Strawberry Plasticulture
Guide for North Carolina

The strawberry is one of the most important berry crops grown in North Carolina. In 1992, the state's strawberry crop had a form-gate value of approximately $7 million, a close second to blueberries. In the last 10 years, there have been major changes in the way strawberries are produced and marketed in North Carolina.

Pick-your-own (PYO) is still the most popular method for marketing strawberries, but new 'plasticulture' acreage continue to replace matted row plantings in all regions of the state. Strawberry plasticulture, as it is referred to in North Carolina, is the annual hill training system in which 'green' strawberry transplants (freshly dug plants or plugs) are planted in early fall in double rows at densities of approximately 17,400 plants per acre on fumigated, raised beds that are covered with black plastic mulch. The key to strawberry plasticulture is the Chandler strawberry variety developed in California. The variety's early maturity, tolerance of cool temperatures, and long harvest period make it responsible for the growth and success of plasticulture in North Carolina.

In this system, North Carolina growers plan to harvest berries 7 to 8 months after planting & compared to approximately 12 months for matted row (MR) methods. The picking season is normally six weeks, but in cooler-than-average spring temperatures, the harvest can last 2 months. The Chandler cultivar bears about 7 to 10 days earlier than the MR strawberry Earliglow, and the harvest period extends one to two weeks past the late-midseason Cardinal. After the strawberry season ends (early to mid- June), Chandler plants are destroyed and the plastic beds are sometimes reused for summer or fall vegetable crops. One acre of strawberry plasticulture produces roughly the same annual yield of 2.5 acres of matted row production. Commercial strawberry plasticulture is recommended for all regions of the state.

Plasticulture Advantages and Disadvantages

Cash expenses for a plasticulture system are approximately $4,400 per acre (Table 1). This production method requires both an overhead sprinkler system for frost or freeze protection of blossoms and buds; and a drip irrigation system to supply water and fertilizer in the prebloom, bloom, and fruiting periods. Sandy loam and clay loam soils are ideal for forming the 10-inch-high beds with bedding machines. Drawbacks associated with plasticulture are that it requires a higher initial investment than for matted-row production and only one fruiting season is possible with the anthracnose-susceptible California strawberry variety Chandler grown in North Carolina. Plasticulture growing methods and use of the Chandler variety allow strawberry picking to proceed at three to four times the rate possible with matted row culture. This advantage may be the most important reason to consider plasticulture production. In addition to being able to market strawberries earlier than matted row producers, the plasticulture grower generally can plan on five to six weeks of steady picking compared to matted row culture, which produces for only two to three weeks. A week of bad weather is not likely to be as costly to the plasticulture grower. Matted row varieties concentrate so much of their crop during just a few weeks that a single rainy weekend can't really hurt sales. Plasticulture provides steadier picking over a longer season. This feature can be helpful in attracting and keeping good pickers. The trend in recent years has been toward an increase in prepicked strawberry sales at the farm, and a reduced reliance on customer harvesting (pick-your-own marketing). Potential markets may include those available through wholesale channels, but this option is not very profitable compared to direct marketing by the grower to the consumer. PYO prices generally fall between $0.65 and $0.85 per pound; 'ready-pick" fruit is priced $0.25 to $0.40 higher than PYO berries. Wholesale prices have averaged only about $0.50 per pound or $5.50 per flat (12 pints) because of the large volumes of inexpensive strawberries available in April and May from the southern and central coast areas of California. A typical Chandler plasticulture planting of 3 to 4 acres in North Carolina is not very large by comparison to those in California, but just a few acres can still provide a good supplemental income for small and part-time growers.

The Chandler Strawberry

The Chandler cultivar was named in 1982 for the late William Henry Chandler, a prominent author and horticulturist of the University of California. Chandlers one of the most important strawberry cultivars in the world; it is grown on a large percentage of the southern California strawberry acreage and is a major cultivar in Louisiana, Mexico, Central America, Italy, France, and Spain. Its high yield compared to other cultivars tested on black plastic and resistance to rain and frost led to its widespread adoption by plasticulture growers in North Carolina and other states in the Midsouth. Chandler is popular with consumers in North Carolina because of its good fresh and processed quality.

Many people have questioned the wisdom of North Carolina's 100 percent dependency on a plasticulture strawberry cultivar that was bred to be grown in Califfonia--not the humid Southeast. In fact, the North Carolina plasticulture industry was nearly ruined in the spring of 1990 as field after field of Chandler berries became severely infected with anthracnose fruit rot (Colletotrichum acuatum). It became nearly impossible to grow anthracnose-free Chandler nursery plants in North Carolina with the 1987 loss of the emergency Section 18-C registration for Difolatan (captafol), the fungicide needed to control anthracnose.

