Ranking the shade tolerance of forty-five candidate groundcovers for agroforestry plantings

Several large-scale screening trials evaluating native and introduced herbaceous ground covers have been conducted in the last half century. Most trials have used shade cloth to evaluate growth of potted plants under moderate shade (45 to 55 percent of full sunlight) similar to what might be found in many agroforestry practices and heavy shade (20 to 30 percent of full sunlight) similar to what might be found in a well managed hardwood forest. It is difficult to compare results from different trials because there are few species in common, variable environments, and different harvesting times. Most of the larger screening trials appear to have included herbaceous forbs and grasses spanning the shade tolerance range from very tolerant to intolerant species. Using data from published and unpublished reports, we determined a percentile value for each species from the most shade intolerant (value = 0) to most shade tolerant (value = 100) within each screening trial. Percentile values were averaged across multiple screening trials to determine relative ranking of forty-five introduced and native herbaceous forbs and grasses. In general, cool-season forages tend to be more shade tolerant than warm-season forages with some exceptions such as eastern gama grass and the native tickclovers.