Roy, My sympathy! Horsetail is notorious as one of the most difficult weeds to eradicate and I believe it has managed to hang on since the age of the dinosaurs. Trying to pull it up or dig it out is useless but the following treatment works well: Firstly, do not treat until the plant is fully grown in late summer because you need as much growth as you can get to absorb the herbicide. Then thrash (bruise) the stems and leaves with twigs (this breaks up the waxy surface). Then mix a systemic herbicide with a mix of light cooking-oil and white-spirit or turpentine (any light oil will do) until it is thin enough to spray and use it in place of the recommended water quantity. Finally, spray the mixture thoroughly over the whole plant. The oil/spirit mix will ensure that the herbicide is in contact with the plant for a sufficient period to kill it and it is rain-proof. There is also a giant, non-invasive form of this plant which looks absolutely magnificent. Good luck. Den.