No effective fungicides are available for anthracnose control, and growing disease-free Chandler crops basically depends on raising the plants in very northern areas in the United States and Canada where the summers are cool. These plants are then transplanted in North Carolina during the fall. After the spring strawberry season ends, Chandler plants are destroyed because of this cultivar's extreme susceptibility to anthracnose. Furthermore, growers are encouraged to rotate planting fields and minimize overhead watering during plant establishment. Thus, growing the anthracnose-susceptible Chandler cultivar in North Carolina depends on sanitation and manipulation of the plant environment to avoid the disease or limit losses.

Sweet Charlie is a new variety from the University of Florida that show considerable promise in coastal plain North Carolina as an earlier season plasticulture strawberry (7 to 10 days earlier than Chandler). Many consumers actually show a preference for Sweet Charlie berries that have a high sugar to acid ratio.

Equipment Needs

Specially equipped and trained custom applicators should be considered for fumigation. They can form beds, inject fumigant, apply plastic, and lay trickle tubes in one operation. Where custom services are unavailable for shaping beds, fumigating the soil, and laying plastic, the grower must purchase equipment costing approximately $5,500 and become certified to apply methyl bromide.

Strawberry plasticulture also requires overhead irrigation for plant establishment and for frost or freeze protection by sprinkling. The expense for solid-set irrigation can run form $1,200 to $2,500 per acre, depending on whether cheaper PVC or more expensive aluminum pipe is used. This cost does not include the expense for the diesel or electric pump or for a new pond. A pumping capacity of at least 60 to 70 gallons per minute is necessary for each acre that is to be protected from frost.

More than 75 percent of the approximately 700-acre strawberry plasticulture industry in North Carolina uses drip irrigation. The higher 10-inch beds require drip irrigation because capillary movement of water is poor. The proper use of plastic mulches and drip irrigation can provide maximum control of the environment and the most efficient use of water and fertilizer. Drip irrigation systems can be designed by a specialist to irrigate strawberry production units of any size. A line source emitter with prepunched holes is used to maintain a continuous moist strip along the row. Because strawberries grown on plastic mulch are considered annuals and are grown for only one season, thin, disposable tubing (4 to 8 mils thick) is commonly used.

Water may come from wells, ponds, lakes, municipal lines, or pits. Well water is generally fairly clean and requires only a screen filter to remove particles. However, the presence of precipitates or other contaminants in the water should be determined by a water quality test before considering a drip system. Any surface water source, such as a stream, pond, pit, or river, will contain bacteria, algae, or other aquatic life, and sand filters are therefore a necessity.

Because the Chandler berry is the most important marketing tool for attracting customers to the farm, the remainder of this publication discusses important preplant, planting and postplant considerations for growing this cultivar in North Carolina. Horticultural Information leaflet 205-G, Matted Row Strawberry Production, gives further detail on the matted-row system.

Preplant Planning
Site Selection

Strawberry plasticulture has been examined under a variety of site and soil conditions in North Carolina since 1982. This production method is best suited to the milder coastal plain and lower piedmont, but growers in the upper piedmont and mountains of North Carolina are experiencing success with the system as well. Some of the best strawberry plasticulture sites are those with a wooded area or a windbreak on the north or northwest side of the field. Windbreaks will help reduce losses associated with wind-borne freezes in late winter and early spring. However, too much wind protection and restricted airflow locations often increase the likelihood of fungal diseases. Air movement must be sufficient to dry the plants quickly after dews and rainfall. A north-to-south orientation is recommended for rage more uniform plant strawberry beds to encourage development and ripening on both sides of the double-row bed. If rows are oriented east-to-west, plants on the north side of the bed will be partly shaded by the plants on the south side during the winter.

Soil

Soil type has a decided influence on how well the planting beds are formed. Sandy loam and clay loam soils are ideal for building and shaping the 8- to 10-inch raised beds. However, soils with a high clay content or those that are rocky or very stony are more difficult to prepare. As a general rule, the physical conditions of the soil as they relate to its bed-making characteristics and internal drainage are more important than soil fertility. High clay content, stones or rocks, and underlying hard- pan are more difficult conditions to manage than low pH or a lack of minerals, which can be balanced through the application of lime and appropriate fertilizers.

