By Ramdas Kanissery
By November, most Florida citrus groves are carrying what the rainy season left behind. Guineagrass has pushed seedheads into the tree skirts, Spanish needles and sedges have filled in the gaps, and milkweed vine and balsam apple are climbing the trunks. Before the fall preemergent herbicide is applied, those weeds need to come out. A residual sprayed over a mat of weeds and vines lands on leaves, not soil.
Preemergent herbicides work in the top couple of inches of soil and do little to weeds that are already up. When a canopy of grass or a mat of vines covers the ground (see Figure 1), much of the spray never reaches the soil So, the weed control you paid for comes up short. Clean ground first, residual second.
MATCH THE TANK MIX TO THE WEEDS
Walk the rows first. Most carryover stands mix grasses (guineagrass, bermudagrass), sedges, vines (milkweed vine, balsam apple, possum grape) and Spanish needles that have grown tall and stemmy. Glyphosate does most of the work because it moves down to the roots. Established guineagrass and nutsedge come back from crowns and tubers, so use the label rate for established perennials, not the rate for small annuals. Glyphosate products also differ in strength, so check the acid equivalent on your jug before you mix.
For vines and broadleaf weeds, add a second mode of action. Saflufenacil with methylated seed oil and ammonium sulfate helps on broadleaves but not grasses. In established groves, 2,4-D adds punch on vines. Glufosinate is a good rotation option but expect perennials to regrow.
TIMING, COVERAGE AND TREE SAFETY
Spray while weeds are still growing. Right now, most groves still have warm days and some soil moisture, so weeds take up herbicide well. Once dry weather and cool fronts arrive, uptake drops. Do not mow the row edges just before a systemic spray since glyphosate needs leaf surface to reach the roots. For vines already in the canopy, cut the stems near the ground and spray the regrowth when it is a few inches tall.
Use enough spray volume for good coverage and add ammonium sulfate if your water is hard. Keep every product off green bark, leaves and fruit. In early varieties such as Hamlin, glyphosate drift can cause fruit drop, so spray on calm days with the boom low. Glufosinate carries a 14-day preharvest interval. As always, read and follow the product label.
CHECK YOUR WORK
Give a systemic burndown two to three weeks before you judge it, then walk the rows again. Dead guineagrass should be brown right down to the crown, with no green shoots at the base. Sedges need a closer look, because new shoots can push up from tubers after the tops have died back. Spot treat whatever survived now, while it is small, instead of carrying it into the cool season.
Look at the row middles while you are out there. Middles full of mature seedheads can undo your work, because a mower throws that seed right back into the clean strip under the canopy. Keep the discharge pointed toward the middle and mow where you can before the seed drops.
Summer’s weeds have had their run. Take the tree row back now, while weeds are still green enough to take up herbicide, and your fall preemergent goes down on clean ground instead of a mat of grass. Wait, and you will spend the cool, dry months chasing the same weeds plus a new flush. Clean rows in November mean fewer weeds in March.
Ramdas Kanissery is an associate professor at the University of Florida Institute of Food and Agricultural Sciences Southwest Florida Research and Education Center in Immokalee.
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