can you tell the difference between the nasties and the not so nasties when it come to a handfull of roots, at this time of year? if so fine burn or rot down in bag all the nasties but dont start a compost heap with anything your not sure of ...if not, i would advise a) double dig potato beds, inverting the soil over the previous forked over botom spit, usually gets rid of the not so nasties and you can spot weed any really nasties during the seasons cultivation of the potatoes when they rear thier ugly heads and you can see them for who they really are!. b) fork over onion bed and remove every weed and bit root you can ( burn them and save the bonfire ash to add to the soft fruit or the compost heap) add leaf mould & sand and dust with year old soot fork over or hoe again in three weeks, then tread to consolidate, three weeks before planting sets. c) spray the weeds with a systemic weedkiller on the brassica bed when temperatures are above 7 degrees and weeds are actively growing, spraying again 3 weeks later to get the real nasties (do not put these poisoned weeds on the compost heap ..burn them whatever they are), then hoe before planting seedlings which are easier to weed and more able to cope with a little competition from any weeds which may germinate later in the season d) or shave off with a spade the top couple of inches of weed infested top soil and put this at the bottom of your 2 ft deep bean trenches along with and other part composted materials, shredded newspaper etc.. if you continue to do this every year removing every weed you see above ground every sunday for ten years you will only have to cope with the wind bourne weed seeds and you might have a weed free allotment we all dream of. i still live in hope, and ive had my allotment 15 years.