It is best to rotate strawberry field sites as often as possible, but the general practice in North Carolina is to crop strawberries continuously on the same land because of existing irrigation lines and market location. Eliminating the use of methyl bromide as a soil preplant fumigant will likely change this practice, and growers may be forced to evaluate the benefits of field rotation, especially for controlling soil-borne strawberry diseases, nematodes, and insect pests that can build up in continuously cropped (untreated) fields.

Water Drainage

The raised beds enhance internal soil water drainage, but plasticulture growers frequently encounter problems with getting rid of excess surface water. This problem occurs because 50 percent of the strawberry field is covered with an impermeable plastic film. Therefore, fields should have enough slope so that surface water flows away uniformly and gently after heavy precipitation without causing erosion or leaving puddles. On fields with more than 2 percent slope, continuous overhead sprinkling during the period when freshly dug plants are being established may cause severe soil erosion. Unlike highly perishable fresh-dug transplants, plug plants do not require intense overhead sprinkling for postplant care and survival. (See the section on planting.)

Soil Preplant Fertility

The soil should be tested several months before planting to provide an indication of how much dolomitic lime is needed to raise the soil pH to 6.0. Approximately 33 percent of the nitrogen (N), 50 percent of the potassium (K), and all of the recommended phosphorus (P) (based on soil testing) is applied before planting. The remaining nitrogen and potassium are applied through the drip irrigation system.

For sandy soils that do not retain potassium or nitrogen well, the usual recommendation for a preplant fertilizer program is to broadcast 120 pounds of potassium per acre in addition to the 60 pounds of nitrogen per acre. The recommended fertilizer source for potassium is potassium sulfate. Usually, ammonium nitrate is the preplant nitrogen fertilizer, and it is either broadcast or banded 4 inches deep in the bed center.

Essentially, these preplant fertilizer applications of nitrogen and potassium supply early nutrition to Chandler strawberries in the fall and early winter when North Carolina growers typically do not run their drip irrigation systems. Soil tests for boron are generally not very accurate, and as a rule growers apply small amounts of boron through the drip irrigation system. Tissue samples are taken in late winter and early spring to determine the need (if any) for supplemental boron injections).

Fumigation

Methyl bromide or chloropicrin fumigation has been a standard practice in California and Florida where annual hill plastic mulch culture has been practiced since the 1960s. Beds are fumigated with methyl bromide for weed, nematode, disease, and insect control. Annual hill production on the same site year after year is not advisable without preplant fumigation. The amount of material actually applied per acre depends on row width and will be a percentage of the broadcast rate. The air temperature should be at least 50 degrees F, and the soil should be well worked, be free from plant debris, and have adequate moisture for weed seed germination. In order to have good soil moisture for forming the beds, it is frequently necessary to use the sprinkler irrigation system to apply approximately 16 inch one to two days before forming and furnigating the beds.

If both weather and soil are warm, most fumigants escape from the bed in about seven days. An additional precaution to reduce the likelihood of rnethyl bromide injury to the strawberry transplant is to cut or punch plant holes through the plastic mulch a day or two before planting.

Strawberry Beds

Most growers in southeastern North Carolina use 10-inch high 'super beds' to promote the development of larger root systems and more branch crowns. The super beds provide a nearly ideal air, soil, and water environment for vigorous strawberry root development. The beneficial effect of these beds on root growth is easily observed by excavating the soil beneath the plastic mulch film at the end of the season (Figure 2).

A few roots may be found as deep as 24 inches, but the largest concentration is in the upper 10 to 12 inches of soil (Figures 3, 4, and 5). Yields of Chandler strawberries on fertile sandy loam soils with 10-inch-high beds have exceeded 36,000 pounds per acre per year for three years in a row on several sites in southeastern North Carolina. The 10-inch-high plastic mulch beds are typically 32 inches wide at the base and 30 inches wide on the top. The beds are slightly crowned so water will run off and not rest on the plastic (A bed with a 30-inch top should slope from the center to the edge with a drop of 1.25 inches.) With lower bed height, the long fruit trusses of Chandler plants come in direct contact with the soil in the row aisles. Thus it becomes necessary to apply straw mulch to the aisles to keep the berries clean, a practice that is not necessary with 10-inch-high beds. Beds are on 5-foot centers.

Drip Tubing Installation

Install drip tubing with the orifices facing upwards. The tubing is typically buried 1 to 2 inches deep in the bed center. During installation, several workers should be watching to ensure that the tubing maintains its orifice- upward orientation, to assist in case tubing becomes tangled in the injector, and to signal when the drip-tape reel is empty. Tubing ends should be closed off by kinking or knotting them until the tubes are connected to the system. Generally, the drip system is not used until late winter or early spring, and fall irrigation for plant establishment is applied by overhead sprinklers.

