Soil fertility provides the foundation for productivity in any crop system, and it is especially important in perennial crop systems where there are few options for supplementing nutrient availability in the deeper root zone, because direct placement of fertilizer into the rhizosphere is difficult and can damage roots. Furthermore, stringent soil, climate, and infrastructure requirements of orchards usually cause growers to replant the same or similar fruit crops repeatedly in the same locations over many decades or centuries. Long-term maintenance of soil fertility and structure is especially important because serious soil problems can develop over time. Orchard soils are prone to wind or water erosion; compaction by tractors, sprayers, and harvest operations; and gradual increases in soilborne pathogens that in fect tree roots. Groundcovers provide a renewable surface layer of biomass that protects soil from weathering and compaction and influences populations of beneficial and detrimental soil microorganisms. As this biomass decomposes into the mineral soil, it replenishes organic matter—promoting microbial activity, sustaining soil nutrient reserves, and increasing soil pore volume and water-holding capacity (Hogue and Neilsen, 1987).
Deciduous fruit trees in cool-climate regions remain dormant for almost half of the year, and there is little uptake of essential nutrients from soil by dormant trees. The potential for soil erosion and leaching or runoff of nitrogen, phosphorus, and pesticide residues is greatest during the dormant season. Cool-season grasses such as Festuca and Lolium species, and broadleaf groundcovers such as brassicas and legumes that continue growing when fruit trees are dormant, can serve as "green manure" or "relay" cover crops that fix or retain nitrogen and other essential nutrients in biomass residues during the winter months. Mowing or tillage of these groundcovers in late spring releases nutrient reserves at a time when they are readily assimilated by fruit trees. Growing dormant season groundcovers that are tilled or killed with herbicides the following spring is increasingly popular in fruit-growing regions with mild winters where soil and nutrient conservation are high priorities (Marsh, Daly, and McCarthy, 1996; Tagliavani et al., 1996).
Groundcovers growing between or within the tree rows can also be managed to help control tree vigor, enhancing fruit quality and tree winter hardiness. Excess soil nitrogen and water availability during late summer and early autumn can prolong shoot and canopy growth, delay fruit maturation, increase the potential for winter cold injury when woody tissues fail to harden-off sufficiently, and make dormant season pruning more difficult and costly (Elmore, Merwin, and Cudney, 1997; Merwin and Stiles, 1998). Encouraging moderate groundcover growth and competition in early autumn can be accomplished either by seeding fast-growing cover crops in late summer or by timing nonresidual herbicide applications or cultivation so that groundcovers naturally reestablish from seeds or root propagules at the desired time of year. The advantages of managing, rather than eliminating, groundcover competition for water and nutrients during critical times of year have been widely recognized by the wine grape industry, where market incentives for fruit qualities essential to make high-value wines offset the losses in gross yield that may result from late-season groundcover competition with vines (Elmore, Merwin, and Cudney, 1997). The tree fruit industry could benefit from similar efforts to develop OFM practices that control canopy vigor in order to improve eating quality and consumption of fresh fruit.
Permanent grass and broadleafgroundcovers also facilitate access by orchard customers, workers, and machinery during wet/muddy or dry/dusty conditions. Pick-your-own growers need to consider subjective factors that encourage patrons to visit their farms, and a well-mowed green sward is generally more attractive to the public than the bare soil of "weed-free" plantings. In orchards where dropped fruit are gathered for processing or fermentation, it is especially important to minimize mud splashing and soiling of fruit beneath trees, and grass or clover groundcovers are often maintained over the entire orchard floor.
Certain groundcover species can suppress pest insects, fungi, and nematodes that parasitize fruit trees. For example, preplant cover crops of marigold (Tagetes patula) or cereal wheat (Triticum aestivum) can suppress pathogenic nematodes and fungi (respectively) that damage apple roots and cause soilborne "replant disease." Flowering groundcovers that provide habitat, pollen, and nectar food sources for predatory insects, such as hover flies (Syrphidae) and assassin bugs (Reduviidae), can increase populations of these beneficial insects in orchards and help to control leaf-feeding pests such as aphids and caterpillars, reducing the need for pesticides. Selecting and utilizing specific groundcovers to promote beneficial insects, fungi, and soil microbial activity in orchards is an integrated pest management (IPM) tactic that merits renewed attention from researchers (Brown and Glenn, 1999).
