The ground cover in plantings of walnut and other hardwoods can substantially affect tree growth and seed production. The number of alternative ground covers that have been suggested for establishment in tree plantings far exceeds the number that have already been tested with walnut and other temperate hardwoods. Knowing how other hardwood species respond to ground covers and which have responses statistically similar to that for walnut would greatly expand our knowledge base for making predictions and recommendations for ground covers in hardwood plantings. Data from over 110 reports of which nearly half included walnut species were compiled into a spreadsheet. The spreadsheet database compares multi-year growth and nut or fruit production of trees growing in different ground covers to that of trees in plots either with little or no management of the weedy vegetation or vegetation-free plots maintained through cultivation, herbicides, or mulches. For most hardwoods with moderate growth rates, growth as a percentage of growth in vegetation-free treatments is similar to the reduction observed for walnut saplings and pole-sized trees managed with forage legumes and grasses. In general, forage grasses have a greater impact on tree growth than do forage legumes. Although frequently used as a method of competition control, mowing does not alleviate the negative impacts of most ground covers on tree growth. Additional studies will likely need to be done before predictions similar to those for growth can be made on the effect of ground covers on fruit or nut production.
Van Sambeek, J.W. 2010. Database for estimating tree responses of walnut and other hardwoods to ground cover management practices. In: McNeil, D.L., ed. Proceedings of the sixth international walnut symposium; 2009 February 25-27; Melbourne, Australia: Acta Horticulturae. 861: 245-252.