Horsetail, Morning Glory, Blackberry--Weeds as Thugs!
by Mary Robson, WSU Area Extension Agent
Imagine what would happen if persistent people didn't chop back weeds--the Columbia Tower in Seattle would probably be wreathed in morning glories and surrounded at its black base with blooming blackberries. Unwanted plants like horsetail, morning glory, and blackberry, called "perennials," are particularly troublesome to gardeners.
Perennial flowering plants--the dandelion is another good example--have roots that store plant food (carbohydrates) over winter, and send the plant into vigorous growth when weather warms. Unlike annuals, which don't live over the winter and have to start every growing season as tiny, vulnerable seedlings, perennials can hoist themselves into growth with reserved fuel. These plants have formidable powers of regrowth. Anyone who has ever watched a blackberry turn from a hidden 3-foot branch into a thicket of 12-foot armed stalks has experienced the power of perennial growth.
So what does the gardener do to manage weeds that can regrow from a root when they are chopped off? (This characteristic is reminiscent of the old myth about the "Hydra," a creature that grew two heads back for every one removed.) As with annual weed management, a combination of techniques will help.
All weeds have vulnerable seedling stages, including perennial weeds. Perennials, such as blackberry and morning glory, do produce copious seeds. Keep the garden free of small, newly-emerged weeds. Blackberries are often planted by animals; morning glory seeds sprout vigorously. Remove seedlings of all weeds by scuffling them out. Using a mulch on the garden after weeding will help to prevent germination of perennial weeds as well as annuals.
Mulch doesn't deter adult perennial weeds. However, a mulch does make spotting them easier. Morning glory is a good example; the plant will send shoots under mulch, staying underground until the tip growth reaches a spot to emerge and grow straight up, perhaps enveloping a peony or bluberry bush in rampant growth. Controlling morning glory must be done by reducing the ability of the plant to build up food reserves in the root. Hoeing or pulling them out won't end the problem, but it does weaken the plants.
Do not allow a morning glory to flower and go to seed--easier said than done, I know. Cut them off before flowers mature, because the seeds will create endless problems. Don't try to get rid of morning glory by tilling an infested plot--morning glory can regrow from a bit of root 1/4 inch in length, and tilling just chops up the rots into hundreds of potential plants.
Perennial weeds like morning glory, which are persistent and recurrent problems in the garden, may also be managed by using herbicides properly in combination with other methods of control. Do not use weed killers--herbicides--as the only method of garden weed control.
Herbicides aren't all alike--it's important to identify the weed and select an herbicide that will work for that particular weed. Understanding the timing of application is also vital. Weed killers must be applied during the correct stage of plant growth for best control. Morning glory can be killed with glyphosate (sold as Round-Up and under a number of different trade names.) But the timing and application technique has to be right.
Glyphosate (Round-Up) works by being taken into the plant through leaves and altering the plant's metabolism. Morning glory is most vulnerable to glyphosate after mid-summer, and in early fall. At this time, the plant is storing carbohydrates in the roots for winter. The chemical, if properly applied to the leaves, is carried down to the roots and will weaken or kill the plant. Glyphosate (Round-Up) is non-selective--it will damage or kill any green plant it touches. So don't use it carelessly.
One way to get glyphosate on to a morning glory vine in teh fall is to unwind as much of the vine as possible from surrounding plants, winding the morning glory around a hand like a piece of yarn. Get as close to the ground as possible, and don't break the vine--it must be connected to the roots. Spray or dip this "ball of vines" in glyphosate (Round-Up). Avoid other plants. The sprayed morning glory will die back slowly--over a period of 3 weeks or so. This method does weak--or kill--the roots, but only if the application happens in mid-summer or later. Sometimes a second application is necessary on re-growth in morning glory. Read and follow all label instructions on all pesticides, and yes, an herbicide is a pesticide.
For blackberries, grubbing the plants out with a mattock will help, but there is often re-growth. Blackberries can be controlled by fall applications (September and October) of glyphosate (Round-Up) to the leaves and stems. Another method is to chop the plants back to about an 8-inch stem. Immediately paint the stem with a brush-killer type product for woody shrubs, without diluting the product. Wear gloves and take care not to splash the material. Again, not the timing--do this in fall. The plant may not show signs of being under control until the following spring.
Horsetails, everyone's toughest week, aren't like ordinary perennial weeds. They don't flower--they are related to ferns, and spread by spores, windborne fine particles. Horsetail doesn't respond to applications of most herbicides--for instance, spraying glyphosate (Round-up) on it simply doesn't work. Chopping the plants out in fall will reduce their root reserves of food somewhat, and should be done.
It's possible to get some chemical control with a soil-applied product used in mid-winter. Mark or note the location of horsetails, and treat the area in mid-winter, January, with dichlobenil (Casoron.) This is a pre-emergent herbicide that keeps the plant from coming up--the first growth of horsetail is a swollen fertile stem that emerges very early spring. Follow label instructions exactly, do not over-apply the product, and cover the area with a mulch after applying to keep it working best.
Dichlobenil (Casoron) can only be used where no planting will be done for at least 6 months. It is safe to use over the roots of established woody shrubs (plants that have been in for at least one season). It is damaging to all young plants, including young shrubs. The product can't be used where bulbs are planted, or on vegetable garden areas, or where annuals or perennial flowers will be planted. Only use it where no planting is planned and where shrubs or trees have been established for some time. Read and follow the label instructions.
Persistence, caution, and knowledge are all required for perennial weed control success. Persistence is the most important, since all gardeners have experienced what happens when we "miss" a weed and it grows to a suffocating monster!
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