Using Roundup or other glyphosate weedkiller repeatedly, it will probably take years. It's usually called mares tail.

Will it, be ok to plant there after useing it.

The makers say that glyphosate is absorbed through the plant's leaves (and stem) and that it is inactivated in soil. They say that it is safe for pets and children to be allowed access to the area when the weedkiller is dry. You need to apply it on a still day so that if you spray it it does not drift onto other things. I often apply it by paintbrush when spot weeding deep-rooted weeds.
According to the manufacturers website -
http://www.roundup-garden.com/how_roundup_works.aspx
it is safe to replant / resow after 7-10 days. This allows time for the weedkiller to work.

usually found where the ground is damp or waterlogged for some time, try improving the drainage of the ground also..

I suggest you will never get rid of it? Weedkiller is difficult with this as it grows through shrubs and plants. I have found the only way is to keep pulling it out and eventually you get on top of it. Till next year then it starts again! It likes poor soil so I get it in my rockery as well.

Horsetail has a waxy coating pour boiling water on it first this will help the weedkiller to penetrate it, then spray it with roundup or weedol. if it is near any plants use a plastic pop bottle. cut it in half put it over the horsetail and spray down the bottle top.