Careful preparation of your fields for preplant treatments and planting can make pest management easier. Important considerations include soil type, crop residue, bed design, proper drainage, and whether you plan to fumigate the soil before or after beds are formed.
- Do not work the field when the soil is wet or when it is very dry because the soil structure will be damaged. Prepare beds with proper slope (at least 0.75%) so that water does not stand in the field and drains off during rainy weather without causing soil erosion. Shape beds properly so that water drains away from the tops of the beds. On hillsides, form beds on a contour to minimize soil erosion.
- Prepare fields far enough in advance of fumigation that residue from the previous crop will be decomposed. Pathogens present in crop residue may not be killed by fumigation. Work reasonably dry soil until it is free of clods; fumigants do not penetrate clods, and working wet soil causes compaction that interferes with successful fumigation. It may be necessary to sprinkle-irrigate dry soil before fumigating because fumigation is less effective in dry soils. These guidelines also apply if you are planning to use soil solarization or drip fumigation.
- If sulfur or gypsum is needed, apply it several months before planting and thoroughly mix it with the soil. Rain or irrigations will then leach excess salts from the root zone. If gypsum is being used to improve water infiltration, apply it to the soil surface without mixing it into the soil.
Good drainage is essential to keep salts from building up in the root zone and reduce root disease problems. Always rip the subsoil several times to provide adequate drainage. Perched water tables, compacted soil layers, and stratified or layered soils must be corrected during field preparation. Deep subsoiling or chiseling to a depth of 30 inches may remove these obstructions and should be repeated every year.
In some cases, organic amendments can improve drainage. Be careful to choose amendments that do not contribute to salt problems. If using manures or composts, mix them into the soil far enough in advance that rains or irrigations will ensure that excess salts are rinsed from the root zone before planting.
High beds improve drainage, so increasing bed height may help alleviate drainage problems. Be sure to provide for adequate drainage from the field during rainy weather; standing water favors the development of root and crown diseases. Drip-irrigated fields should have a uniform slope of 0.75 to 1%. Ideally, water from rains should not be allowed to stand in strawberry fields for more than about 6 hours.
Planting beds may be formed before or after soil fumigation. If you are planning to use solarization for weed control, you must form the beds first. Two-row or four-row beds may be used. Four-row beds are most common in the Santa Maria Valley and Southern California areas, while two-row beds are most common in the Watsonville-Salinas area. Salinity management, pest control, and harvest may be more difficult with four row beds.
When setting up planting beds in hilly areas, plan to leave natural draws free of plants—for example, by making roadways between planting blocks. This will allow cold air drainage and help reduce the risk of low temperature injury.
Covering the planting bed with polyethylene mulch helps regulate soil temperature, which in turn helps regulate plant growth and fruit production. Mulching also conserves soil moisture and reduces salinity build up on the soil surface and is very important in reducing decay problems by limiting fruit contact with soil and irrigation water.
Preplant weed control is critical, unless opaque mulch is used, because clear and translucent mulches do not control weed growth. The type of mulch used and the timing of application depend on cultivar, planting and harvest seasons, and other management practices.
Polyethylene mulch may be applied by machine before planting or by hand or machine after plants are in place. When applied after planting, the plastic is unrolled lengthwise over the bed and secured to the bed shoulders with metal pins placed every 6 to 8 feet or with a layer of soil turned over the edge of the mulch. Either rotating drums with tines that slice the plastic are used to create a hole for the plant or a special burner is used to heat a metal cylinder that punches a hole in the plastic over each plant and the plants are pulled through the holes. An advantage of the tines is that they create a smaller hole and there is less weed growth, but the larger holes created by the burners allow more water to get to the plant if overhead irrigation is used. If bed fumigation is used, mulch is applied before planting. Plastic bags filled with soil are placed as needed to keep wind from damaging the mulch.
Clear mulch allows sunlight to heat the soil during short winter days, stimulating crown development and thereby enhancing early and total yield.
Santa Maria Valley and Southern California
Apply clear polyethylene to fall-planted strawberries before or as soon as possible after planting. Clear mulch with opaque sides that covers the bed shoulder is also used in Southern California. It provides the advantage of warming soil on the bed top while controlling weeds on the sides of beds, where drip-applied fumigant concentrations are often insufficient for optimal weed control.
Short-day cultivars may be planted through clear mulch or clear mulch applied soon after planting to encourage winter growth. For day-neutral cultivars, clear mulch is usually applied in mid- or late December.
Central Valley areas
Clear polyethylene may also be used when summer plantings are planted later than recommended; it is important to apply the clear mulch in early November to stimulate crown growth during late fall and winter. Applying clear mulch in late December or January when day length is increasing stimulates runner growth and shortens the fruit production season.
White or white-on-black mulch cools the soil significantly, slowing early growth, and favoring production of larger fruit and prolonging the fruit production season of some cultivars. In the Central Valley, apply white polyethylene to summer plantings immediately after pruning. White mulch is least likely to burn fruit during hot weather. Most white mulch is translucent and does not inhibit weed growth. White mulch with black backing does control weed growth. Reflection from white and silver-colored mulches helps repel some insect pests such as greenhouse whitefly.
Opaque polyethylene, such as those colored black, brown, or green, provides considerable soil warming (but less than clear) and controls weed growth on the planting bed. However, shoots of yellow nutsedge can puncture holes and grow through opaque mulch. Black mulch can lead to problems with fruit burn when temperatures are high (above 90°F).