The orchard floor can be divided into two distinct areas: the area between the tree rows (generally planted with a permanent cover crop), and the area directly underneath the trees. The ground cover in an orchard must be managed just as the tree canopy is managed.
Row middle management
Row middle management in Pennsylvania orchards traditionally has fallen into three broad categories: clean cultivation, trashy cultivation and planting a permanent cover crop. The first two are no longer recommended because they tend to destroy soil structure by increasing soil compaction and erosion. They also create an ideal seedbed for the establishment of broadleaf perennial and annual weeds, which serve as a reservoir for viruses.
The vast majority of commercial orchards in Pennsylvania are grown under a ground management system of a sod row middle with a vegetation-free zone underneath the trees. Sod between the rows prevents soil erosion, provides traction for equipment, adds organic matter to the soil, improves soil moisture and structure, and can be a site for beneficial predatory insects.
Grass covers used in row middles tend to grow rapidly and require frequent mowing. Perennial ryegrass, K-31 tall fescue and clover are covers that have been traditionally grown in Pennsylvania orchards; however, a number of newly developed turfgrasses have been found to perform well as slow-growing cover crops. Because they grow more slowly, these new grasses require less frequent mowings. In addition, they tolerate low-fertility soils, poor growing conditions, and heavy traffic, and they grow densely enough to crowd out weeds.
The addition of clover or other legumes is not recommended for orchard row middles. While they may provide additional nitrogen to the orchard, the release of that nitrogen is unpredictable. Legumes can also serve as reservoirs for tomato ringspot virus, which causes stem pitting in peaches and apple union necrosis in pome fruit.
A relatively recent criterion of grass cultivar quality is the presence of endophytes. Endophyte-enhanced varieties are recommended over those without endophytes. Endophytes are fungi
that live within the grass plant and deter certain turf insects from feeding. Some species and varieties have naturally high levels of endophytes. Penn State researchers have been evaluating these grasses primarily for use in ornamental nurseries. This research and other observations made around the state indicate that hard fescue, chewings fescue, creeping red fescue, and slow-growing, turf-type perennial ryegrass can be used with success in orchard row middles. Each type may contain many acceptable cultivars. New cultivars regularly become available; check with your local supplier.
Using these grasses successfully depends on proper establishment practices. Cover crops that aren’t well established may be too sparse to be effective.
Tree row management
The area underneath the trees is important in the development of an orchard. Numerous studies have shown that excessive vegetation underneath trees competes with the tree for water and nutrients and can reduce the growth and cropping of the trees. Research in New York showed that apple trees grown in a mowed sod were nearly 25 percent smaller than trees grown under an herbicide program six years after planting. Penn State research on peaches also showed that the width of the vegetation-free strip affected tree growth and yield, with a narrower 2-foot-wide strip producing less fruit and smaller trees. Ideally, the vegetation-free strip should extend out the width of the tree’s canopy.The width should be established early in the life of the orchard and should not be reduced on newly planted trees.
The timing of weed control has also been shown to be critical. A study with Gala/M.26 showed that the first crop in the life of the orchard was much larger when weeds were controlled early in the season. The weed competition also affected fruit size. Based on this work, it is believed that the critical period for weed competition in apples runs from bloom to 30 days after petal fall.
At first thought mulching might seem to offer some attractive potential benefits for orchards. Mulching usually results in greater moisture retention, increased organic matter, and can help to suppress weed growth. However, mulch also provides an ideal habitat for meadow and pine voles, which can feed on tree trunks and roots.
Cultivation under the trees has some potential with the development of improved machinery. However, it can increase erosion and requires frequent cultivation and a skillful tractor driver.
Herbicides are the primary tools used to manage vegetation under the tree row, but they also have risks. Young trees can be very sensitive to herbicides, and drift onto green bark or foliage can stunt or kill the tree. Continual usage of herbicides can also build up residues of the chemical in the soil, resulting in sterilization. Herbicides are, however, the most cost-effective means currently available to control vegetation under the tree.
Source: Penn State Tree Fruit Production Guide: http://extension.psu.edu/plants/tree-fruit/tfpg
Robert Crassweller Ph.D. is a professor of Tree Fruit at the Penn State University Extension, College of Agricultural Sciences. He can be reached at firstname.lastname@example.org.
PHOTOS COURTESY OF PSU EXTENSION, COLLEGE OF AGRICULTURE SCIENCES