Is it chain link fencing or weldmesh panels. Chain link fencing can be obtained in green. l presume that the total area of the allotments is being enclosed which means a lot of fencing. Anything growing to hide it will need maintenance in its own right. Who will do that, and how many will refuse to do anything? So anything growing will need minimum maintenance. A suggestion of something fast growing, Virginia creeper or like, but there are people here who can offer better advice than me.