2018 ASHS Annual Conference

Many pinebark suppliers now organize inventories based on particle size (e.g. 2 or 1 cm, etc.) to create custom blends for growers. Experiments were conducted in 2016 and 2017 to determine the influence of pinebark particle size on weed growth and preemergence herbicide efficacy when subjected to similar irrigation regimes. Pinebark was obtained from a local supplier in Apopka, FL and separated into three particle sizes including 6.3 (large), 2.8 (medium), and 1.4 mm (fine) using soil sieves. Standard amendments (fertilizer, lime, micronutrients) were incorporated in each substrate. Substrates were then treated with two rates of prodiamine [0.84 and 1.7 kg active ingredient (a.i.) ha^-1], dimethenamid-P (0.84 and 1.7 kg a.i. ha^-1) or indaziflam (0.04 and 0.08 kg a.i. ha^-1) using a CO₂ sprayer at 468 l ha^-1 application volume. Seeds of Eclipta prostrata (eclipta), Digitaria sanguinalis (crabgrass), Oxalis stricta (oxalis), and Pilea microphylla (artilleryweed) where then surface sown onto pots. Eclipta were sown to pots treated with indaziflam, oxalis were sown onto pots treated with dimethenamid-P, and artilleryweed and crabgrass were sown onto two separate groups of pots treated with prodiamine. Pots were kept inside a greenhouse and irrigated 0.8 cm per day. Data collected included weekly weed counts and shoot fresh weights at 8 weeks after seeding. The trial was designed as a 3 × 2 factorial with three particle sizes and two rates of herbicide. In nontreated pots, substrate particle size significant for all four weed species with growth increasing as particle size decreased. Shoot fresh weight increased by 33, 117, 49, and 318% for crabgrass, eclipta, oxalis, and artilleryweed, respectively, when comparing weeds growing in small particle substrates to those growing in large particle substrates. This was likely due to higher water holding capacity of the smaller particle substrates as irrigation was not adjusted for different particle sizes. In herbicide treated pots, particle size did not influence herbicide efficacy for any species. Rate was significant in both eclipta and oxalis, however both rates provided over 90% control, and there were no rate × particle size interactions for any species. Based on these results, normal particle size ranges used in nursery production will likely have little effect on herbicide efficacy if proper herbicides and rates are applied for problematic species. In outdoor production where total water applied (irrigation and rainfall) is more difficult to manage, weed growth will likely increase in substrates with higher water holding capacities.