You have my deepest sympathies :-) I also battle horsetail but it doesn't sound like my situation is quite as severe as yours. But no doubt about it, this is a very tough weed to control. Lecturing on weed control is one of my many duties as a horticulturist and this is what I recommend.

1) Remove any top growth as soon as you see it. Mow if it appears in the lawn; pull if it appears in any planting beds. The repeated removal of the top growth will - in time - weaken the root system due to lack of ability to photosynthesize. But you have to keep on top of it. I take an almost daily turn around my garden and pull out any I see emerging that are tall enough to grasp. I also hunt for the little nubbins of a new sprout and dig down to grasp those and remove. Be certain you remove any of the spore bearing shoots immediately upon their apperance. They usually are only present in early spring and look quite different - very pale in color with a brownish yellow top and no radiating green parts. Get them before they release their spores.

2) If you have a large area to control, cutting all the plants back almost to the ground and then painting the cut stems directly with a herbicide can be helpful. I recommend a product containing Triclopyr (sometimes sold as brush and vine killer) as being the most effective. Glyphosate (RoundUp) and many other herbicides tend not to have as much effectiveness. Painting directly with the herbicide (preferably in a concentrate) on the cut stems gets the herbicide down into the root system and avoids contaminating any other nearby plants. Casoron also has reportedly generated good results and can be used with other plantings but I would use it only where you have got the problem somewhat under control.

Neither of these is a one time fix. You will have to use repeat treatments and the manual removal can take years to accomplish fully. Unfortunately, there is very little you can do to keep your neighbors' weeds from migrating to your garden. In the area you intend to put the patio, you could try digging the roots out. But horsetails have a deep and interconnected root system and just leaving a piece of root in place opens the potential for regrowth. Doing this right before you lay the patio could be helpful, though. Excavate, dig out all the roots you can locate, grade the area and lay down several layers of landscape fabric, then put down your stone or sand base and lay the pavers. I'd also suggest mortaring the pavers for more impenetrable coverage or use concrete for your patio surface.

Persistence is the key here. Any plant that has been around for 230 million years is not going to go away easily.

Cutting the plants down only encourages more growth and can spread the spores that produce them further. Take a good look at the soil they are growing in and make corrections to that soil into something the horse tail does not want to grow in.

Horse tail prefers an acidic (pH in the 4.5 to 5.5 range), wetter than normal soil, although it is adapting to other soil types. Around here it grows mostly in sandy, but moist soils (high water table).

If you are planning a patio where the horse tail is growing now the excavation for that may eliminate it if this patio is to be ground level. If a deck is planned then other steps will need to be taken.

kimmq is kimmsr

"Cutting the plants down only encourages more growth and can spread the
spores that produce them further. Take a good look at the soil they are
growing in and make corrections to that soil into something the horse
tail does not want to grow in."

Statements like this make it very clear that a first hand, up close and personal relationship with horsetails is lacking. While the plants may very well have soil preferences, they will grow anywhere - dry soils, higher pH soils, highly improved/amended garden soils, in the middle of a cracked asphalt parking lot, adjacent to a salt water beach. Exactly what kind of soil won't they grow in??

And the very clear steps jean outlined IS the recommended course of action to effect control.

Thank you all! These are very helpful tips - I actually hadn't thought about trying to excavate the area but that might be one of the best ideas to try and stop this stupid plant.

Thank you to Gardengal and Jean! I was trying to find out if mulching would stifle it (because of photosynthesis), and this is the only place I found the answer. Now I can make a plan and move on with my life! ;)