Two Herbicides Recommended For Pesky Horsetail

Q. I have a severe horsetail infestation. Is there any way to get rid of it?

A: Horsetails were around when the dinosaurs roamed the earth. The dinosaurs are gone, but the horsetail is still with us! It makes a person wonder, doesn't it?

Horsetails are primitive, non- flowering plants that resemble ferns in their sexual reproductive strategy; however, unlike most ferns, horsetail has the ability to spread vegetatively via a wide-spreading rhizomatous root system. Only one of its fern relatives can reproduce in this fashion - bracken fern. Both of these species are generally considered noxious weeds.

Horsetail and bracken fern can both be controlled by constant cutting or pulling. If the process is consistent, an infestation of either one should ultimately be starved out; however, since neither species respects property lines, any mechanical or cultural effort to rid oneself of either pest must be a community one.

Herbicides registered for control of the two weeds are available. An extensive area of horsetail infestation might be brought under control using triclopyr. This can be found in two products labeled for emerged horsetail control, Brush-B-Gon and Crossbow. Both of these will injure or kill any broadleaf plants they contact, so they must not be used in the vicinity of desirable plants, with the exception of grasses. A pasture, infested with horsetail, or a vacant lot with other undesirable brush on it, can be sprayed after the horsetail is in full foliage, and some control will occur. Repeat treatments may be necessary.

Horsetail can be prevented from growing in the vicinity of shrubs and trees by applying Casoron, a pre-emergent herbicide, during the winter or very early spring before the horsetail starts to grow. If the horsetail is already up, it can be hand weeded, and the Casoron applied and watered in. This chemical does a very good job of suppressing the horsetail's growth, but it will not kill it.

Horsetail coming up in a vegetable or flower garden or in the vicinity of perennials may only be hand pulled or hoed to keep it from taking over.

Bracken fern should not be treated with triclopyr, since it is resistant to this chemical. Glyphosate (Roundup), on the other hand, will effectively kill bracken fern, but not horsetail.

The Roundup is applied to actively growing bracken foliage, but, since Roundup will injure or kill most plants it's applied to, one must take this into consideration when using it. Interestingly enough, horsetail is not in the least harmed by Roundup; in fact, it may get worse if control is attempted with Roundup.

If Casoron is applied to control other weeds in shrub areas, it may also effectively prevent bracken from emerging. Casoron must be watered in after application during a non-rainy period, otherwise it's likely to go off as a gas, and may injure the shrubs it's being used around. Another way to prevent it from volatilizing is to cover it with an inch or so of organic mulch.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by George Pinyuh and Holly Kennell, Washington State University/King County Cooperative Extension agents, Mary Robson, Master Gardener program assistant and volunteer Master Gardeners.