governing bodies have developed classifications and noxious weed control laws. These may differ from region to region. In Manitoba a listing of noxious weeds and a Tier Level categorizing the severity of infestation can be found on the Government of Manitoba Agriculture website.
For the purpose of this listing of weeds, we have chosen those weeds that are most commonly found on or near public and private gardens. We have also included a listing of plants that are still sold or exchanged as ornamentals, but can be very aggressive and spread easily into neighbouring properties.
Identification: Identifying the “weed” can be difficult because many are very similar in appearance. Also identification depends on what stage of growth you are dealing with. It is important to examine leaves, stems and roots at the early stages of growth. As the plant matures you will have flowers and maybe seed pods to examine. Also consider where the plant is most plentiful, is it in sun or shade, dry or moist soil, tilled or compacted soil. Identifying the plant provides the information required to decide the best control methods.
Control Methods: Early detection and removal of unwanted plants is the first line of defence. Making sure your desired plants are healthy and vigorous will impede the growth of weeds. Using mulches inhibits the growth of some weeds. Tilling and hand weeding removes most annual weeds. However, some weeds are very persistent and require repeated digging and discarding of all plant parts. If dealing with a large area, smothering or solarizing can be used. This is where dark sheeting, heavy cardboard or old carpeting is spread over the area and left for several weeks or even months during sunny, dry weather. This smothering will kill the roots and once the cover is removed the dead plant material can be removed to insure that there is no regrowth. If repeated attempts to eradicate fail and the problem is increasing, it may be time to consider some form of chemical control. There are more organic products being developed so try those before resorting to the more toxic chemical products. Always follow the instructions and take the necessary safety precautions when using any product even ones listed as “organic” or “natural”.
1. Dandelion – Taraxacum officinale
Easily recognized by its bright yellow flower heads or fluffy gray seed heads, it is a perennial that spreads by seeds and branching taproots. Dandelions can be found almost everywhere and if left unchecked form thick colonies in lawns, parks and along roadways. Seedlings appear in early spring with smooth oval shaped cotyledons and grow into a basal rosette with irregularly toothed leaves. The flower buds form in the middle of the rosette. The hollow stalk which supports the flower leaks a sticky white sap when cut. Repeatedly digging out as much of the taproot as possible will weaken young and established plants and may cause their demise. If you know seeds have blown onto your lawn, you can apply corn gluten, a pre-emergent, in early spring. This will reduce the number of seeds that germinate, but do not use corn gluten if you want to reseed your lawn. Heavy infestations may require spraying; however, use spot spraying rather that general broadcast.
2. Canada Thistle – Cirsium arvense
A perennial that spreads by horizontal root stalks and seeds. Canada Thistle is found in home gardens, back lanes, fields and especially along roadways and undeveloped properties. Seedlings form in an oval and develop into a basal rosette of spiny irregularly toothed leaves. Often found in groups since they grow from the spreading root system. As the plants mature, erect stems form with prickly notched leaves topped by purple flowers that open in mid-summer. Gray seed heads follow and copious amounts of seeds are spread by the wind in late summer. Roots can go down several feet, so persistent, repeated digging and removal of as much of the root and any seedlings is the best way to keep control of this aggressive plant. Do not till as this will only spread the weed. Repeated mowing along roadways and undeveloped properties can control seed production as long as mowing is done before flowering.
3. Perennial Sow Thistle – Sonchus arvensis
Perennial and annual Sow Thistle are hard to distinguish. However, Perennial Sow Thistle has extensive light-coloured creeping roots that are easily broken. Seedlings form irregularly spaced elongated leaves with soft prickles along the margins. Erect, hollow stems contain a milky sap and form several branches that produce clusters of bright yellow ray flowers in mid-summer. Gray seed heads follow and seeds are spread by the wind, but also provide food for some seed eating birds. Perennial Sow Thistle is found in home gardens, field crops, along roadways and undeveloped properties and prefers moist conditions. In home gardens dig out and discard as much root as possible as it will regrow. This will require repeat digging so a thick, organic mulch can be applied once the plants are weakened; however, the thistles may still find their way through or around. Do not till. Sow thistles act as a host for viruses that can attack desirable plants.
