2019 ASHS Annual Conference

Many ornamental plants are notoriously sensitive to preemergence herbicide applications, which creates challenges for growers who must keep their crops weed free. Beginning in 2016, a series of experiments have been conducted at the University of Florida Mid-Florida Research and Education Center in Apopka, FL to determine effective methods of weed management through strategic fertilizer placement. Four placements including topdressing, incorporating, subdressing, and dibbling were evaluated using a controlled release fertilizer applied at the recommended rate to determine how placement influenced growth and reproduction of large crabgrass (Digitaria sanguinalis), spotted spurge (Euphorbia maculata) and eclipta (Eclipta prostrata) in greenhouse experiments. A similar study was conducted to determine how these different fertilizer placements applied at two rates (low and high recommended rates) influenced efficacy of preemergence herbicides including dimethenamid-P (for spotted spurge control), flumioxazin (for eclipta control), and prodiamine (for large crabgrass control). The greatest shoot and root weights were recorded for all three weed species when fertilizers were either topdressed or incorporated. Dibbling or subdressing fertilizers at a depth of 5 cm resulted in limited to weed growth or seed production and weeds fertilized using these alternative methods were similar to size to weeds that received no fertilizer. When two fertility rates were evaluated, weed growth increased at higher fertility rates in topdressed and incorporated treatments but fertilizer rate did not influence growth of spotted spurge or large crabgrass when fertilizers were topdressed. Fertilizer placement had little effect on preemergence herbicide performance, indicating that weed control efficacy should be consistent across different fertilizer placements. Overall, subdressing fertilizers could be an effective method of weed control in container grown nursery crops. Dibbling resulted in similar or less weed growth than subdressing, but may be associated with crop phytotoxicity in some cases based on previous reports. When using preemergence herbicides, control will likely be influenced by herbicidal active ingredient, rate, and application practices to a greater degree than fertilizer placement.