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Ask the Gardener: Untangling the mystery of how to kill knotweed

Ask the Expert
Japanese knotweed.
Japanese knotweed. adobe stock

What to do this week: Wrap the lower trunks of young fruit trees to protect them from hungry rodents that gnaw bark. Spray prized evergreens with a repellent where deer are a problem. Fill bird feeders with black oil sunflower seeds to attract cardinals. Hang suet bricks for woodpeckers. Top off Christmas tree stands with water frequently. Move houseplants into south- and west-facing windows if they need more light. A humidifier benefits both houseplants and people in winter.

 

Q. I have tried weed killers and pulling at a 30-foot stand of 6-foot tall Japanese knotweed to no avail. Every year it grows farther into our lawn. Do you know how to get rid of it?

A.L., Dedham

A. Japanese knotweed is the Godzilla of the plant world. If people realized how almost unkillable it is, they wouldn’t be so casual about spreading it around through infected fill and careless mowing. Perhaps the single best tip I can give my readers is not to buy or move soil from another site for fear of introducing this towering plant (or another invasive) into their yard. The good news it that the frothy-white late summer seeds seem to be sterile, but a piece of stem or root can start a new colony of bamboo-like stalks. It took me more than 20 years of decapitating and poisoning to defeat just two measly stalks. In England, professional knotweed removal is an industry because it’s hard to sell or get a mortgage for an infected property. But in the United States, you’re pretty much on your own.

So what can you do? Many have concluded that the only way to kill knotweed is to apply glyphosate (Roundup, Rodeo) between August and the first hard frost — year after year after year — until it stops resprouting. Most attempts fail because people don’t know that timing and persistence are essential. The tops are relatively easy to kill, and at first it looks like you’ve had total success with a single spraying. But the roots are your target, and you can reach them only when the sap is retreating downward in the fall. Glyphosate is widely considered the most effective herbicide for knotweed, but it will kill any other greenery it contacts. Be careful not to get spray on yourself or desirable plants. Some people inject it directly into the knotweed stalks. If your abutter has knotweed, too, you need to work together for total control, as it can travel 20 feet underground before popping up.

Many people abhor using glyphosate or other herbicides. Others make an exception for Japanese knotweed, having concluded it is very difficult to control and perhaps impossible to eliminate by organic methods. One alternative is to cut down the stalks as soon as they appear, and to keep cutting them down before they can photosynthesize. Treat each living stalk like toxic waste to be incinerated. Bag stalks rather than composting them, and, when in doubt, call your local Department of Public Works for disposal instructions. The New England Wild Flower Society recommends leaving black plastic bags of cut stalks in a sunny place for several months to fry them before disposing of them. Do not try to pull on, dig up, or weed the deep, massive roots. You can’t succeed, and you may stimulate more spreading. Do not disturb or move soil that’s within 20 feet of knotweed; that may encourage spreading, too. Think of this as a sleeping dragon you don’t want to awaken.

Clear the top growth of Japanese knotweed (Fallopia Japonica) away during the winter, after it has been killed by cold. Then carefully cut down and bag any new growth as soon as you see it. Think of knotweed control as your new hobby.

 

Q. I read somewhere that peanut shells make a good fertilizer for vegetable gardens. If so, how should one apply them? Mix them into the soil? I have been saving bags of shells of unsalted, roasted peanuts.

M.A., Lexington

A. Pioneering black scientist George Washington Carver discovered more than a century ago that growing peanuts can help replenish exhausted soils. Rather than mixing the shells directly into your soil, however, add them to your compost pile. They take about three years to break down — less if you grind or break them up first. So crush those bags of shells. You can also use them as a nutrient-rich, weed-free mulch, perhaps combined with cottonseed meal first to help keep them from compacting when it rains.

Note to Readers This is my last garden column until March, so hold your questions until then, please. See you in 2019.

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