Pulling them, rather chan clipping, will get more of the root, and slow them
down more. I've had some luck spot spraying with weed-b-gon or roundup.
Spray the new growth, then let them be. Spray again if they don't die back.
Use a sharp flat bladed spade to slice 12-14 inches deep all around your
yard, to cut off the nutrient supply coming in from roots outside your yard.
That, and constant pulling, will get them eventually. Don't let them get
tall, that just lets them build up the roots. It is not a one year project.
It might take several to clean it up, but you will see gradual improvement
if you keep after them.

Bob

Thanks for the tips, Bob. It's nice to hear somebody with at least a
little optimism that I can get the horsetail under control; I'll cling
to the illusion of hope, even if I lose the battle in the long run.

I did find it interesting that your advice about pulling the roots
contradicts what I've read in some other messages. Many people have
said that pulling the plant up by the roots stimulates it to grow even
faster -- for some scientific reason that eludes me -- and have
suggested clipping that plants at ground level is a better idea since
it will theoretically help starve them via denial of photosynthesis.
I'm assuming you disagree with this and suggest that I just go
straight for the roots?

Last but not least, I'm curious whether anybody out there has an
opinion about whether a commercial gardening service would be of any
use. In other words, would a professional yard care firm have access
to chemicals and such that might be more effective than what I can get
my hands on? Would it be worth $200-300 to have them come out and
spray the weeds or would it simply postpone the problem for a few
short weeks? I'd be willing to pay for professional help with the
horsetail problem, but only if it results in at least a semi-permanent
improvement and not just a few weeks of relief.

Does anybody have thoughts on whether this would be a good/bad
investment?