I've found this
This is the toughest weed to eliminate from a garden. Bindweed,couch grass and ground elder are easy in comparison. The roots go down 3 feet or more so digging out isn't really an option.It spreads by deep runners or by spores in spring. Once you've got one piece you'll soon be over run and if it gets among your shrubs/plants its virtually impossible to eradicate
I've tried glyphosate(Roundup) and bruised the stems to let the chemical do its work but even after spraying almost every week for months on an infested gravel area it was only partially successful. The weed looked sad but didn't die. (don't worry... read on there is a method that works much better!)
An organic method would be to cover the weed with weed supressing membrane or old carpet but will take at least a year to be effective and there is no guarantee of success.
Since writing the above I've learned that tthe weed killer Kibosh( now Kurtail) will kill mares tail. It is very expensive but the bottle you get is concentrated and will dilute to about 20 litres so it will probably last you a lifetime unless you have an enormous garden! (or you could share it with a friend) Once diluted it compares in price with the hand held spray weed klllers common in the shops
On edit : I've now tried Kibosh and after 7 days all the marestail I've sprayed is black and the top growth at least is very dead. No other weed killer I've used has had much visible effect at all. Time will tell if it has killed the roots completely but it is looking good!
March 2011 ... still . no sign of any marestail.
I will report back again when plants start growing in spring.
On edit : 20 th April 2011. There are some signs of regrowth of the marestail :^(
I have sprayed each new shoot and they're all turning black. I'll stick with it to see if I can eradicate the evil stuff.
On edit 15th October 2011. The Kurtail has generally been a success. I've had some regrowth but i suspect most is coming from the field next door which I cannot treat. I've also tried it on a gravel garden with great success... no regrowth at all .
I've also taken the liberty of copying this from Kopex in the comments section below
"Timbrel works an absolute dream in treating horsetail on lawns. 25ml Timbrel in 5l of water didn't harm new grass and the horsetail was dead in 2 days. Timbrel is selective and kills everything apart from grass so obviously it can't be used around trees and plants"
I'd just like to add I have no links whatsoever with any of the products I've mentioned. I'm a professional gardener and I'm just describing what works for me