4. Quack Grass or Couch Grass – Elytrigia repens
Recognized as coarse, slightly hairy blades attached to a tough whitish rhizome. This perennial weed spreads aggressively through these rhizomes that secrete a toxic substance which suppresses growth of surrounding plants. You want to keep it away from shrubs and trees. This grass grows everywhere and can quickly take over garden beds or areas of lawn if not controlled. Roots can penetrate deep into soil and creep under shallow barriers or through thin mulch. Persistent, continuous removal by digging out the root system and discarding is recommended. Do not compost or till as even small pieces of root will regrow. Solarize large patches. A last resort is chemical application, but this means you will also lose your adjacent plants. In undeveloped areas it becomes a ground cover that can be mowed to keep it from going to seed; however, if this area is ever reclaimed it will be very difficult to get rid of the extensive root system.
5. Broad-leafed Plantain – Plantago major
Perennial plant that reproduces by seeds produced on long stalks that form above a basal rosette of ribbed, oval leaves. Tough, fibrous roots make it difficult to pull out once it is mature. It thrives in moist, compacted soils along roadways and in lawns and quickly forms colonies if not controlled, so it’s best to find the seedlings and dig them out. Seed production continues throughout the growing season so mowing will spread the seeds. Birds like the seeds and Plantain leaves were used to treat insect bites.
6. Creeping Charlie or Ground Ivy – Glechoma hederacea
Aggressive low growing perennial with green scalloped round leaves and small blue flowers with a mint like smell when crushed. This weed reproduces by seeds and stolons that spread and send down roots forming new plants. It prefers moist, shaded areas and grows in gardens and will infest the lawn forming dense mats. Pull out and discard all plant parts. Do not mow or till as it will regrow. Use a thick mulch in beds, for heavy infestation in turf try to solarize. Can make cats or dogs sick if they eat this plant.
7. Orange Hawkweed – Hieracium aurantiacum
Oval elongated leaves covered on both sides with fine hairs form a basal rosette. Clusters of flat topped bright orange ray flowers are produced on top of single, hairy stems that contain a bitter, milky sap. Flowers are produced all summer. A creeping perennial that spreads through leafy runners that root, underground rhizomes as well as seeds. It grows along roadways, in hay fields, meadows and undeveloped land. Because the flowers make this plant attractive, it has found its way into people’s gardens and become a problem because of its aggressive nature. It is classified as invasive. Eradication requires aggressive, persistent digging and removal of all plant parts. Do not allow to go to seed since the dandelion like seeds can spread quickly. Do not till or compost.
Annual, Winter Annual and Biennial Weeds
1. Purslane – Portulaca oleracea
An annual with succulent elongated reddish leaves that form prostrate mats around a central taproot. Leaves are veinless and alternately arranged. Small yellow flowers grow at the ends of stems and open on sunny days. It reproduces by seeds and stem cuttings. Germinates during warm summer weather. Purslane grows in gardens, along roadways and in sparse turf. Purslane can be hoed in the early stage; however, since all parts of the plant can regrow especially after a rain, this may not be successful. It is best to remove the complete plant including the root and discard. Do not compost or till. Seeds can remain viable for several years.
2. Common Groundsel – Senecio vulgaris
Classified as an annual or winter annual Common Groundsel reproduces by seeds. A prolific re-seeder if produces seeds all summer and fall rosettes can survive the winter. Irregular rosettes of elongated, notched leaves form along multiple short erect stems. Small yellow disk flowers form in clusters at the ends of stems. Tan coloured seeds are covered with white hairs and look like tiny, closed thistle heads. This weed grows in gardens, lawns, along roadways and undeveloped properties. Pull or dig out the entire plant including the tap root as it will regrow. Discard plants as flowers may still produce seeds if dropped and left in garden. Plants contain alkaloids that are poisonous to cattle, horses and pigs.
