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Gardening Answers Knowledgebase

Search Results for: Noxious weeds | Search the catalog for: Noxious weeds

Plant Answer Line Question

Keywords: Polygonum cuspidatum, Noxious weeds

I have a non-native bamboo. It's in a marshy area. It is soft light green. It dries to wood every year. And I cut it like firewood and chip it. Then suddenly it grew back and is growing to an acre size. It even flowers: soft light white vanilla flowers for the bees. Can I rototill it under and seed in native Northwest groundcovers?


It is difficult to do plant identification by description alone, but it sounds like you may have Japanese Knotweed (Polygonum cuspidatum) It is extremely (!) hard to control this plant. Rototilling it will make hundreds of new plants because it grows from the tiniest root fragment, so do not do that! There is a lot of good information on it on the Internet, but here are two good links:

King County, which lists it as a Class B noxious weed (control recommended but not required by law)

Date 2017-05-11
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Plant Answer Line Question

Keywords: Noxious weeds--Washington, Noxious weeds, Invasive plants

Can you provide me with an extensive list of resources for checking whether a plant is invasive or a noxious weed?


Here is a list of helpful resources:

Washington State Noxious Weed List from the US Department of Agriculture

State noxious weed list and schedule of monetary penalties from the Washington State Legislature

Class A, Class B, and Class C

Washington Department of Ecology (aquatic plants)

Washington Invasive Species Coalition and their GardenWise handbook

King County Noxious Weed Lists

National Invasive Species Lists

Plant Conservation Alliance Alien Plant Invaders list

Invasives in British Columbia

The lists which are national in scope are useful too, as some plants not yet officially listed as invasive here may still be plants to watch out for.

There are a great many books on this subject. A recent one, co-authored by a faculty member here, is Invasive Species in the Pacific Northwest edited by P.D. Boersma, S.H. Reichard, and A.N. Van Buren; Rebecca L. Gamboa, photo editor. University of Washington Press, c2006.

Date 2017-04-15
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Plant Answer Line Question

Keywords: Polygonum, Noxious weeds

I'm having difficulty removing knotweed from my garden. Can you give me advice on how to eradicate it?


I'm sorry to hear you are struggling with knotweed. Manual methods include cutting, mowing, pulling, digging, or covering. Dig surface roots in loose soil. Check frequently for new shoots and dig them up as soon as you notice them. Cut stems close to the ground every two weeks throughout the growing season. After cutting completely, you could instead decide to cover the area of the knotweed patch and the immediate area around it with black plastic or other impermeable material. This is a long-term process, and it may take several years to eradicate the Polygonum. Here are some helpful links on knotweed:

Invasive Knotweeds from the King County Noxious Weed Control Program

Knotweed Biology and Control from the King County Noxious Weed Control Program

Controlling Knotweed in the Pacific Northwest (a large file!)

Japanese Knotweed from the PCA Alien Plant Working Group

Date 2017-04-15
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Plant Answer Line Question

Keywords: Cuscuta, Noxious weeds, Invasive plants

I'm doing a science fair project on dodder plant and I'm seeking information about the plant, and a source of seeds or plants for the project.


Dodder is a parasitic plant that lives on crops, ornamentals, native plants, and weeds. Because it has limited chlorophyll, it can't make enough food to support itself, and so relies on the plants it colonizes for nourishment. It belongs to the genus Cuscuta, in the family Convolvulaceae (same family as morning glory). It was formerly referred to as Grammica.

Perhaps the reason that seeds and plants are not readily available is that dodder causes great damage to the plants it parasitizes. The U.S. Department of Agriculture's plant protection and quarantine office states that "products, including foods, containing whole dodder seeds (Cuscuta spp.) are prohibited entry into the United States. APHIS regulates whole dodder seeds, both as a parasitic plant pest under Title 7 Code of Federal Regulations (CFR), Part 330 and as a Federal noxious weed under Title 7 CFR, Part 360."

Here is additional information from University of California, Davis's Integrated Pest Management website. Dodder is sometimes referred to as the "Vampire Plant," as this University of Florida Extension page explains. Although your project, safe within the confines of a lab or classroom, might pose no threat, it is not legal to sell Cuscuta seeds or plants in the U.S.

Date 2018-06-14
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Plant Answer Line Question

Keywords: Ailanthus altissima, Noxious weeds, Invasive plants

There seems to be a plant invasion in my Seattle neighborhood. I think what I've been seeing are larger specimens of Ailanthus altissima (Tree-of-Heaven) surrounded for many blocks by smaller seedlings of this same tree. It sprouts up through the middle of landscape plants and lawns, and right up against concrete foundations. I feel I should be warning people, but I'd like to know what the local status of this tree is, and I want to be sure I have identified it correctly. I already know it's aggressive here, and I know it's been designated invasive in other parts of the U.S. and the world.


You are correct that Ailanthus altissima has quite a track record for invasiveness. As you say, the U.S. Department of Agriculture's National Invasive Species Information Center lists it. If you want to show people in your neighborhood some clear images of the tree in various stages, the Invasive Plants Atlas of the United States has good information.

If you aren't completely certain what the plant is, you can bring samples to the Herbarium here at the Center for Urban Horticulture, or you can compare and contrast what you have observed with some close look-alikes:

Tree-of-Heaven is mentioned in a pamphlet on alternatives to invasives for Eastern Washington gardeners, but it has not yet achieved official invasive status. The Washington State Noxious Weed Control Board is considering a nomination to designate it as a Class B noxious weed (second highest priority) for the 2012 state weed list.

You could encourage your neighbors to eradicate it when possible. It spreads by seed (which can be dispersed by birds but especially by wind), and by root sprouts. It is a very fast grower, and it is important, when digging it up, to get every last bit of root, or you will soon find more of it sprouting. The California Invasive Plant Council has excellent, detailed information on its history, its growth and reproductive habits, and several methods of controlling its spread.

Date 2018-03-14
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Garden Tip

Keywords: Hedera helix, Noxious weeds, Invasive plants

Did you know that one English ivy plant removed from a tree in the Olympic National Park weighed an estimated 2,100 pounds? The King County Noxious Weed Control Program has a great deal of information on how to control ivy. If you would like to receive the information in other formats, call them at 206-296-0290.

Date: 2002-09-18
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May 31 2018 13:14:08