How to Manage Common Noxious Weeds

There are four knotweed species across British Columbia, all similar in appearance, their impacts, and methods of control, so they will be discussed here under the general title of “knotweeds”.

Knotweeds grow 1-5 metres in height and in large, dense patches. Stems, or canes, are hollow, green, varying thicknesses, and grow upright like bamboo. Stems have reddish-brown/red speckles. In winter, they are bare, grey or straw-coloured hollow stalks. Leaves are mostly heart or triangular-shaped on all species except Himalayan, which has long and tapered leaves. All species, except giant knotweed, have leaves 8-10 cm wide and 15 cm in length. Giant knotweed leaves are generally twice as large as the other three species. A distinguishing feature of Japanese knotweed is the zigzag pattern of leaves arranged along the plant’s branches. The flowers of knotweeds are generally small, white/green in showy clusters along the stem and where leaves are joined. When mature, the creeping roots, called rhizomes, are thick and woody and can spread up to 20 metres laterally. The rhizomes have roots that travel into the soil with penetrating force. Seeds are typically dark, glossy, 8 to 9 mm long and have three wings. Not all seeds can grow.

Biological

A sap-sucker psyllid or lice, has been studied as a potential biological control in the Pacific north-west.

Manual

Important: Mechanical control on its own is not an effective management tool. Manual control is recommended under specific circumstances and done with extreme caution as root and stem fragments are easily spread and will re-grow. Mechanical control is time-consuming and will require dedication of frequent removal over numerous years. All removed plant material should be disposed of properly.

See: https://bcinvasives.ca/wp-content/uploads/2021/01/ISCBC-Factsheet-Knotweeds-180216-WEB.pdf

Cutting or mowing may be effective for small populations if repeated several times a year with constant monitoring. Cutting should be repeated for several years. Cutting is most effective when followed up with herbicide application

Digging has shown to be effective on very small and recently established populations if all root tissue is successfully removed and followed by restoration of native plant communities.

Cultural

Burning: is not effective as the plants contain high water content.

Grazing: may result in short-term reduction of above-ground plant matter. Grazing requires trained animals continuously throughout the growing season for numerous years.

Herbicide

Application of herbicide is best done when there are enough leaves to accept the herbicide spray during active growth. Plants are actively growing early in spring to July/August when plants typically mature and set seed. In September, warm moist conditions promote regrowth of perennial plants and germination of seeds and is a good time to treat.

Contact your local invasive plant specialist for herbicides registered for the control of knotweed species

Prevent the spread of Scotch thistle by reporting any new infestations to the Regional District of the North Okanagan, the Invasive Species Council of BC, or with the ReportInvasivesBC app.

bcinvasives.ca/wp-content/uploads/2021/01/ISCBC-Factsheet-Knotweeds-180216-WEB.pdf

Japanese knotweed

https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Reynoutria%20japonica&redblue=Both&lifeform=7

Bohemian knotweed

https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Reynoutria%20x%20bohemica&redblue=Both&lifeform=7

Giant knotweed

https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Reynoutria%20sachalinensis&redblue=Both&lifeform=7

This plant lives from one (annual) to many (perennial) years. It grows up to 0.7 metres tall, with the whole plant covered in star-shaped hairs. The upper leaves are oval (egg-shaped) and clasp the stems. The flowers are white with deeply cut petals. Seed pods are oval, 5 – 8 mm long, sort of flat, located close to the stems and are lush and green when immature and transparent and web-like when mature.

Biological

No biological control

Manual

Cutting/Mowing

A single mowing once a year is not an effective control as most plants are short-lived perennials. However, annual plants may be controlled with one mowing or cutting if done late in the season. Mowing sites low to the ground before July can prevent seed production. Combined mowing and subsequent herbicide treatment of re-growth appears to be an effective control method. Treatments must be repeated over several years.

Hand Pulling

Hand pulling is effective and feasible. Hoary alyssum has a single taproot that is shallow-rooted and will easily come out intact when pulled. Wear gloves and other protective clothing to prevent skin irritation. Pulling hoary alyssum is best when plants have developed stems. Effective pulling is best in moist soils to remove as much of the root as possible to ensure it does not regrow. If the plant snaps off at the root crown, regrowth will occur.

