Do note that horsetail is less likely to become established in dry soils. You see, its wind-blown spores will only germinate under moist conditions. However, once the plant is established, it will readily tolerate dry conditions, although it will grow more slowly under dry conditions than in moist ones. Typically the home gardener accidentally carries a section of rhizome from a moister setting to a drier one, sometimes in soil purchased from a nursery, leading to a new infestation.
In a garden watered regularly for the benefit of other plants, horsetail will be very much at ease and will spread rapidly.
Cultivating Makes Things Worse
Another reason that cultivating doesn’t work is that horsetail tubers tend to be produced deep in the ground, up to 5 feet (150 cm) down, well out of range of cultivation tools!
If you ever decide you do want to try to control horsetail by cultivating the soil, at least make sure you clean your tools before using them in another area, otherwise you risk spreading sections of rhizomes or tubers by accident. The rototiller can be the horsetail-haters worst enemy on this level: not only does it slice the rhizomes into numerous small pieces and spread them hither and yon, but some almost always stick to the blades, transporting them to the next section of garden. A rototiller really needs a good cleaning after each use!
Herbicides
One type of biological herbicide, though, is fairly efficient. Vinegar-based herbicides such as Ecoclear or Weed B Gone will effectively kill horsetail foliage, but not its rhizomes. You therefore have reapply them when the plant regenerates from underground, so they will require a bit of followup.
Note that It is difficult to use herbicides, even biological ones, in a flower or vegetable garden without also accidentally killing desirable plants nearby.
Liming the Soil
The belief that you can control horsetail by adding lime to the soil to render it more alkaline is simply a horticultural myth. Horsetail grows just fine in alkaline soils. Look here for more info.
No Sun, No Horsetail!
You can however eliminate horsetail by preventing it from carrying out photosynthesis. In other words, by cutting off its only supply of energy: sunlight. If you keep its leaves from being exposed to the sun, the plant will quickly stop spreading and will eventually exhaust itself and die.
Or try densely planting with taller plants: tall ferns, big perennials or thick shrubs, anything that will cut off the sun. Horsetail does best in full sun to partial shade. It weakens in deep shade. By creating dense shade, you’ll be ensuring that it will eventually disappear. It may take 2 or 3 years to get total control… but right from the start, taller plants will pretty much hide the horsetail from view.
The Dig and Sift Method
Obviously, this method is most practical for localized infestations. It will be hard to carry out on a large scale.
The Laidback Gardener Method
I decided years ago I was no longer going to fight horsetail. I reasoned it was not stealing minerals from my plants (horsetail is not a heavy feeder) and, with its sparse foliage, it lets most of the sunlight through, so my plants still get their share of sun. And unlike dense growing weedy plants such as goutweed (Aegopodium podagraria), it rarely seems to crowd out desirable plants, just to blend in among them. To be honest, I find horsetail attractive. With its beautiful feathery foliage, it can give an attractive misty look to your landscape. If you can learn to accept horsetail as an ornamental, that will save you all the effort of trying to get rid of it.
Oddly enough, once I had decided to accept horsetail as a friend, it began to slowly disappear from my gardens. As they matured and became fuller, denser and shadier, there was no longer any room for horsetail.
Sometimes (and even often), tolerance is simply the easiest way to go… and it’s certainly the most laidback!