By Ramdas Kanissery
When the summer heat rolls in and the rain gets spotty, growers tend to shift their focus to irrigation, nutrition and HLB management. Weed control often gets pushed to the back burner. But here’s the thing: Heat and dry spells do not stop weeds. They change them, and those changes can make your weed management program a lot more complicated if you are not paying attention.
DON’T UNDERESTIMATE STRESSED WEEDS
Under heat and drought stress, most weeds slow down and go into survival mode. They may look small or half-wilted, but do not let that fool you. They are not giving up. Sedges, for example, tap into underground tubers for moisture and can keep growing long after the soil surface has dried out.
The bigger problem is what stress does to herbicide uptake. A weed that is not actively growing is not moving product through its system. Systemic herbicides like glyphosate or saflufenacil (Treevix) need to travel from the leaf down to the roots to do their job. In a stressed, slow-moving plant, the herbicide can just sit in the treated tissue and go nowhere. You might see some leaf burn and think the application worked, then watch regrowth pop back up two or three weeks later.
TIME YOUR APPLICATIONS PROPERLY
Spray weeds when they are actively growing, not when they are sitting there stressed and shutting down. In the summer, that often means timing applications after a rain event or irrigation cycle. A weed that has had 24 to 48 hours of good moisture is going to take up and move a systemic product much better than one that has been sitting in dry soil all week.
Weeds such as Spanish needles add another wrinkle during Florida’s hot summer months. This weed can grow surprisingly fast when conditions allow, but heat or drought stress will cut down on how well it takes up herbicide (Figure 1). That combination means a delayed or missed application on large plants can turn into a real headache. Catch Spanish needles small and actively growing, and your products will do a much better job.
Early morning applications can also help. Temperatures are cooler, weeds are less stressed, and humidity is higher, which slows drying on the leaf surface and gives the herbicide more time to absorb before the heat of the day takes over.
USE CONTACT HERBICIDES AS A BACKUP PLAN
When weeds are too stressed for systemic products to get the job done, contact herbicides provide a more reliable option. They do not depend on translocation, so plant stress matters a lot less. Systemics will not knock out established perennial weeds with deep root systems, but for annuals and seedlings, a contact product can clean up a block without waiting around for ideal systemic conditions.
Just keep in mind that contact-only programs usually mean more follow-up trips compared to a well-timed systemic. Factor that into your decision when you are planning what to use and when.
WATCH FOR WEEDS AROUND DRIP EMITTERS
Even during a dry stretch, drip irrigation creates localized wet zones that weeds find quickly. Nutsedge, Florida pellitory and other moisture-loving species can concentrate around emitters and compete directly with the tree at the root zone. Check these areas regularly because weed pressure there does not follow the same pattern as the rest of the grove during dry weather.
Directed sprays work well in the tree row during dry periods since there is no surface water to carry product off target. Just make sure to shield applications away from green bark in young groves and avoid contact with feeder roots that may be close to the surface near emitters.
ADDRESS ROW MIDDLES DURING DRY WEATHER
Bare or sparse row middles during dry weather are not necessarily a problem, but they can become one when rain returns. Weed seeds that have been sitting dormant in the soil are going to germinate fast once conditions improve. If you let weed pressure in the middles go unmanaged during the dry season, you may be dealing with a heavier flush than expected when summer rains pick back up.
Mowing row middles before the rains return can help reduce the seed bank and make post-rain management easier. Some growers also use low rates of systemic herbicides for chemical mowing to slow regrowth without full burndown, which can extend the interval between applications.
Heat and dry weather make weed management more unpredictable, but they do not make it optional. Staying on top of what is happening in the grove during dry stretches, timing applications to catch weeds when they are at least partially recovered from stress, and understanding how your herbicide modes of action respond to those conditions will keep you ahead of problems rather than chasing them after the rain returns.
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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