[blurb needed]
- information on some of the most commonly asked about invasive plants/ noxious weeds. More information at BC Invasives.ca
Knotwood Species
(Japanese knotweed (Reynoutria japonica), Giant knotweed (Reynoutria sachalinensis), Bohemian knotweed (F Reynoutria x bohemica), and Himalayan knotweed (Polygonum polystachyum).
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 knotweed 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
Giant knotweed
Hoary Alyssum
(Berteroa incana)
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 hoary alyssum 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
Burdock
(Arctium spp.)
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 burdock 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
Scotch thistle
(Onopordum acanthium)
