Asparagus takes a few years for the roots to spread and mature. If this is their first year in the ground, you don't want to do much with them. You'll need to wait until the second or third year to harvest. Even then, you'll not want to harvest the whole season, just a month or so. For now, let them fern out. Don't cut them back, the ferns collect energy and transfer it to the roots for the next season.

If these are mature plants that are suddenly sprouting out thin, it might mean that they were harvested for too long last spring, that they aren't getting enough fertilizer, or that some impertinent bug is chewing on the roots.

Thanks Lainedru/tugbrethil ;) they are new plants i was given 3 weeks ago in tiny pots ,so i repotted them into larger pots partly because we were still getting hard frosts which seems to have subsided now,in the meantime they've shot up from 3 inch to 15 in the greenhouse, so im gonna get the soil prepared and planted this week, fingers crossed thanks again ;-)