2018 ASHS Annual Conference

Mulch can reduce weed growth but it not clear how physical properties of mulch affect weed emergence. To determine how different mulch materials and physical properties affect weed emergence, outdoor and greenhouse experiments were conducted in Apopka, FL. For the greenhouse experiment, containers (3.8 L) were filled with standard substrate and divided into two halves using plastic sheets. Crabgrass (Digitaria sanguinalis) or garden spurge (Chamaesyce hirta) seeds were sown to half of each container either above or below pinestraw, pinebark or hardwood mulch at depths of 0, 1.3, 2.5, 5.1, or 10.2 cm. For the outdoor experiment, nursery containers (11.4 L) were filled, mulched, and seeded in a similar manner. A square transparent plastic tube was inserted in the center of each container, below the mulch layer and light intensity measurements were recorded using a LICOR^® LI-191R sensor. Particle size analysis of mulch materials was conducted separately using soil sieves ranging in size from 1 to 25 mm. Moisture retention by mulch was also recorded using moisture sensors and by recording water retention in the mulch layer following 2.5 cm of irrigation at 1, 4, and 24 hr after application using Buchner funnels. Data collection included biweekly weed counts, light measurements under mulch layers, moisture levels in the mulch layer, and drainage through mulch layers. Weed emergence decreased 37 to 90% when seeds were placed below the mulch layer in all mulch types for both weed species compared with seeds placed above mulch. Mulch type had no influence on emergence when weeds were placed below layers of 2.5 cm or greater. When seeds were placed above the mulch layer, pinestraw or pinebark had 63 to 83% fewer weeds compared to hardwood. Particle size analysis showed hardwood was composed of smaller particle size and had 60% greater moisture holding capacity compared with other mulch materials. Light sensor data showed mulch depths > 2.5 cm excluded 99.5% of light and there was no difference in light readings at higher mulch depths. Results from these trials suggest that for the species evaluated, weed emergence will likely increase when seeds are present (introduced) above mulch layers compared with seeds already present in the soil. Pinebark and pinestraw may provide greater weed control compared with hardwood mulch due to less moisture holding capacity but mulch type will have less influence on emergence of seeds already present in the soil when as depths increase.