Physical and Chemical Effects of Mulch on Weed Seed Germination

Weed seed germination and subsequent growth results in a major cost to nursery owners, landscape contractors, and homeowners. The objective of this study is to determine how physical and chemical characteristics of different mulches impact weed seed germination. A mixture of three broadleaf (pigweed, spurred anoda, and morning glory) and three grassy (signal grass, crabgrass, and johnsongrass) weeds were broadcast over a nursery container containing soiless media mulched at several different mulches and at different depths. The media was a 3:1:1 bark:sand:peat:media (v/v), amended with a Osmocote 15-9-12 fertilizer at 2 lbs N/ cubic yard and 8 lbs/cubic yard dolomitic lime. Mulch materials included pine bark mulch, pine bark chips, hardwood chips, shredded pine straw, and a control. Particle size distribution was measured using USDA sieves ranging from 1" to < #30 and chemical analysis (pH, EC, P, K, Ca, Mg, Mn, Fe, Mo Cu, Zn) were tested at the LSU Soil Testing Lab. Three mulching depths including 0, 2, and 4 inches were evaluated in this study. Nursery containers were evaluated over an eight-week period irrigated overhead using 0.5" of water twice daily as needed. There were significant differences measured between mulch material physical and chemical characteristics. Larger sized particles and shallow depths allowed greater light penetration. Hardwood bark pH was greater than both pine bark and shredded straw. Nutrient levels were also different among mulch materials. Generally, greater depths improved weed control and reduced plant growth. Future research will expand weed species and mulch materials in production systems.