3. Stinkweed – Thlaspi arvense
Stinkweed is an annual or winter annual that is sometimes confused with Shepherd’s Purse; however, the unpleasant smell produced when crushed help to identify this weed. Oblong, smooth leaves form a basal rosette. Branched stems 20 to 50 cm high produce racemes of tiny white flowers which will produce circular, flat seed pods. Stinkweed is a prolific re-seeder and the seeds germinate throughout the growing season. Seeds can survive up to 20 years. This weed can grow in most conditions and if unchecked will form thick colonies that will compete with your spring seeded plants. Plants that have survived the winter and flowering plants are difficult to pull, but spring and fall seedlings can be tilled. Larger plants need to be pulled out and discarded. This plant acts as a host for Tarnished Plant Bug.
4. Shepherd’s Purse – Capsella bursa-pastoris
Shepherd’s Purse was introduced to North America because of its medicinal and culinary uses. A prolific re-seeder that continues to germinate throughout the growing season, fall seedlings survive the winter which makes it a problematic weed. It grows in gardens, lawns, crop land, along roadways and in undeveloped land. A basal rosette of elongated, deeply lobed leaves produces multiple sparsely branched stems 15 to 50 cm tall. Racemes of tiny white flowers form at the tips of stems and produce heart-shaped flat seed heads. Hoeing fall and spring seedlings and pulling and removing larger plants before seeds form will help control this plant. In lawns it will be harder to eradicate because of its tough, long taproot. Keeping lawn healthy will reduce germination of these and other annual weed seeds.
5. Annual Sow Thistle – Sonchus asper
There are several types of annual Sow Thistle and it is often difficult to distinguish them from Perennial Sow Thistle. Annual Sow Thistle leaves are more deeply lobbed and the roots are tap roots rather than underground runners. Pale yellow ray flowers form at the tops of stems and seeds are spread by the wind. Annual Sow Thistle grows in gardens, fields and undeveloped properties. It prefers moist conditions so will grow in ditches if moisture is available. Seeds continue to germinate throughout the growing season so it is best to hoe or dig out small plants before they form a deeper root system. The tap roots break easily at ground level if you try to pull out the plant and it will send up new growth. Sow thistles act as vectors for some viruses that can affect beneficial plants.
6. Thyme-leaved Spurge – Euphorbia serpyllifolia
Thyme-leaved Spurge forms prostrate mats of ovate leaves oppositely arranged on reddish stems which bleed a milky sap when cut.Tiny, green flowers form at the ends of stems. This annual reproduces by seeds that germinate throughout the growing season. It grows in gardens, lawns, along sidewalks and on undeveloped properties. It will grow more erect in shaded areas. Plants are fairly easy to pull out, but be sure to get the entire tap root and remove the plants since seeds can still mature if plants are dropped.
7. Prostrate Knotweed – Polygonum aviculare
Prostrate Knotweed is an annual that grows well in dry compacted soils along roadsides, walking paths and will find its way between paving stones. Narrow blue-green leaves form in clusters along wiry stems that form irregular mats and will form thick colonies along sidewalks. A tough taproot and the fact that this plant grows in compacted soil, makes it hard to pull out; however, you need to kill the entire plant as it can regrow if the root is left. If Knotweed becomes a problem in your lawn, aerate and water to encourage healthy growth of the lawn. This plant acts as a host for powdery mildew.
8. Redroot Pigweed – Amaranthus retroflexus
Redroot Pigweed is identified by the red colour on the base of the stem and underside of leaves of seedlings. An erect, hairy stem with ovate, alternately arranged leaves can grow 30 to 50 cm tall. Tiny, green flowers form a bristly spike at the top of the stem. Thousands of black seeds that are produced can survive for several years. Pigweed grows in gardens, fields, along roadways and in undeveloped properties. To control pigweed, hoe or pull out seedlings. If the plant has reached the flowering or seed stage it will be more difficult to pull out the tough taproot. Discard plants as they can go to seed or re-root if dropped. This plant acts as a host for Tarnished Plant Bug that can affect berry crops such as strawberries.
