Controlling Horsetails

Of all the weeding challenges facing new gardeners—and not-so-new ones as well—the one most likely to make anyone feel like throwing in the trowel for good is horsetail. This is one of the best articles we’ve found for methods to tame these wild creatures.
Equisetum spp., also called mare’s tails (and a good many other names that are unprintable), horsetails are those 20–30cm (8–12”) high, primitive looking green or brownish stems with tufts of thin, wiry green leaves arranged in whorls at each stem
joint. Because they’re perennial, fast growing and spread quickly via underground runners, two or three harmless-looking horsetails can quickly overwhelm a large planted area.

Once established, a patch of horsetails seems destined to remain there forever, because these plants are virtually indestructible. Note: they were documented among the first organisms to reestablish in the blast zone of Mt. St. Helens!

HERE’S WHAT NOT TO DO

Pull Stems: A new plant will appear at each new scar on the underground running root.

Dig Out Roots: Besides the futility of excavating your entire garden, any tiny piece of root left will resprout. In fact, digging invigorates them.

Cover with Black Plastic: No matter how long it’s left on, you’ll end up with a forest of tangled roots beneath it waiting to regrow. Horsetails thrive in warm, dank, oxygen-starved conditions.

Cover with Bark Mulch: While it appears to work temporarily, horsetails soon reappear stronger than ever. A deep covering of bark mulch aggravates the problem (and may even help create it) by providing a moist, airless, acidic “wet blanket” with no nutrient value.

Spray with Weed Killers: Besides being impervious to all but the most toxic chemicals, horsetails are “killed” only on the top growth and will eventually regrow.

HERE’S WHAT TO DO

The only permanent way to rid a garden of horsetails is relatively simple, but takes time and effort. You must improve the drainage, raise the pH, and increase your soil’s fertility. Horsetails must be admired for their tenacity. These living fossils, virtually unchanged for millions of years, thrive in conditions that mimic those that existed millions of years ago—acidic soil (low pH), low oxygen (minute air pockets), and very low nutrients. Moist or boggy soil provides all three conditions, but in many cases dry soil is also acidic, compacted and lean. Changing the soil conditions preferred by horsetails seems to give them an evolutionary signal that their reign is over, and it’s time for other kinds of plants to take their place. When this is done, believe it or not, the horsetails will gradually disappear all by themselves. Sounds farfetched, but it works. The first thing to tackle is poor drainage. Fill in any depressions that hold water, and make sure heavy rains can drain away quickly. For severe drainage problems you may have to dig a perimeter ditch to divert water from your garden, or even lay drainpipe.

Next, remove any mulch and/or plastic from the soil (there’s no need to dig up your plants) and apply dolomite lime at the rate recommended on the package. Wait at least two weeks before adding any fertilizer, since lime and fertilizer tend to cancel each other out if applied together. Meanwhile, water in the lime. For large areas, spread fish compost, homegrown compost, or aged steer or horse manure in a layer at least 5cm (2”) deep. For smaller areas, a balanced blend of organic meals will also do the trick. If you’re concerned about weeds (other than horsetails), consider a non-acidic mulch such as stones or gravel [although this can be a challenge to maintain due to leaf litter, etc.]. The best weed suppressor of all is a living one, so now is an excellent time to get some good ground covers established.

You should see considerable improvement in one year. Don’t be discouraged if some horsetails reappear – this is a long process, and it may be five years before you’re 100% horse-tail-free. If you have a severe problem, you may even have to re-lime and fertilize. But since these steps can only improve your soil, you have nothing to lose (except horsetails) and everything to gain.

To avoid introducing horsetails into your garden accidentally, keep an eye out for the shorter, leafless stems of fertile horsetail plants in April – a good month before the yearly appearance of the leafy, infertile horsetail plants just described. The pinkish-yellow pointed domes atop these fertile stems contain zillions of extremely small spores. Given the right conditions, these spores become new horsetail plants, so avoid walking through patches of fertile stems. If you find the spore-containing domes before they ripen, carefully cut them off, contain them securely in a zippered baggie and dispose of them in the garbage.

— by Carol Hall, Gardens West, April 2004