Allelopathic effect of landscape mulches on the germination of weed seeds and growth of ornamental plants

Weed seed germination and ornamental plant growth response to selected mulch materials (33 types) were tested in the field and greenhouse at the University of Tennessee, Knoxville in 1987 and 1988. Two field (experiment 1 and 2) and two greenhouse (experiment 3 and 4) studies were set up using morningglory, large crabgrass, and pigweed seeds. Ornamental plants tested in experiment 1 were Lagerstroemia indica L. (common crapemyrtle), Cotoneaster dammeri C.K. Schneid (bearberry cotoneaster), Salix triandra L. 'Brilliant' (willow), Acer rubrum L. (red maple), Tagetes sp. (marigold), and Helianthus sp. (sunflower). Ornamental plants tested in experiment 2 were Ilex x Meserveae S.Y. Hu. 'Blue Girl' (holly), Euonymus alatus (Thunb.) Siebold. 'Compactus' (euonymus), Cornus florida L. (flowering dogwood), and Cotoneaster congestus Bak. (pyrenees cotoneaster).

In the greenhouse experiments only the three weed seeds were tested. For experiment 3, a layer of mulch was spread over the seeds which were on mined quartz sand in four-inch round pots. For experiment 4, leachate derived from the mulch materials was poured over the weed seeds which were also on sand in pots. Soluble salt and pH levels of the leachate were obtained in experiment 4.

The results of all experiments were analyzed with ANOVA and the means separated according to Duncan's multiple range test at the 5% level of probability and significance was compared to the control(s).

In experiment 1, the most significant mulches that inhibited germination of the weed seeds were black walnut leaves against morningglory, barley straw against large crabgrass, and rye straw against pigweed. Growth of the red maple test plant species was significantly inhibited by wheat straw.

In experiment 2, the most significant mulches that inhibited germination of the weed seeds were eastern hemlock foliage and wheat straw against morningglory, and sugar maple sawdust and black walnut wood shavings against pigweed. Euonymus growth was significantly inhibited by sugar maple sawdust, and holly growth was significantly enhanced by eastern red cedar wood chips.

In experiment 3, the most significant mulches that inhibited germination of the weed seeds were alfalfa against morningglory, red maple leaves against large crabgrass, and eastern red cedar wood chips against pigweed.

In experiment 4, the most significant mulch leachate that inhibited germination of the weed seeds were red maple leaves against morningglory and pigweed, and eastern hemlock foliage against large crabgrass. The greatest correlation between soluble salt readings and germination occurred with morningglory (r = -.58). The greatest correlation between pH readings and germination occurred with large crabgrass (r = .47).

Due to the lack of supporting research in this topic further experiments should be done to provide definite conclusions. This research could provide valuable information and guidelines for additional research.