Horsetail, which is the almost unchanged descendants of Horsetails from the Carboniferous Period, have two parts to their life cycle.
In spring the first phase grows. This is a shoot with a cone at the top. This cone can shed many spores but these spores rarely produce new plants as they require a lot of moisture. But cut them off before they ripen just to be on the safe side. After the fruiting bodies the stems with leaves in whorls around them are produced. The outer wall of the leaves and stems are thick and contain silica deposits causing it to be difficult to kill with herbicides.
The underground part of the plant (the rhizome) can go down for six feet or more into the soil, making it very difficult to dig out. And it will regrow from the smallest piece of root.
The growing conditions that horsetail likes are those that mimic the conditions that existed millions of years ago – acidic soil (low pH), low oxygen (wet or compacted soil), and very low nutrients. Moist or boggy soil provides all three conditions, but in many cases dry soil is also acidic, compacted and lean.
So, we now know the enemy - how can we combat it? Simple, just create conditions that it doesn’t like.
First of all, acidic soil can have the pH raised by the addition of 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.
Then dig as much compost, manure or other organic material into the soil as you can. This will open up the soil structure and aerate it. It will encourage worms, that will also aerate it and will improve the nutrient content. These treatments will create a soil that Mare’s Tail will not like. Repeat these treatments on badly infested areas again in subsequent years.
While you are digging to improve soil structure and incorporating organic material be careful to remove as much of the dark brown rhizomes as you can. Then don’t forget the gardener’s law “Never let is see a Sunday” - Frequently hoe off or pull out any top growths that pop up, and you will gradually weaken it.
There is no good chemical treatment available to the amateur gardener, although if you bruise the fronds of the Mare’s Tail some glyphosate herbicides like ‘Round Up’ may help. Professionals who have had training in the use of herbicides can use other products which some consider are much more effective and work systemically.