Me and my fellow growers are experiencing this problem too. While it's possible to decrease the potency loss by picking out pollinated ovaries, it is a procedure that has to be done often. When such masses of ditchweed present near your garden, it would be wise to consider planting sativas. Wild cannabis (including ruderalis) are related mainly to indicas, so indicas can be pollinated by them quite easily. Sativas cannot, and the more sativa genetics are in a plant, the more resistant it is against wild pollen. This is my first reply, I hope it'll help you.