Prune out spent flower stalks or leave them on to develop mature seeds if you want Verbena bonariensis to lightly seed itself around the garden. After blooming, plants flower stalks along with leaves will eventually begin to look leggy and ragged. When this happens, cut almost all the way to the ground, leaving just a few buds at the base of the plant. Plants will grow back reinvigorated. At certain times of the year, these plants often develop a white fungus on the leaves called powdery mildew. In general, there is no need to resort to fungicides, as the plants are simply prone to the infection and can usually survive it with no problems. If the powdery mildew is significant, simply cut back to the base, as explained above, to remove infested parts of the plants and stimulate new growth. It is best to throw out mildew infested material rather than compost it or keep it on site as part of the mulch layer. Good air circulation around plants and keeping foliage dry helps discourage powdery mildew (S). References