Dr. Nathan Boyd, weed scientist at the UF/IFAS Gulf Coast Research and Education Center and I have been working on tropical ornamental crop tolerances to preemergent herbicides as part of an FNGLA research grant. He sent this picture of firebush BoradStar herbicide damage to me and thought I would share. BroadStar can be applied to ornamental plants growing in containers, the field and in landscapes. Mild spotting is a common reaction of tolerant plants with symptoms showing up 1-2 days after application. Do not apply BroadStar to wet foliage, stressed plants, or some plants that can funnel the herbicide into leaf bases. Below is his description of what is going on.
The active ingredient in BroadStar is flumioxazin. This herbicide is a PPO inhibitor that is generally applied preemergence (before weeds emerge). It can, however, damage plants when the herbicide comes in contact with green foliage. When applied preemergence, the emerging seedling absorbs the herbicide and subsequently dies when the shoot is exposed to light. Flumioxazin can also be absorbed by the root system of larger plants. When this occurs the roots generally are not damaged as they are not exposed to light and do not contain chlorophyll. Flumioxazin can travel through from the roots to the leaves. Once it reaches the leaves where it is exposed to light the veins turn purple and die. Leaf death follows soon afterwards. Symptoms of flumioxazin damage include, purpling of the veins in the leaf, purpling or tissue death moving from the veins outward.