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Weed or Knotweed?

What is a weed anyway? Maureen Sexsmith-West ISA Certified Arborist, PR4600A It is essentially, a plant that is not desired within a certain context. The term WEED is subjective depending upon where the plant is growing. A plant that IS classified as a weed in one location MAY NOT be a weed when growing where it belongs or is wanted. Weeds tend to outcompete native plants through the agressive reproduction through seeds or ‘suckering’ habits (from rhizomes, tubers or root systems) or both, depleting nutriets and moisture to desirable plants in the landscape. Despite the ongoing battle with the common dandelion in the urban landscape, it is not considered a prohitibed or noxious weed under the AB Provincial Weed Control Act.

Life Cycles:

  1. Annual Weeds: Annual plants germinate, grow, flower and set seed in one year or less. Temperature, light, and moisture trigger annual plants to germinate. Annuals spread only by seed. (example:

  2. Biennial Weeds: Biennial plants complete their life cycle in two growing seasons. They germinate and form a rosette the first year. During the second growing season they form a stem, flower, set seed and die. (example:

  3. Perennial Weeds: Perennial plants grow for many years. Simple perennials, spread only by seed. Spreading perennials spread by seed and underground reproductive structures. Quackgrass, thistle and bindweed are examples. Control of spreading perennials may be very difficult because of this extensive underground root system. (example:

Who decides what is a weed in Alberta? Alberta redefined the classification of undesirable plants under their Alberta Weed Control Act in 2010. The new Weed Control Act regulation includes a new schedule of weed designations. Previously there were three weed categories, restricted weeds, noxious weeds and nuisance weeds. These categories were replaced with the new weed designations of Prohibited Noxious and Noxious. ‘Prohibited noxious weeds‘ must be destroyed, while ‘noxious weeds’ must be controlled–i.e don’t let them go to seed or spread).

“Prohibited Noxious Weed: means a plant designated in accordance with the regulations as a prohibited noxious weed and includes the plant’s seeds. This weed designation can be seen as regulatory support for an “Early Detection, Rapid Response” stage of invasive plant management. Plants in this category are either not currently found in Alberta or are found in few locations such that eradication could be possible. Under the Weed Control Act a person has a responsibility to destroy a prohibited noxious weed. Noxious Weed: means a plant designated in accordance with the regulations as a noxious weed and includes the plant’s seeds. This weed designation can be seen as regulatory support for a “containment” stage of invasive plant management. Plants listed in this category are considered too widely distributed to eradicate. A local authority may conduct control programs for these weeds if they feel they may have significant ecological or economic impact on lands within their municipality.” In many communities, additional plants may be included in their local weed by-laws. Contact your local municipal office to determine what is covered. In Lethbridge, the City’s Weed Control By-law identifies the following additional plants they monitor. This by-law is currently under review.


1. Chickweed

2. Dock – Rumex spp.

3. Fixweed – Descurainia sophia L.

4. Goat’s beard – Tragopogon dubius

5. Grass, couch – Agropyron repens L.

6. Kochia – Kochia scoparia L.

7. Lamb’s-quarters – Chenopodium album L.

8. Mallow, round-leaved – Malva rotundifolia L.

9. Milkweed, showy – Asclepias speciosa

10. Pigweed – red root – Amaranthus retroflexus – Russian

11. Ragweed, common – Ambrosia artemisifolia L. false – Iva xanthifolia

12. Stinkweed – Thlaspi arvense L

SO HOW DO YOU KNOW IF YOU HAVE PLANTS THAT ARE REGULATED? The qualified professionals at Ladybud Arborists can help! We offer an Integrated Weed Management Program that involves:

  1. The proper identification of the weed and knowledge of the weed’s biology.

  2. Determine why the weed is a pest.

  3. Evaluate the suitability of various control measures (cultural, mechanical or chemical) which may be available.

  4. Evaluate the results.

To prevent the introduction or spread of undesirable vegetation: don’t use raw manures, deadhead flowers before seeds can mature, and avoid ‘wildflower seed mixes’. When accepting or donating ‘free’ plants – take the time to do some research so you don’t spread prohibited or noxious plants.

Herer are great alternatives to prohibited plants suggested by the Alberta Invasive Plant Council.×9%20WWbrochure.pdf


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