Orchard groundcovers of various types and mixtures are important components in sustainable fruit-growing systems. Properly managed groundcovers can improve soil fertility and water-holding capacity, facilitate access during inclement weather, provide habitat for beneficial wildlife, make orchards more attractive for workers and pick-your-own customers, suppress pathogenic soil fungi and nematodes, and limit excess vigor of mature bearing trees—optimizing fruit quality and reducing pruning costs.
200 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT ORCHARD GROUNDCOVER DISADVANTAGES
Despite the potential advantages of groundcovers, in most fruit plantings the surface vegetation is suppressed or eliminated over all or part of the orchard floor. In some situations this is a matter of practical convenience. For example, in almond groves where the nuts are gathered by vacuuming or sweeping from the ground surface, harvest is more efficient when no groundcover residues are present. Where flood irrigation is practiced, horizontal flow of irrigation water is more rapid and smooth when the orchard floor is completely weed free and uniform, although infiltration of irrigation or rainwater into most soils is enhanced when surface structure and porosity have been protected by groundcovers (Elmore, Merwin, and Cudney, 1997). In most situations, the primary reason for suppressing or eradicating orchard groundcovers is that, without proper management, they become "weeds" that compete with the crop for limiting nutrients and water, reducing tree growth and productivity.
Fruit trees have relatively sparse root systems that do not compete effectively with most groundcovers for water and nutrients. Grass and herbaceous groundcover root systems are more dense and pervasive, and excavation studies show that there are few tree roots in the topsoil beneath orchard groundcovers, compared with weed-free herbicide-treated areas in tree rows (Atkinson, 1980). Isotope tracer studies reveal that nitrogen fertilizers applied to the orchard floor are almost completely assimilated by soil microbes and groundcover vegetation, with little short-term uptake by trees. Soil water content during midsummer is also reduced beneath vegetative groundcovers in comparison with weed-free tree rows, despite greater tree root density in weed-free soil, which indicates that fruit trees use water more sparingly than groundcovers. Heavy irrigation is usually necessary to provide sufficient water for trees that must compete with ground-covers for soil nutrients—even in regions with humid growing seasons. Frequent mowing reduces only slightly the evapotranspiration of soil water by groundcover vegetation, although studies demonstrate a negative correlation between groundcover biomass per square meter and soil water availability during the growing season (Elmore, Merwin, and Cudney, 1997; Merwin and Stiles, 1998). In regions where rodents, rabbits, or hares often cause serious damage to the trunks and lower branches of fruit trees, OFM systems that reduce the height and density of groundcovers help to limit depredation by these pests.
Weedy groundcovers can be especially damaging in young orchards where trees must rapidly establish root and shoot systems that fill their allocated space in the orchard. The optimal type and proportional area of groundcovers in orchards thus depend upon numerous site-specific factors. Determining acceptable levels of groundcover competition—the economic damage threshold at which soil conservation and other pest management benefits compensate for acceptable levels of groundcover competitive interactions with the crop—is complex and variable from region to region, depending upon orchard planting systems, climates and soil types, other management practices or constraints, and marketing strategies (Elmore, Merwin, and Cudney, 1997).
The most common OFM systems in deciduous orchards involve permanent groundcover mixtures of perennial grasses and herbaceous broadleaf species, such as clovers, maintained by periodic mowing in the drive alleys between tree rows, and strips beneath the trees that are treated with herbicides, mulches, or cultivation to suppress groundcovers during the growing season or year-round. The relative widths of the drive lane groundcover and tree row weed suppression strips can be adjusted as appropriate for tree age and vigor or soil nutrient and water supply. In numerous studies comparing different ratios of groundcover to weed-free area, the optimal weed-free area varies according to tree age and soil conditions (Hogue and Neilsen, 1987). In high-fertility soils where tree roots are concentrated by drip irrigation and weed suppression, studies show that fruit production is equivalent in weed-free strips of 2, 4, and 6 square meters per tree (Merwin and Stiles, 1998). Under nonirrigated conditions or in soils with low nutrient reserves, growth and productivity of young trees generally increase as the weed-free area increases, up to 10 square meters per tree, beyond which there is little further gain. Less is known about the importance of weed-suppression timing in orchards, but a few studies suggest that controlling weed competition during the early months of the growing season is especially important for successful establishment of young fruit trees.
