Powdery mild in dry locations or were the summers are very hot and humid. Slugs and snails might feed on new growth. Plants can self-seed heavily if happy. But light frosts do not effect them. If applying a heavy winter mulch - remember to remove it early and carefully so that the new flowering stems are not damaged.
Seed is good sized, black, shiny and ripe in mid to late summer.
After Flowering, the stems lay on the ground, as new leaves are grown. Seed is not easy to collect, each flower might produce seed but only one seed per flower. As the seed nears the point of ripping it is very easily dislodged from the plant making it a rare find to collect ripe seeds from the old flowers. The use of some kind of drop cloth, like cheese cloth or old pantyhose might be helpful in collecting seed as they are dislodged. If you want to collect seed place the drop cloth down around the plants three or four weeks before the seeds ripen and collect them as the fall out of the flowers onto the drop cloth.