Ask a Master Gardener: Summer solstice ideal time to control tough perennial weeds

Summer solstice ideal time to control tough perennial weeds

There’s nothing more frustrating for gardeners than spending both time and money trying to kill tough perennial weeds only to have them resprout after a short period of time. Hand digging can be effective, but only if the entire root system is removed. When it comes to digging out all of the roots of well established perennial weeds like Canada thistle however, most gardeners will attest that it is next to impossible and with good reason.

The biomass of Canada thistle’s creeping root system can be as much as 10 times the visible above-ground parts. The roots, which produce new shoots every six inches, can easily spread horizontally about 20 feet in one year! A single plant is capable of producing a total root length of 1,600 feet in just one growing season! In that time it can produce 154 underground shoots and approximately 800 buds. The root system contains enough food reserves to generate new shoots for more than one and a half growing seasons without being replenished. Root fragments broken off by tillage have enough food reserves to survive for 100 days under adverse conditions.

Although seed is not the most efficient means of spread for Canada thistle, it does occur. The seed germinates best at high temperatures (77 to 86 degrees F.) and can germinate 10 days after maturity. In just three weeks after successful emergence, Canada thistle seedlings can regenerate top growth from their established taproot. Four to six weeks after successful emergence, horizontal roots can initiate new shoot growth. Seeds can survive burial in the soil for 20 years.

The key to successful control of Canada thistle is to understand how it grows. Over 90 percent of the weed’s root system is below cultivation depth. Hoeing off plants serves to stimulate new growth from underground buds. It is the buds of the creeping roots of established Canada thistle plants that largely account for re-establishment after attempts at control. Buds on creeping roots can generate new shoots a year or more after top-growth has been destroyed. 

One option is to dig out plants rather than just hoeing off the top foliage. In order to be effective, you will need to be vigilant in digging deep to get as much of the root system as possible and repeating the process every time new foliage appears.

Another option for eradicating Canada thistle is to use herbicides that move down through the plant and cause substantial injury to the creeping root system. The best time to treat Canada thistle with foliar translocated herbicides is in late June (summer solstice), after the first flowers are formed and before the first flowers open. This is commonly referred to as the “purple bud stage.” Herbicide applications made at this time should be followed up on fall regrowth during September and early October. The fall application is especially important since growth at this time helps restore underground food reserves.

One of the most effective herbicides registered for use to control this weed is glyphosate, which is sold under numerous trade names including Roundup. Glyphosate is readily translocated into the root system. For maximum effectiveness, make sure the thistle leaves are green, not wilted, and free from damage caused by insects, diseases or drought. Since glyphosate is a non-selective herbicide, make sure the spray does not come in contact with desirable plants.

What about using vinegar to control Canada thistle?

Although there are currently three different weed killers registered for use in Washington state with acetic acid (vinegar) as the active ingredient, none have proven to provide reliable control of established perennial weeds. Canada thistle treated with household formulations of vinegar with an acetic acid concentration of 5 percent showed 100 percent burndown, but roots were not affected. Therefore, the shoots always regrew.

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