9. Chickweed – Stellaria media
Is classified as an annual or winter annual. The plant enjoys a shady, moist, damp, nitrogen- rich location spreading close to the ground to a 5 to 50 cm width. The opposite located soft bright green leaves are oval with a pointed tip and the tiny white flowers, which are comprised of 5 two-lobed white petals, are located at the tips of these growing stems. Chickweed plants procreate through prolific seeds or stolons, which mean they root along the horizontal plant stems forming new plants. To keep chickweed under control aerating the location and consistently physically removing the plant will help with control. Do not till as each little piece of the plant will regrow.
10. Creeping Wood Sorrel – Oxalis corniculata
Creeping Wood Sorrel is delicate in appearance enjoying full to partial sun and fertile loose soil. It has low growing, 2 to 8 inch narrow, slightly succulent, creeping stems which root easily. The plants can be either light green or a reddish purple, both with alternate trifoliate leaves along the stems. The flowers grow either individually or in small umbels in the axils of the leaves or at the stem tops. They are a quarter of an inch across and a little longer in length, blooming early summer to fall. The erect seed capsules, one half inch long, split into five parts ejecting seeds up to 10 feet away. Oxalis has a fleshy taproot system which can produce vegetatively along the root stolons. As the plants are very easy to pull out, eradicating Oxalis with constant monitoring and pulling the entire plant before it goes to seed, is required. Any little piece that remains can regrow.
11. Yellow Goat’s Beard – Tragopogon dubius
Goatsbeard is a biennial, occasional annual or a short-lived perennial grown through seed only. The high, 30 to 100 cm (12 to 40 in.) tall stems are round and smooth, slightly fleshy with a deep-rooted thick taproot. Juvenile, first year biennial, plants are stemless and resemble shoots of grass. Goats Beard leaves are long, very grass like, smooth and fleshy with one leaf per stem node. Young leaves are slightly hairy or downy in appearance and touch. Blooming in June/July and occasionally continuing to September, the buttery pale yellow, ray-shaped flowers, 4 to 6 cm. (1 ¾ to 2 ¼ in.,) sit singly at the stem ends. The flower heads open in the morning following the sun until mid-day when they close. The mature seed head, a white fluffy sphere, 7 to 10 cm. (3 to 4in.) is comprised of individual seeds, much like a dandelion dispersed through wind. The entire Goats Beard plant, excepting the seed, exudes a white milky juice. This plant is found in pastures, meadows, roadsides and occasionally in home gardens. Recognizing the young Goat’s Beard plant and digging it out is essential to remove it from growing. All of the deep tap root must be dug out. As it is mostly a biennial, first year recognition is important and immediate removal will prevent it from recurring. If you notice it in bloom, remove the bloom and dig out the plant. Never allow it to go to seed which is a big fluffy dandelion-type head.
12. Pineapple Weed – Matricaria matricarioides
This is an annual weed producing new plants through seed dispersal. The plant grows from 5 to 40 cm. tall with erect multi branched hairless stems. Leaves are alternate and very finely divided and when crushed have a pineapple scent, hence the plant’s common name. The small, 5 to 9 cm. across, yellowish-green flowers, are grouped together and each one is dome shaped, looking much like a mini pineapple. The flowers are without ray florets and can seed spring through autumn. Pineapple Weed grows in compacted soil along pathways and roadways. As this plant has a fairly shallow root system it is readily pulled especially if the soil is moist. Constant pulling and not having the flowers mature, and quickly going to seed, is the best way to control this weed.
13. Common Burdock – Arctium minus
Common Burdock is a biennial weed with basal leaves in the first year, flowering in the second and dying after producing seed, up to 18,000. The leaves are large heart shaped alternating on 1 to 3 m. (2 to 6 ft.) tall thick hollow-branched stems. The leaves diminish in size as the plant grows upwards losing their heart shape and becoming more angular and elongated. Blooming from July to September, the 2 cm. globular purplish-pink flowers, with densely hooked bristles, grow singly or in small clusters at branch ends or at the axils of the leaves. There are no ray florets on the flowers. The mature seed heads, or burs, easily break off adhering to clothing or animal fur, hence dispersed widely. The taproot is thick and fleshy. Burdock is found in waste places, pastures, open woods, roadsides, fence rows, barnyards and occasionally in gardens where in its first year growth can be mistaken for rhubarb leaves but could be differently distinguished by its wooly underside leaves. With Burdock being a biennial entire removal, especially all of the deep root, of the plant guarantees it will not regrow in its location. If the plant does flower, immediately remove the flower head before it matures to seed stage.
