Orchard Floor Management Systems

Orchard floor management objectives and practices differ between rows and alleys. In rows, weed and soil nutrient management are the primary focus. In row weed management tools include herbicides, mulches, and tillage (Table 1). Consider integrating how these tools are used to establish a vegetation free zone of 2.5 to 3 feet on each side of a row of trees, giving a total strip width of 5 to 6 feet. This strip provides an area where roots can grow without competition from weeds or grass sod. In orchards with a vegetation free zone, most of the root growth occurs in the vegetation free area, especially for young trees. Vegetation cover, usually grasses, around mature trees (after establishment) has been used by many growers in the past. It can be effective on steep slopes where erosion is a severe problem and orchards planted are on vigorous rootstocks (MM.111, MM.106, or seedling). Planting cover crops (i.e. living mulches) in rows has not been effective in the Western USA (Granatstein and Sanchez 2009). 

In the alleys, the objective is low maintenance, access to trees. This is typically achieved by establishing perennial, drought tolerant grasses. The grass alley provides a solid path for equipment travel, helps prevent soil erosion, helps maintain soil structure, and aids water infiltration. Depending on the cover crop sown, weed invasion can be minimized and sod establishment can be fairly quick. The alleys may be kept as bare soil with shallow tillage or herbicides. This can be a muddy mess and lead to soil erosion and loss of soil organic matter over time, but can be a useful during the first few years of the orchard when there are established perennial weeds that need to be managed or lots of weed seeds in the soil that make establishing alley vegetation difficult. Bare soil in alleys does not provide good traffic support when soils are wet and may lead to additional soil compaction from heavy equipment. During the driest part of the summer, bare ground in the orchard results in increased dust, which will exacerbate mite problems in the orchard canopy.

Herbicides

Herbicides are the most commonly used weed management tool in orchards since, when properly used, herbicides can be effective, cheap, and require little labor (typically 2-3 applications per year). Herbicides are typically not viable weed management tools for organic orchards since organic herbicides have limited efficacy and are prohibitively expensive, in particular when perennial weeds are present. See herbicide spray tables for herbicide options.

Cultivation

Shallow cultivation can be an effective way to manage weeds and is the most common weed management practice in organic orchards. There are several cultivators designed for in-row weed management in orchards and vineyards (e.g. Weed badger ™, Wonder Weeder ™, Ladurner Cultivator ™, Rinieri EL in-row rotatory harrow). Cultivation may improve water infiltration of some soils, but frequent shallow cultivation damages feeder roots near the soil surface and often results in reduced tree growth and yields (Granatstein and Sanchez 2009). Irrigation systems should be designed to avoid damage if growers plan to use in-row cultivation. In alleys, traditional field cultivators (e.g. disc or sweeps) can be used. Cultivation is most effective on annual weed seedlings and must be repeated every few weeks during the growing season. Frequent cultivation breaks down soil structure, making the soil more prone to erosion and compaction. 

Mulches

Mulches conserve soil moisture, reduce soil temperature and initially inhibit annual weed growth around trees, but mulches (except woodchip or bark) can provide a sheltered habitat for voles (Granatstein and Mullinix 2008). Effective plant or compost-based mulches include straw, sawdust or shavings, hay, leaves, chipped prunings, or shredded newsprint.  The effects of mulches on soil properties and weed control will depend on the type and depth of mulch applied. Mulches are expensive to obtain and apply on an annual or biennial basis. Some growers use clippings of alley cover crop as mulch in the row (mow and blow system) using a side delivery mower. Organic mulches do not control and can encourage perennial weeds like quackgrass, Canada thistle, and field bindweed (Hammermeister 2016). Mulches must be applied thick enough (>3-4 inches) to prevent annual weeds from growing through. Organic mulches may bring in additional weed seeds and/or new weed species. As they decompose, some organic mulch materials high in carbon tie up available soil nitrogen (Billeaud and Zajicek 1989), but have been shown to increase soil qualities and tree growth over the long term (Atucha et al. 2011).

Weed barriers, synthetic plastic film and spun bonded polyester fabric mulches, are an additional management option. These products are typically applied prior to planting and the trees are planted into the weed barrier. They have a limited lifespan that varies from a single season (thin film plastic mulch) to several seasons (spun fabric). They are allowed in organic production but must be removed completely before they break down. Synthetic mulches create a barrier to soil applied fertilizer.  Plant nutrition management may have to rely more on fertigation and foliar application. Alternatively, some growers will apply two strips of fabric mulch on either side of the row and remove in the winter to discourage rodents and amend soils (WSU-organic weed management).

Alternative Weed Control Practices

Other less commonly used orchard weed management practices include mowing or heat. Mowing is used most often when a cover is grown in the row or as a rescue treatment for weeds that have escaped control and can help reduce weed seed production. Heat, typically provided with a propane torch, can be effective but only for small, annual broadleaf weeds. Grasses and perennial weeds will be damaged but will quickly grow back. Flame weeders can damage trees and irrigation systems. Other forms of using heat (e.g. steam, electrical shock) are being researched but are not widely used at this time.

Table 1. Weed management options in orchards

Tool	Advantages	Disadvantages
Herbicides	Effective Easy to apply Can act selectively Typically the least expensive: Less time required and lower cost per acre.	Requires training/education to avoid risks of non-target effects: worker safety, crop injury, environmental contamination Organic herbicides are expensive-not economical
Cultivation	Effective on small annual weeds Can be used in organic systems	May damage soil structure, shallow tree roots, and irrigation system May spread perennial weeds Provides short term control- must be repeated every few weeks
Mulches: Synthetic plastic or woven weed barriers	Effective and low maintenance Can be used in organic systems	Must be applied prior to planting Interfere with soil application of fertilizer Limited lifespan Can provide habitat for rodents
Mulches: Plant/Compost based	Effective on annual weeds if applied to sufficient depth (>3 inches) Increase soil quality and nutrition Can be used in organic systems	Costly Can tie up Nitrogen Often need to be reapplied every few years Can provide habitat for rodents
Mowing	Reduced seed spread Useful as quick/rescue treatment or in established orchards (>4-6 yrs. old) on larger rootstocks	Often used after weed competition has occurred Repeated applications required
Heat (Flaming, steam, electric shock)	Effective on small annual weeds Can be used in organic systems	May damage trees and irrigation system.