The horsetails, botanically the genus Equisetum, are perhaps some of the most distinctive plants in the world with their ridged hollow stems, that lack green leaves, and their cones bearing jumping spores. However they are also a problem to gardeners who do not want them competing with their flowers or vegetables. Unlike Japanese knotweed (Fallopia japonica), a non-native for which there are now many control measures, the common horsetail is a native plant with no biological control.
As a botanist I appreciate this ancient lineage of plants, that is now classified with the ferns, and even grow some of the less invasive species including Equisetum hyemale of the form often sold as E. japonicum. In the University Tropical Glasshouse we have Equisetum myriochaetum, a horsetail that can grow several metres high but is not frost tolerant. At one time horsetails were even thought to accumulate gold!
However, the stimulus for this blog was a phone-call yesterday afternoon asking for advice on how to eradicate horsetail from a rock garden. My initial response, thinking this was a small domestic rock garden, was to suggest moving the rocks to a new clear site and to replant from scratch because the movement of any soil would risk transfer of horsetail too. However it soon turned out the problem was on a rather grander scale, the problem was in the rock garden at a major historic house. At that point I had to think hard. I’ve been trying on-and-off to eradicate horsetail from my allotment for five years but still it comes back every spring. The plant sends rhizomes several metres into the soil, those rhizomes are brown or black in colour and fragment easily. Horsetail can regenerate from small rhizome fragments so digging it out is just not practical. With severe infestations an initial dig to remove the bulk of the rhizomes is worthwhile, but only to help with the next stage.
Bruising of the stems followed by treatment with a glyphosate containing systemic weedkiller is one of two ways I have found to reduce the vigour of horsetail and is also recommended by the RHS. Even then it can take several treatments and several years of dedicated search-and-treat to remove all traces of horsetail. Any small surviving fragments will regenerate rapidly and soon re-establish the plant. The second method is to clear the ground entirely and cover it with thick black plastic for at least five years, and sometimes longer to starve the plant of light and water, even then, there may be small persistent fragments that will re-grow.
On an established rockery, eradication would take a dedicated team with good eyesight and plenty of both time and systemic weedkiller to keep the plant under control, and probably nothing short of a miracle to totally eradicate the horsetail. Gardeners only stay as gardeners if they have persistence and optimism in the face of adversity so don’t let that put you off.
So my advice; if the infestation is heavy, dig and hand weed the bulk of the material then check on a weekly basis for new shoots, bruise them by hitting with a trowel or rake (or any hard object) and paint them with glyphosate weedkiller. Keep doing that for 5-10 years and with any luck the horsetail will go. But even then you need to stay vigilant for the first signs of return.
A weedkiller I have not tried, and that is sold for horsetail eradication, is Glufosinate-ammonium (included in products such as ‘Kurtail‘ and ‘Whippet Horse Tail Killer’) however the environmental safety of this chemical is debated by several organisations including the Pesticide Action Network but I have no personal experience of its use and leave the reader to make their own decision. Legislation controlling which herbicides can be used in Europe is gradually being amended.