14. Common Ragweed – Ambrosia artemesiifolia
This annual weed grows only from seed. The erect stems grow between 6 to 60 inches (15 to 150cm). On young plants the leaves are bright green to slightly yellowish green and the compound finely-divided leaves become grayish green on older plants. The green flowers are unisexual, both male and female are on the same plant, and are very inconspicuous growing between, 1/12 to 1/5 inches (2.5mm). The male flowers are racemes, where the flowers are attached on short stalks to a central stem, growing at branch ends and hanging downward. The female flowers heads are located in the leaf axils near the base of each of the long clusters of male flowers. Each flower head has only one diamond-shaped seed, 3 to 5mm (1/8 to 1/5 inch) long. Bloom period is August to October and found in cultivated land, and is the most common weed in gardens and flower beds. Also prevalent along roadways, sidewalk edges, fence lines and waste places. Ragweed’s high pollen count is the major cause of hayfever and can drift on the wind vast distances up to 125 miles (200kms). Giant ragweed Ambrosia trifida) has similar properties but grow larger and with larger flowers along with leaf surfaces that are usually rough feeling like sandpaper. Learning to recognize Ragweed at its very early stage allows easy pulling out. If it grows to a large size immediately remove the entire plant before the tiny flowers mature and go to seed.
15. Black Medick – Medicago lupulina
This annual, which when young can be mistaken for clover, is in the legume family, and grows by seed only. The plant has alternate, one per node, compound leaves which are comprised of three oval leaflets with the centre one having a definite stalk. The wiry stems grow prostrate on the ground, or on shorter plants a bit taller and more erect. The roots are slender and very difficult to pull or hoe out. The individual yellow flowers are grouped in a dense head-like cluster, 1cm (2/5 in.) in size, located on long stalks with leaf axils. The flower is similar to pea and bean flowers and the black seed pods, hence the common name, drop seed throughout the growing season from early spring to late autumn. As it is very common in lawn, it is the gardener’s nemesis as it is very difficult to eradicate. Black medick also grows in gardens, waste lands and pastures. As dedicated constant pulling is required to control black medick, and as mentioned frequents lawns, a herbicide could be used as a last effort to remove the plant.
16. Lamb’s Quarters – Chenopodium album
This annual grows between 20 to 200 cm (8 inches to 6 ½ feet) high with branched smooth green, or with reddish lengthwise stripes and ridges, on the stems. The initial true leaves are opposite but later grow to be alternate as are the stems. The stalked leaves, 3 to 10cm (1 to 4 inches) long are lance shaped or broad triangles with irregularly shaped shallow teeth. The leaves are green or can be grayish due to a powder-like look and sometimes with a reddish hue on the undersides. The greenish flowers are very small with 5 sepals and no petals. These thick granular flower clusters are found along the main stem and upper branches blooming from June to August. Seeds are small, 1 to 1.5mm (1/25 to 1/16 inch). Lamb’s Quarters are very widespread in gardens, cultivated fields, roadsides, wasteland, pastures and wherever soil is cultivated. Early recognition of the plant is important and at that time easy to remove. If the plant is mature, immediately remove it and throw into the trash, do not compost as the flowers mature to seed stage quickly.
Aggressive and Invasive Ornamentally Adopted Plants
1. Creeping Bellflower – Campanula rapunculoides
Common names: creeping bluebell, European bellflower, garden bluebell, June bell, rampion bellflower, rover bellflower
Creeping Bellflower is a perennial that originated from Europe and was brought over as an ornamental that has become problematic. Creeping Bellflower is a hearty plant that can survive drought and tolerate various light conditions .It is found in gardens, lawns, back alleys and undeveloped areas. It reproduces by seeds as well as vigorous rhizomes that can creep under fences, sidewalks and concrete. Seedlings have heart-shaped hairy leaves but as the plant matures the leaves become lance shaped along erect stems that can reach 1 m. in height. The light purple bell-shaped flowers are arranged along one side of the stem with 5 united petals.
