Green Horizons

Tall fescue is the major grass species used for pasture in the east-central U.S.; millions of acres of tall fescue are grazed in Missouri. However, establishing trees in these pastures, for silvopasture and alley cropping practices, for example, isn’t easy as fescue competes with trees for moisture and nutrients, and may be allelopathic (producing inhibiting chemicals). Thus, producers must implement some form of weed control to limit tall fescue’s influence on growing tree seedlings.

These walnut trees growing at the MU Southwest Center are the same age, but the row on the right grew faster and yielded more nuts when tall fescue was eliminated. (Research initiated by the late Professor Jack Slusher.)

However, the amount of weed control required for successful tree establishment in tall fescue is unclear. Current recommendations for vegetation-free zones around trees vary widely, with some literature suggesting zones extending as much as 7.8 feet (2.4 meters) from tree seedlings. More defined guidelines are necessary to minimize weed control costs and optimize land use; finding the minimum radius needed for optimal tree development is vital to growers.

A study by Jimmy Houx, University of Missouri Center for Agroforestry researcher and MU research specialist in Plant Sciences, was designed to answer the question of how much weed control is enough and to create guidelines on establishing trees in existing tall fescue pastures for ease of tree farmers and agroforestry practitioners.

Houx planted walnut seeds in six different sized vegetationfree zones in tall fescue pastures (a no-herbicide control was also included). The study was conducted two consecutive years at two locations in central and north-central Missouri, differing in soil types. Data were taken on tree seedling height and diameter every two weeks during the growing season and also at the end of the season in October. Black walnut was chosen for its importance in producing both nut and timber crops.

The research showed that a minimum of a 4-foot (1.21 meter) vegetation-free radius should be implemented to optimize tree growth and survival. Although rings as large as 6 feet in radius were studied, results showed 4 feet was optimal. Tree growth increased steadily as weed control increased from 0 to 4 feet from tree seedlings. Growth in larger zones (5 and 6 feet in radius) was not greater than that in zones of 4 feet, suggesting weed control beyond a distance of 4 feet from tree seedlings may not result in greater tree growth, but would require greater weed control costs and remove more land from forage production.

Seedling diameter was affected more than height by the different weed-free size treatments, suggesting this growth parameter is more sensitive than height to tall fescue competition.

Houx also says this study suggests that in the initial years after planting, competing vegetation is the primary factor influencing black walnut seedling growth (i.e., initially, site characteristics may not be as important as weed control for seedling growth).