My sister Steph and I moved most of a huge pile of couch grass roots on the allotment to a heap where we can cover them in light-proof sheets whilst they rot down for a year or two. We also planted three gooseberry bushes she brought us as a present.
Our plot runs along part of the fence which surrounds the allotments, so we are especially vulnerable to vandals. What is worse, the fence at this point is lower than elsewhere, so it is a favourite point for access. I have begun to plant a hedge of thorny bushes alongside the fence - blackberries, hawthorn, loganberries, and gooseberries. I'd like to add holly, roses and blackthorn. When mature this should be a vandal-proof barrier, and also provide food for us and habitat for native birds (who will in turn reduce the population of slugs, snails and caterpillars). If it gets thick enough it will stop passers-by on the footpath from even seeing into the allotments. I like chatting to the friendly people who go past and it would be a shame to cut ourselves off from them, but I think if the local toe-rags can't see what's on the other side of the hedge it will remove the temptation to break in and set fire to the sheds etc.