Herbicides, mulches, or mechanical cultivation can all provide sufficient control of groundcover competition within tree rows, but these three OFM practices differ substantially in cost, convenience, and soil impacts (Elmore, Merwin, and Cudney, 1997; Hogue and Neilsen, 1987). Preemergence residual or postemergence herbicides are the easiest and least expensive methods for suppressing weeds. Preemergence soil-active herbicides can be safely applied beneath established trees in most situations, and a single application can control most groundcover species for the entire growing season or longer. However, the residual soil persistence and activity of these herbicides can exacerbate the risk of chemical leaching or transport of eroded soil particles and prolong selective pressure for weed genotypes with herbicide resistance. It may not be desirable or necessary to keep tree rows weed free during the dormant season when soil weathering and nutrient losses are most likely (Merwin and Stiles, 1998; Tagliavanni et al., 1996).
Postemergence nonresidual herbicides suppress groundcovers for a relatively brief time (usually four to eight weeks) during the growing season and permit surface vegetation to reestablish later in autumn, thereby providing soil organic matter and surface protection during the dormant season. Long-term OFM studies indicate that tree growth and productivity are as good or better when sparse ground-covers are allowed to reestablish in tree rows treated with nonresidual herbicides earlier in the dormant season, compared with soil maintained completely weed free year-round with residual herbicides (Marsh, Daly, and McCarthy, 1996; Merwin and Stiles, 1998). Considering that soil porosity, nutrient and water retention, and organic matter content are better conserved in the sparse seasonal groundcover that results from nonresidual postemergence herbicide treatments, the use of residual herbicides to maintain the orchard floor continuously weed free arguably constitutes "overkill" in many situations.
Cultivation with rototillers, disks, and harrows or moldboard plowing to mechanically suppress weeds was a common practice for several centuries, and it is still common in orchards with deep, coarse-textured soils. Over the long term, mechanical cultivation often depletes soil organic matter, degrades soil structure, increases erosion and nutrient runoff, and ultimately fails to control weeds that regenerate from rhizomes and other root propagules. Cultivation within crop rows also destroys part of a tree's upper root system, which can be a serious problem for young trees and in shallow soils. Specialized orchard cultivation equipment has been developed to minimize some of these problems, and periodic tillage is still a useful OFM system when used in conjunction with dormant season cover crops in regions with mild winters and soils that are not prone to erosion.
Synthetic fabrics, or "geotextiles," plastic films, and various natural biomass mulches provide nonchemical alternatives for suppressing groundcover vegetation in tree rows. They can serve to protect roots and soil from deep-freezing in the winter and to raise soil temperatures earlier in the spring (in the case of black plastic or fabrics). Mulches are rather expensive to apply and may require supplemental hand weeding to control certain species. They can also increase damage to trees by voles (Microtus species), and most of the synthetic fabric or film mulches are not biodegradable. Biomass mulches increase organic matter and supplement nitrogen, phosphorus, potassium, calcium, magnesium, and other essential nutrients in the soil— traits that make these mulches especially useful on coarse-textured, droughty soils with low water-holding capacity and fertility (Merwin and Stiles, 1998).
So-called "living mulches" of clovers, vetches, and other legumes have been evaluated as sources of nitrogen and weed suppression for orchards. In regions where legume groundcovers can be grown in the dormant season and suppressed or tilled into the soil at the start of the growing season, they are a useful tactic for weed control and soil conservation. In colder regions where living mulches complete growth and development during summer months, they are usually too competitive for water and other limiting nutrients to provide much benefit to fruit trees.
Orchard floor management is an important part of a fruit-growing system. Groundcover vegetation has both detrimental and beneficial roles in orchards. Effective OFM involves recognizing and understanding the positive and negative interactions between fruit trees and groundcovers within a complex agroecosystem. No single OFM program is optimal for all situations, but various effective strategies do provide a satisfactory balance between groundcovers as a tactic for IPM and soil conservation and groundcovers as problematic weeds. Short-term gains in fruit production and management convenience must be evaluated with due regard for long-term priorities such as soil and water quality and the sustainability of fruit-growing systems.
Related Topics: NEMATODES; ORCHARD PLANNING AND SITE PREPARATION; PLANT-PEST RELATIONSHIPS AND THE ORCHARD ECOSYSTEM; SOIL MANAGEMENT AND PLANT FERTILIZATION; SUSTAINABLE ORCHARDING; WATER RELATIONS; WILDLIFE
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