This plant is resistant to most herbicides and because it can grow from root fragments, persistent and continuous digging up as much of the root system as possible requires years of effort in order to be successful.
Slugs like this plant and can do some damage, but not enough to allow slugs to set up in your garden. It is best to diligently dig out the plants and roots when the plants are small and before the problem gets out of hand. It is not preferred by deer.
2. Dame’s Rocket – Hesperis matronalis
Common names: Dame’s Violet, Sweet Rocket
Dame’s Rocket is a biennial or short-lived perennial that is native to Europe and South-west Asia. It was introduced to North America as an ornamental. Dame’s Rocket and Garden Phlox look very similar except that Garden Phlox has 5 petals whereas Dames Rocket only has 4. There are several stems per plant that can grow up to 1 m. Leaves are lance shaped with serrated edges, dark green and hairy on both sides. Fragrant flowers range in colour from white to pink to purple and form loose clusters at the top of the stem. Dame’s Rocket is a prolific re-seeder that will out-compete native vegetation and take over moist shady areas including meadows, ditches and undeveloped properties. Hand pulling is an effective means of control as the roots are easy to remove.
3. Ribbon Grass – Phalaris arundinacea
Common names: reed canary grass, gardener’s garters
Ribbon Grass is a perennial ornamental grass that has the reputation for being very invasive. This grass could have a place in the garden as a ground cover in tough sites or where nothing else seems to want to grow. It needs to be used with caution as it spreads by rhizomes and can creep between rocks and under barriers. The leaf blades are usually green, but may be variegated. Ribbon grass is tolerant of a wide range of soils from dry to wet in sun or shade. When planted in the shade, the plants tend not to be as aggressive and are better behaved. Persistent, repeated digging and removal of all root parts or installing a strong, deep barrier can keep this grass under control.
An invasive species of Canary Grass which is green and grows up to 2m. has become problematic in grasslands because it spreads by seed as well as rhizomes.
4. Himalayan Balsam – Impatiens glandulifer
Common names: Policeman’s helmet, Himalayan Impatiens
Himalayan Balsam is an annual herb, native to the western Himalayas which was introduced to North America as a garden ornamental. Himalayan Balsam has a pink orchid shaped flower resembling a British policeman’s helmet, which gave rise to its common name of “Policeman’s helmet”. Leaves are toothed and opposite on a pink-tinged stem that can grow several feet in height. Mature seed capsules explode when touched sending seeds flying several feet from the parent plant. Himalayan balsam can completely cover an area and crowd out native vegetation. It is still found in private gardens but has escaped into mostly riparian areas, especially river edges and wetlands. The shallow root system makes hand pulling an easy control method, but do this before any seed heads appear.
5. Bishop’s Goutweed – Aegopodium podagraria
Common Names: goutweed, bishop’s weed, ground elder
Bishop’s Goutweed is commonly grown as an ornamental, used as a ground cover, and best known for its foliage. It can be green, but the variegated form of bluish-green leaves with white edges is more popular. Mature plants tolerate full sun or part shade, and a wide range of moisture conditions from dry to medium wet. It spreads aggressively through rhizomes and will easily outcompete other plants and lawn. It must be dug up carefully and discarded with care, since it has made its way into natural habitats where it will choke out native plants and form dense patches. Bishop’s Goutweed is still sold in greenhouses so plant with caution. This plant can only be controlled by using a deep wide barrier and careful monitoring to keep it from escaping.
6. Ox-eye Daisy – Chrysanthemum leucanthemum syn. Leucanthemum vulgare
Common names: Dog daisy, field daisy, Marguerite, moon daisy, moon-penny, poor-land penny, poverty daisy and white daisy.
Introduced from Europe in the early 1800’s primarily as a grass seed contaminant, and subsequently spread as an ornamental. Ox-eye daisy has become a serious invader of pastures, ditches and natural areas throughout North America crowding out native plants. It is a short-lived perennial that reproduces both by seed and shallow creeping roots. Seeds remain viable throughout the growing season and for several years. Seedlings form a basal rosette of green, oval wavy leaves. In the second year tall stalks emerge from a thick crown to produce white ray flowers with prominent yellow centers. All parts of the plant have a strong, unpleasant odour.