Cover Crops on Sloping Terrain

As soon as the land is fumigated and mulched, it is advisable to broadcast approximately 50 pounds of annual rye grass per acre over the entire field. The annual ryegrass can be spread the same day you finish fumigating. The ryegrass will reduce soil washing in the aisles after heavy rains or irrigation on sloping terrain. It is vital to dress the land immediately after fumigating so that surface water can flow easily out of the rows. It is much more difficult to deal with surface drainage problems once the ryegrass is established. Obviously, you will not apply any preemergence herbicide to the aisles if you decide to grow a 'living mulch" for the fall and winter season. In fact, very few herbicides are labelled for strawberry plasticulture. Check with your county Cooperative Extension Service agent if you have questions about available herbicides, application rate, and timing.

Planting Considerations
Plant Quality

The success of a strawberry plasticulture planting depends in large part on the health and vigor of the planting stock. Purchase plants from a reputable nursery. Chandler plants should be true to variety and not a tissue culture mutation of the cultivar. The plants should be free of insects, diseases, nematodes, and viruses. The North Carolina Strawberry Association, Inc. (Box 7609, Department of Horticultural Science, North Carolina 5tate University, Raleigh, NC 27695-7609) maintains a current listing of Chandler nursery plant sources in the United States and Canada for interested growers.

Plant Type

Plug plants are generally more expensive to purchase than fresh-dug strawberry plants, but they do have the advantage of being suitable for mechanical transplanting with a water-wheel or pot-mulch planter, whereas freshly dug plants are most often transplanted by hand. Successful establishment of the highly perishable, freshly dug transplants depends on intense overhead sprinkling for one to two weeks, depending on weather.

North Carolina commercial grower experiences with plug plants indicate that these do not require intensive overhead sprinkling for postplanting care. Plugs are definitely recommended over freshly dug plants for part- time growers who do not have the time to oversee the continuous overhead watering of freshly dug plants during the first week after field transplanting. Also, less- experienced growers are encouraged to consider plant- ing plugs because they are more 'mistake-proof' than the highly perishable freshly dug plants. Transplanting dates for plugs can also be slightly later than for freshly dug plants without as great a yield reduction. The reason is that plugs establish more quickly than freshly dug plants after transplanting. Plug plants are comparable to freshly dug plants in both marketable yield and fruit size.

Planting Period

Until recently, plasticulture growers could not obtain freshly dug (green) plants until sometime near the end of September or early October from nurseries in the north- em United States and Canada. That is the best time of year for transplanting in the lower piedmont and south- eastern areas of North Carolina, but earlier transplanting dates are needed to achieve full crops in colder areas, including the upper piedmont and mountains of North Carolina (USDA zones 7a and 6b, respectively). Growers in western North Carolina are now able to achieve target planting dates in mid-September by using fresh 'container-grown" strawberry plug plants.

Plant Spacing

You should set two plant rows per bed ('a double-row hill'), and plants between rows are generally offset for improved light and air circulation. Plants are usually spread 12 to 14 inches between the plant rows, and 12 to 16 inches between plants in the row. With 5 feet between bed centers, 17,400 plants will be required per acre at the 12-inch-by-12-inch spacing; 14,900 plants per acre at the 14-inch-by-12-inch spacing; and 13,000 plants per acre at the 16-inch-by-12-inch spacing.

In North Carolina, the influence of Chandler in-row plant spacing on marketable yield and fruit quality of Chandler strawberries has not been adequately investi- gated. The 12-inch-by-12-inch plant spacing is usually recommended but a number of growers have recently adopted a wider in-row spacing because of problems with excess plant vigor. The 14- and 16-inch in-row spacing allows easier picking, improved spray penetra- tion, and fewer possible problems with botyyfis fruit rot than with closer plant spacing. Strawberry flavor and fruit color may also improve with the 14- and 16-inch spacings. Many growers are inadvertently creating a plant spacing problem by planting too early, applying excessive amounts of nitrogen preplant fertilizer, and overwatering with drip irrigation systems.

Freshly Dug Plants

Freshly dug plants may be stored in a cold room at 40F for one to two days before setting. Storing them for a longer period can make the plants more difficult to establish. Plants in a nursery box or crate are packed tightly enough to make them prone to what is called a 'heat' during shipment, making them unfit for subsequent planting in the field. It is very important to cool the plants before transit. During hot weather it may be necessary to run water through the crates to keep plants cool.

