How do you get rid of those hardy horsetails?

Continued experience with landscaping problems over the past 20 years has caused me to realize that what humans want versus what nature insists on may be very different things. And one of the most convincing cases of this is the brushy and tenacious horsetail, a fern-related plant that originated millions of years ago and once grew to the size of trees.

The ground-covering horsetails are considered one of the most troublesome, invasive and downright maddening of our garden weeds. They're tough, though, and admirable.

Tough may be too mild a word. Horsetail was the first vascular plant to return after the Mount St. Helens eruption. Bad soil and poor growing conditions don't faze them. (Amazingly, some nurseries on the East Coast sell them as garden ornamentals.)

These field horsetails (Equisetum arvense and other species) have fascinating jointed stems. Though they like moist locations, dry soil really doesn't set them back once they get going. They're not only beautiful, but the stems contain enough silica — actually silicon dioxide — to polish a pile of kitchen pots, pewter, even wooden tools.

They aren't a friendly pasture weed: Some types are poisonous to livestock — particularly horses.

Like their fern cousins, horsetails reproduce from spores rather than from seeds. The earliest spring growth on field horsetail is "fertile stems," short, stubby and yellow-orange, that produce no foliage but only orange spores. Removing fertile stems will reduce spore load, but the real enemy is its root system.

Dark woolly-brown roots, looking a little like unraveled yarn, travel to at least 6 feet down, though about 50 percent of the roots inhabit the first foot of soil. Rhizomes generate new plants. It's difficult to rid a garden of them, especially if untended stands prevail in your neighborhood.

Coexistence has appeal where this weed is concerned. One choice is to allow the plants to spread where they will in some spots. Mowing keeps them from advancing through lawns. But if you've yanked them out, you know that rapid regrowth resembles three-day stubble. Keep pulling these, digging deeply to get roots, which will to some extent prevent the weed from new growth.

In a vegetable garden, hoeing is the only solution. Mulch, often a good weed suppressor, simply encourages perennial horsetail.

Some people remove it, then spread weed cloth barrier (geotextiles) which will work for one or two seasons. Some vigorous plants compete. Gardener Ciscoe Morris says he's had good luck with a hillside of rock rose (Cistus species). It helps if the shrubs you're working to establish are taller than the horsetail.

Chemicals aren't generally the answer. Actively growing horsetail doesn't die out with glyphosate (Roundup) or even 2-4, D.

There's one weed killer that suppresses the growth if applied to the dormant plants in winter. However, you want to think carefully before using this environmentally unfriendly stuff. It is a pre-emergent herbicide called dichlobenil (Casoron), which will, if applied properly, suppress growth of horsetail.

I mention it here only because people do use it, and there are ways to minimize its dangers.

Casoron works by releasing a toxic gas that retards plant growth. Put a mulch over the area immediately after application and keep the chemical out of water. Don't overuse it, and don't use it for general weed control. Don't apply it to anything edible, recently planted or over spring bulbs.

Casoron is overused by gardeners who hope it will get rid of weed problems throughout the year but instead it often causes plant damage.

Obviously this is a last-resort tool. Any inexperienced user is safest when seeking professional help to handle it right. For my money, coexisting with horsetails is far preferable.

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension.

She shares gardening tips every Wednesday. Her e-mail is gardeningtips@seattletimes.com.