Digging

Digging is usually not necessary, except in hard-packed soils, as the plant root usually comes out intact. Removing the entire root will not result in regrowth, but any remaining root will likely regrow.

Deadheading

Constant cutting of plants before flowering dramatically reduces or eliminates seed production in most plants. Hoary alyssum produces viable seed from early to late growth stages. Seeds in pods low on the stem mature first and seeds in pods at the top of the plants mature as it grows. Bag deadheaded material and dispose of in a landfill. This is very labour-intensive. There is no information on the longevity and viability of seeds.

Herbicide

Application of herbicide is best done when there are enough leaves to accept the herbicide spray during active growth. Plants are actively growing early in spring to July/August when plants typically mature and set seed. In September, warm moist conditions promote regrowth of perennial plants and germination of seeds making it a good time to treat.

Contact your local invasive plant specialist for herbicides registered for the control of hoary alyssum.

Prevent the spread of Scotch thistle by reporting any new infestations to the Regional District of the North Okanagan, the Invasive Species Council of BC, or with the ReportInvasivesBC app.

Resources

• bcinvasives.ca/wp-content/uploads/2021/01/Hoary_Alyssum_Factsheet_20_02_2019.pdf 
• linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Berteroa%20incana

Burdock grows its’ first year as a leafy seedling plant and does not grow tall to produce seed. Leaves are large and resemble rhubarb leaves and are often called wild rhubarb. As a second-year plant, it will develop a stem which can grow 3 metres high with rounded pinkish flowers that produce seeds. Flowers have hooked spines that easily attach to clothing and animals. The seed head will turn brown when mature, and heads easily become attached to passing animals and humans or can trap birds and bats.

Biological

No biological control

Manual

Cutting/Mowing

Burdock is not controlled with a single mowing once-a-year as plants live two years and will re-grow after being cut. Mowing or cutting plants very low to the ground before July can prevent seed production in the second year of growth and may extend its’ life cycle to three or more years. Combined mowing and subsequent herbicide treatment of re-growth appears to be an effective control method. Treatments must be repeated over several years.

Hand pulling

Hand pulling may be used in areas if the soil is moist and loose. However, because burdock roots are large and deep it is not usually feasible to pull with the entire root. Burdock has a single taproot that is deep and will not easily come out intact when pulled. Gloves and other protective clothing should be worn to prevent skin irritation. Pulling burdock is most easily done when plants have developed stems. Soil should be very moist to effectively remove as much of the root as possible to ensure it does not regrow. If the plant snaps off at the root crown, it will regrow.

Digging

Digging is usually best, except in hard packed soils, as the plant root more easily comes out intact. Removing the entire root will not allow plant re-growth, remaining root will likely allow plant to regrow

Deadheading

Constant cutting of flowers and developing seed structures dramatically reduces or eliminates seed production in most plants. Burdock produces viable seeds from flowers that mature late August into the fall. Flowers are at the ends of branches and easily detach. Bag deadheaded material and dispose of it in a landfill. This is very labour-intensive. The viability of seed in the soil can be up to 40 years.

Herbicide

Application of herbicide is best done when there are enough leaves to accept the herbicide spray during active growth. Plants are actively growing early in spring to July/August when plants typically mature and set seed. In September, warm moist conditions promote the regrowth of perennial plants and germination of seeds and is a good time to treat.

Contact her local invasive plant specialist for herbicides registered for the control of burdock.

Prevent the spread of Scotch thistle by reporting any new infestations to the Regional District of the North Okanagan, the Invasive Species Council of BC, or with the ReportInvasivesBC app.

Resources

• linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Arctium%20minus&redblue=Both&lifeform=7  

Scotch thistle a perennial plant that lives two or more years. It is a listed noxious weed in the Regional District of North Okanagan and grows prolifically in the Vernon and surrounding areas. It grows along roadsides, disturbed areas, rangelands, and irrigation ditches. Scotch thistle only reproduces by seeds that can remain viable in the soil for 39 years.

This tall and vigorously thistle grows up to 3 metres. Stems have many branches, with sharp, spine-edged wings along the sides. Leaves are large, have irregular projections, and have sharp spikes. Both the stem and leaves have woolly hairs, giving the plant a silvery grey-green appearance. Rosette leaves can be small to large, up to 60 cm long and 30 cm wide. Flowers are pink, purple, or sometimes reddish in colour, 2.5-5 cm in diameter and have sharp, spine-tipped flowers.

