Weed Control and Organic Mulches Affect Physiology and Growth of Landscape Shrubs

Landscape mulches are widely promoted to improve soil moisture retention, suppress weeds, and improve the growth of landscape plants. The objective of this project was to determine the effect of common landscape mulches (pine bark, hardwood fines, cypress mulch, color-enhanced ground pallets) on soil moisture, soil pH, weed control, and physiology and growth of landscape shrubs. Two additional treatments were not mulched: no mulch + no weed control and no mulch + weed control. Growth was measured on eight taxa (Euonymus alatus ‘Compactus’, Spiraea ×bumalda ‘Goldflame’, Weigela florida ‘Java red’, Taxus ×media ‘Runyan’, Thuja occidentalis ‘Golden Globe’, Hydrangea paniculata ‘Tardiva’, Viburnum dentatum ‘Synnestvedt’, Viburnum trilobum ‘Compactum’). Leaf gas exchange [net photosynthesis and stomatal conductance (g s)] were measured on Hydrangea paniculata, V. dentatum, and V. trilobum only. All mulches increased soil moisture compared with no mulch + weed control. There was no difference in soil pH or foliar nitrogen among treatments. All mulches, except cypress mulch, increased plant growth of most shrub taxa compared with no mulch without weed control. Mulches increased g S relative to no mulch without weed control. Photosynthetic rates of plants mulched with cypress mulch were less than the other mulches and not different from no mulch. Overall, the result suggests that, except for cypress mulch, the organic mulches tested are equally effective in improving growth of landscape plants. Reduced photosynthetic efficiency and growth of shrubs with cypress mulch suggest potential allelopathic effects.