Ox-eye plants can be found sold through nurseries and as seed in wildflower mixes. Closely related cultivars, such as Shasta Daisy and originally sterile, can revert back to being fertile. The two plants can cross pollinate, resulting in an invasive hybrid that is difficult to distinguish from either parent. This fact makes public awareness critical to prevention and control. Do not purchase nursery plants or seed labelled as Ox-eye daisy. Removing plants by hand pulling in the first year will help eliminate plants from garden beds. Digging is needed if they establish in turf.
7. Common Tansy – Tanacetum vulgare
Common names: bitter buttons, garden tansy, golden buttons
Common Tansy is a perennial herb that is native to Europe. It was introduced to North America as an ornamental and medicinal herb. Leaves are fern like and hairy later becoming smooth and forming a large clump. Several erect stems emerge from the crown and are topped with clusters of bright yellow button-like flowers. Mature plants can grow up to 2 m. in height. All parts of the plant have a strong odour. The plant contains alkaloids which can cause illness to cattle and humans. Common Tansy spreads by rhizomes, but mostly through seeds that can remain viable for many years. It is best to avoid growing this plant since it has spread into meadows, fields and ditches. Carefully remove it from your garden and discard properly to limit its spread.
8. Purple Loosestrife – Lythrum salicaria
Purple Loosestrife was introduced as an ornamental that spread into the wild. It is a perennial that spreads by rhizomes and seeds. It grows in moist, marshy areas and ditches forming large colonies that choke out all other plants. It has become problematic in wetlands and efforts to control it have ranged from organized “pull and remove” to the introduction of biological controls by introducing leaf-feeding beetles. The fleshly root system produces tall erect stems with lance shaped leaves. Clustered spikes of magenta flowers form at the tops of stems and produce millions of reddish-brown seeds. This plant is often confused with Fireweed that produces less dense racemes of flowers with four petals while Purple Loosestrife has five to seven petals.
Exchange programs were introduced to have gardeners remove Purple Loosestrife from their gardens and replace it with comparable plants such as Garden Phlox. If you find Purple Loosestrife in a garden or public space carefully dig up the entire plant and allow it to completely dry out before properly discarding it. Report colonies of Purple Loosestrife to the Invasive Species organization in your region.
9. Absinth – Artemisia absinthium
Common names: Wormwood
A fragrant perennial herb that regrows each spring from a large taproot. Introduced from Europe where it was used as a medicinal as well as a base for intoxicating drinks. Absinth is now widespread throughout northern regions of North America. It grows in meadows, hayfields, along roadsides and undeveloped properties. Seedlings form rosettes of oval, scalloped leaves before developing into a clump of light greyish-green silky leaves that give off a strong sage-like odour. Small yellowish flowers form on top of drooping stalks in midsummer. Established plants can grow 30 to 50 cm. in height. Plants spread mainly by seeds. Control by pulling or digging in early stages. Cultivation can control large colonies. Pollen can cause allergic reactions in humans. This plant can often be confused with sage or domestic cultivars of Artemisia so be careful where you source your plants.
10. Birdsfoot trefoil – Lotus corniculatus
Low growing perennial that forms sprawling mats of clover-like branching stems from a strong taproot. Yellow pea-like flowers bloom most of the summer. Seeds develop in long pods resembling a bird’s foot. Plants spread by seeds, rhizomes and stolons.
Birdsfoot trefoil has been used in forage crops and for erosion control along roadways. However, it is becoming invasive since it has started spreading into lawns, boulevards and parks. It is tolerant of a wide range of soil conditions including dry compacted soil and because of its aggressive behavior it will quickly take over an area. Control can be achieved with early identification. Hand pull or dig out and discard the entire plant before seeds start to form. Large areas can be mowed to stress plants, but since it produces lots of seeds and all parts of the root can regrow, this provides moderate control.