Late September through October is the best time to control weeds like dandelion, clover, and chickweed with an application of a broadleaf weed herbicide. Broadleaf weed herbicides are systemic and move through the plant in the phloem, which also transports photosynthate. The more areas of the plant that the herbicide translocates to, the more effective the kill. In the fall, perennial plants are “preparing” for winter, translocating photosynthate and storage products to the roots. If herbicide is applied in the fall, it will translocate with the photosythate throughout the plant and to the roots, thus usually causing a complete kill. In early Spring on the other hand, the plant tends to be moving storage products from it’s roots to reinitiate leaves and flowers. Thus a herbicide applied in the early spring tends to remain in the leaves, not translocating throughout the plant, and does not provide nearly as effective long-term control as a fall application.