Introduction

The purpose of my blog is to share with you what I have learned based on my experience as a practicing forester in California and Washington and as the general contractor in our former homestead in Mendocino County, California and our current homestead in Kittitas County, WA. As a forester, for more than a decade, I have practiced forestry within the context of a strong land ethic that endeavors to balance economic return with the beauty, clean water, clean air, wildlife habitat, recreation and carbon storage offered by well managed forests. As home and property owners, my family and I challenge ourselves to make our footprint smaller, through conservation, sourcing quality materials from well managed sources as close to home as possible and use of alternative technologies within a budget. Thank you for visiting my blog and I hope that the information provided will help you as a steward of the forest and in the place that you call home.

August 12, 2011

Controlling Invasive Species without Chemicals

By Thembi Borras

Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare) and prickly lettuce (Lactuca serriola) serve a purpose. Adjacent to our land, they were some of the first opportunists that colonized the acres of disturbed soil, which were bared during the course of development. When the housing market crashed in 2008 the development stalled, then stopped and will soon be in foreclosure. In the meantime, these species have done what they do very well…spread.

Canada thistle can spread 12’ to 15’ per year, vegetatively, via horizontal stems; located 6” to 12” below the soil surface and which send up frequent vertical stems. They are also very effective seed producers; a single plant produces an average of 1,500 seeds. Bull thistle and prickly lettuce are even better at producing seed. Mature bull thistle can produce up to 4,000 seeds per plant and prickly lettuce may produce more than 46,000 seeds per plant. In another adjacent area that is grazed, diffuse knapweed (Centaurea diffusa) has become prevalent. A single diffuse knapweed plant can produce up to 18,000 seeds, which are dispersed, in part, after the plant dries out, breaks off, tumbles assisted by the wind dispersing seed as it rolls until it gets stuck against a fence or shrub where it drops more seed. If I was not so busy trying to keep these species in check, without using chemicals, I would marvel at their mechanisms for survival; producing large quantities of seed, thriving in disturbed soil or in over-grazed pasture, growing aggressive root systems that spread long distances and producing seed that is effectively transported by wind, water, birds, animals and humans.

Per RCW 17.10.140 it is the landowner’s responsibility to eradicate all Class A noxious weeds and to control and prevent the spread of designated Class B and Class C noxious weeds from their property. Diffuse knapweed is identified as a Class B weed and Canada thistle and bull thistle are identified as Class C noxious weeds on the Kittitas County Noxious Weed List. Canada thistle is given special attention given the importance of the export hay market in Kittitas County and the adverse affect of Canada thistle on it. Prickly lettuce is not classified as a noxious weed; we choose to control it because we would prefer to replace it with a native plant that does not have stiff bristles that get lodged in fingers like splinters.

Chickens around the perimeter of our property are our ultimate plan; acting like little soldiers keeping the weed seeds out. However, until we find the time and money to build enough fence to realize this vision we continue to use somewhat conventional control methods. We control each plant depending on its life cycle. Using tarps and recycled pool liner we cover the patches of Canada thistle. Since Canada thistle readily propagates from stem and root fragments plowing or other soil disturbance is ineffective and can increase thistle densities. Mowing Canada thistle is effective if repeated at 7-28 day intervals for up to 4 years; mowing Canada thistle once a year is not effective. To prevent production of viable seeds, stems must be mown before the flowers open; stems with flowers that have been open 8-10 days can develop viable seeds. The bull thistle is the easiest to control, it has a two year life cycle. Plants grow vegetatively their first year as rosettes and in the second year the stems elongate and flower. Digging out or pulling up the taproot and disposing of the seed heads, if they have already gone to flower, is an effective control method. Prickly lettuce is an annual, which, this year, I pulled and weedeated before the flowers opened. However, after doing research for this narrative, I realize the plants I weedeated may produce new stems and flowers. So next year, we may have a sheep do the work for us or emphasize pulling and maybe some tarping over weed eating. Diffuse knapweed is a biennial, annual or short lived perennial that reproduces by seed. This year I pulled the plants and if the taproot did not accompany the pulled plant, I dug it up. Rototilling or plowing are effective methods of controlling knapweed if you don’t mind disturbing the soil. However, mowing is not, plants are able to resprout and flower again in the same season and plants that are regularly mowed can persist as short lived perennials or flower below the level of the mower. Grazing is not an effective control method for diffuse knapweed because it is generally unpalatable to livestock, and the spines around the flower heads may injure the mouths and digestive tracts of grazing animals. Also, the ground disturbance created by grazing creates ideal habitat for knapweed to spread. As a precaution, anyone working with diffuse knapweed or other knapweed species should wear protective gloves and avoid getting knapweed sap into open cuts or abrasions. Workers should wash their hands and exposed skin with soap and water following contact with this plant.

A word about disposal, if you can remove the plants before they go to flower, they can be left on the ground to be desiccated by exposure. If they have gone to flower, there is a chance that even after being cut or pulled, they can go to seed. It depends on the species and how long the flower has been open. To be safe if a plant has gone to flower I treat it as if it has gone to seed. This year, I took the removed plant and/or seed head to the local compost facility, where they get their piles hot enough to kill weed seeds. Last year, I bagged the material in 30 gallon heavy duty black garbage bags and let it cook in the sun for a year.

The proverb, an ounce of prevention is worth a pound of cure, is worth repeating in bringing it back around to the disturbed soil that started this narrative. If you bare the soil, and don’t take care to restore cover, nature will and it probably won’t be with desirable species. Preventing soil disturbance, in the first place, is far more cost effective than eradicating the invasive plants that may occupy it. Finally, the Kittitas County Weed Board is an excellent resource for information about identifying and controlling noxious weeds. Following is their contact information along with other relevant resources.

Kittitas County Noxious Weed Control Board (509) 962-7007 http://www.co.kittitas.wa.us/noxiousweeds/laws.asp
http://www.co.kittitas.wa.us/noxiousweeds/Weed-list.pdf

Washington State Noxious Weed Control Board. http://www.nwcb.wa.gov/

Invasipedia houses information on invasive plants and especially how to best manage them. http://wiki.bugwood.org/Invasipedia
http://wiki.bugwood.org/Cirsium_arvense
http://wiki.bugwood.org/Centaurea_diffusa

King County Noxious Weeds Index to Specific Weed Pages for Identification, Photos, Control and Other Information
http://www.kingcounty.gov/environment/animalsAndPlants/noxious-weeds/weed-identification.aspx
http://your.kingcounty.gov/dnrp/library/water-and-land/weeds/BMPs/bull-thistle-control.pdf
http://your.kingcounty.gov/dnrp/library/water-and-land/weeds/BMPs/Diffuse-Knapweed-control.pdf

University of Washington Herbarium Image Collection: Plants of Washington
http://biology.burke.washington.edu/herbarium/imagecollection.php

Washington State University Extension
http://extension.wsu.edu/nrs/noxious/Pages/default.aspx
http://www.whitman.wsu.edu/documents/Weeds/PricklyLettuce2006.pdf.

A portion of this production was gleaned from the above listed resources.

I welcome responses as well as ideas for future Forestry and the Homestead narratives. You can also get an email version by contacting me at thembi@mcn.org.

Thembi Borras is a forester and aspiring permaculturalist living and working in Kittitas County, WA USA.