BIOCONTROL         

Currently, there are no biocontrol agents available for Scotch thistle.

MANUAL

Smaller infestations of Scotch thistle can be manually dug, ensuring the taproot is entirely removed. For larger plants, cut back branches and stem first, then follow by digging the taproot or severing it as low as possible below the soil. Sharp stems, branches, and leaves require wearing leather gloves and appropriate clothing when handling plants. It is important to conduct manual controls before the plant goes to seed. Note that deadheading seedheads or cutting and/or mowing at ground level will usually in regrowth, especially if done early in the growing season. Manual controls, such as mowing or hand-cutting, are most effective in combination with other methods. For example: Cutting or mowing followed by taproot cutting to 2.5-5 cm below ground surface.

CULTURAL

In areas at risk of potential Scotch thistle infestations, it is important to maintain a strong, competitive perennial plant cover. Rehabilitate disturbed areas quickly with native or domestic perennial grasses, and legumes if no herbicide has been applied, to reduce likelihood of new infestations.

Sheep, goats, and horses – but not cattle – have grazed on young Scotch thistle plants in the early stages of infestation. Note that overgrazing can promote Scotch thistle.

When leaving an area infested with Scotch thistle, it is crucial to remove plant parts and seeds from clothing, pets, gear, vehicles, and machinery to prevent the spread of new infestations.

HERBICIDE

Application of herbicide is best done when there is enough leaves to accept the herbicide spray during active growth. Plants are actively growing early in spring to July/August when plants typically mature and set seed. In September, warm moist conditions promote regrowth of perennial plants and germination of seeds which is a good time to treat.

Contact her local invasive plant specialist for herbicides registered for the control of Scotch thistle.

Prevent the spread of Scotch thistle by reporting any new infestations to the Regional District of the North Okanagan, the Invasive Species Council of BC, or with the ReportInvasivesBC app.

Resources

• bcinvasives.ca/wp-content/uploads/2021/01/Scotch_Thistle_TIPS_2017_WEB.pdf
• https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Onopordum%20acanthium&redblue=Both&lifeform=7

Purple loosestrife stems are woody at the base and grow between 0.5-2.0 m tall. They are square in cross-section and may have short hairs. The leaves are long, lance-shaped and arranged opposite to each other in a whorled formation along the stem. Leaves are 3-10 cm long, stalkless (sessile) and sometimes covered in fine hairs. Flowers are individual with 5-7 pinkish-purple petals (approximately 10 mm long) arranged along vertical spikes at the stem. This perennial has a rhizomatous root system which can produce 30-50 erect stems annually. The fruits are dark brown, ovoid-shaped capsules approximately 2-3 mm in length and contain many tiny seeds.

Biological

The root-boring beetle and two species of leaf-eating beetles are effective against purple loosestrife. These beetles damage the root system, stunt growth and reduce plant seed production.

Mechanical Control

Mechanical control for large infestations has been unsuccessful.

Cutting

Deadheading or cutting of the flowers prior to maturing can reduce the seed production of a plant during the current year. This is a top-growth or suppression management tool.

Hand pulling/Digging

Small infestations, if caught early, can be removed by hand pulling or digging. Purple loosestrife reproduces through fragmentation, so special attention must be paid to ensure all plant parts are removed. Use this method before it goes to seed set to prevent scattering

Cultural

As purple loosestrife is a semi-aquatic plant, it typically grows on shorelines and in adjacent waters. Therefore, direct revegetation methods would be specific and may require permits. Any techniques to encourage an increase in the growth of native species could outcompete purple loosestrife and resist its invasion.  

Herbicide

Herbicide use is difficult for purple loosestrife due to it being found in wet soils. Contact her local invasive plant specialist for herbicides registered for the control of purple loosestrife species

Prevent the spread of Scotch thistle by reporting any new infestations to the Regional District of the North Okanagan, the Invasive Species Council of BC, or with the ReportInvasivesBC app.

Resources

• bcinvasives.ca/wp-content/uploads/2021/01/Purple_Loosestrife_TIPS_2017_WEB.pdf
• https://linnet.geog.ubc.ca/Atlas/Atlas.aspx?sciname=Lythrum%20salicaria&redblue=Both&lifeform=7