There is no easy answer to this. Chemicals will only burn off the foliage and it will regrow. If you dig it out, it will regrow also. You can control it by waiting until the stems are fully expanded and then pulling it or spraying it. This will eventually weaken the plant and it will become less of problem.

The other drastic step is to cover the whole area with black plastic (or similar) and leave it for more than a year.
You will probably have to repeat this. It is probably the worst weed you can have in a garden.

Nice slug by the way which looks as if it is grazing the mares tails.

In a "Hobson's Choice" type of question Vincent, I think I'd rather have mares tails than Japanese Knotweed. Maybe mares tails are the second worst weed to have.

Dont see how you can spray as they grow up in the plant. I found pulling them out works,m as I get less and less each year. They like poor soil so as you enrich your soil for your plants you will also find less except in places like the rockery. But even there I find pulling them up is the best way.

Let it grow until about this time of the year, if you can bear to, and then pull it out with as much root as possible. Although it will come back this sets it back for quite a while and you will have several weeks while the majority of flowers are out without seeing it growing there. Then keep pulling it out as it comes through. If you try to weed it out earlier, I find two or three grow where each of the individual stems was pulled.
The other way to reduce it to insignificance is by constantly cultivating the soil and pulling out as much as you can each time. This is fine in vegetable growing but not a lot of use with flowerbeds unless you want to grow annuals like a 'crop'.
I don't agree that it's such a terrible weed as it doesn't overwhelm other plants like bindweed, for example, or make huge leaves like docks, or towering plants like thistles.
Just learn to live with it, or decide to move!
I've noticed this year that quite a few horsetail stems have turned brown and died. Now if we could only find out what causes that!

ha! interesting thought...i'd take a bit of whatever it is for my garden :P
I hadn't realised which time of year you pulled it out affected the effectiveness of getting rid of it, i'd just tried to pull it up as i saw it, but when you yank 19 out of about 4m squared of lawn you start to get a bit worried...

I think i will give it a go spraying/painting weedkiller onto the extended ones like vincentdunne said, maybe not in the flowerboarders though... there i'll just keep pulling it up (sadly we have too much for black plastic to work): it will be interesting to see which method is most effective :P

Thankyou all of you for your help :)

If it's in a lawn, you should be able to effectively get rid of it by mowing more regularly. Absolutely nothing can survive being cut down over and over again. Try increasing the frequency of mowing to at least once a week.

well, except maybe grass :P we are rather inconsistent with our lawn mowing, so i shall give it a go doing it more often :) thanks for that tip :D

I've just been reading up on the internet about it and the general consensus seems to be that its a bit of a hopeless case :'( although apparently if you bruise the stems you can break up the waxy coating on the leaves and help weedkiller penetrate more easily...

*sigh* seems as if i've got a long hard battle ahead of me

oh, i also found out that i've been getting it wrong and that mares-tails are actually an aquatic plant.... woops :$

I used to have them growing in a gravel car park. Roundup several times a year seemed to have got rid of them. Perhaps being driven over by the occasional car helped get the weedkiller in.

Equisetum comes in other species and one is sold as an aquatic plant, which is perhaps what you are referring to. (This one is not a pernicious weed though). The common weed, unfortunately, does best in wet and marshy soil, but also thrives in very dry soil, as we have discovered.
The only place we got rid of it completely was the 'patio', where we covered a completely infested area with a water permeable weed suppressing membrane, followed by gravel. Even so, the mares tail came up through it in lots of places, but I spent the first year pulling it out as soon as it appeared. There was still some in the second year, but now, six years' later, there is never ANY! Wonderful.

fantastic! sheer persistence pays off then?! :D i can't help being impressed by the stuff though, that it takes 6 years of weeding, covering and gravel... if i had a hat i think i'd tip it ;) or doff it...
looooong road ahead of me... but its encouraging to hear that eradication is possible :P