The freshly dug strawberry plant is manually transplanted,through the plastic mulch in 2 1/2-inch slits cut by specially constructed spacing wheels that also open a narrow hole for planting (Figures 6, 7, and 8). This equipment can substantially reduce the number of hours required to set I acre of freshly dug strawberries (approximately 40 hours per acre). Some root pruning may be needed to shorten roots to 5 to 6 inches before transplanting.

Set the plants so that the midpoint of the crown is level with the soil surface. If plants are set too deep, they become unthrifty and the crowns may rot and the plants die. If planted too shallow, the root system is exposed, which can result in poor rooting and shifting of the plants. Often plants may be set at the right depth but either be in a small depression or have soil ridged around the crown. When irrigation is initiated to establish plants, the depression can fill and bury the crown or the ridge may erode and expose the roots. A firm plant bed assists in preventing the bed from settling or eroding.

Initiate overhead irrigation as soon as plants are set - no more than 1 hour should elapse. The plants will require irrigation for five to seven days after transplanting. Each morning, start irrigation when the plants show moderate wilting and continue to irrigate until the hot part of the day has passed. After a few days, irrigation can be initiated a little later in the morning and can be discontinued earlier in the afternoon. The primary purpose of irrigation is to prevent foliage loss until the root system can develop and absorb sufficient moisture to sustain the plant. Plants should have three or morefully green leaves remaining at the end of the establishment period (Figure 9). This mist cooling requires only a relatively small volume of water. Successful results have been obtained using 0.10 inch per hour.

Plugs

Extended storage of the plantlets or tips is generally not needed. Northern commercial nurseries can harvest fresh tips weekly from early July through mid-October. The tips are shipped by refrigerated truck to the grower's farm for delivery approximately 35 days before field transplanting. Tips can be stored at 34F for up to two weeks without deterioration in quality. The boxes containing approximately 1,000 plantlets must be stacked 'loose' so that cool air can circulate freely around the boxes. The strawberry tips are a living, respiring plant and must be kept cool until you are ready to root them under mist. The cooler should be kept at 75 to 80 percent relative humidity. Before rooting, additional plantlet preparation is needed to trim away excess runner-cords. An approximate 3/8 to 1/2-inch runner 'stub' serves to anchor the plantlet until new roots develop. Fresh strawberry tips are best rooted under a fine mist that will wet the foliage yet put very little excess water on the soil. The goal is to keep moisture on the leaves until the plant is well rooted, about 7 days. As the roots form, the plants can be weaned from the mist and allowed to get their moisture from the soil. Misting can be gradually reduced over a two- to five-day period. One week after transplanting it should be possible to pull most plants from the cell and have the root ball remain intact (Figure 10). When that occurs, misting can be stopped.

Strawberry plants raised in containers are grown in specially prepared growing media. Many different. media are available, but a soilless medium based on peat, sand, grit, vermiculite, perhte, polystyrene, or other materials is recommended. You will need about 4 cubic feet of media for approximately 1,000 tips in 60-ceU rigid plastic trays 2 3/8 by 12 by 20 inches. The 60-cell tray is suggested for small- and medium-size strawberry tips.

If plantlet lengths of the tips you receive from your supplier are quite variable, it is best to grade the tips into large- medium- and small-sized lots. The large tips should be in 50-cell trays, the medium tips in 60-cell trays, and the smaller tips in 72-cell trays. Sticking large tips (longer than 5 inches) in the same tray with small tips (2 to 3 inches) will result in light competition and irregular root growth of the smaller, shaded tip plants. During misting, shaded tips are susceptible to botrytis infection.

After the misting cycle is complete, trays are moved outside to a fully exposed gravel pad for another two to three weeks of growth and acclimation before they are transplanted to the field. During this final field-conditioning phase, a single daily watering is suggested along with a weekly supplemental drench of a complete fertilizer. A slightly root-bound plug is desirable for mechanical transplanting; plugs for hand transplanting can be set before this stage is reached. It is not advisable to plant plugs that have been left in the tray for more than 5 to 6 weeks. In transplanting plugs, the plant crown (bud) should be just above the soil line, not buried (Figure 11 a, b, c).

Plug plants pose less serious problems than freshly dug plants for field transplanting. Pot-mulch planters or vegetable water wheels can be used to transplant and water strawberry plugs mechanically. Careful grading of tip plants by size will produce more uniform plugs for efficient machine transplanting. A few hours of overhead sprinkler irrigation after transplanting is also recomended.

Postplanting Care
Postplanting Fertility

In southeastern North Carolina, drip irrigation begins at first bloom (late February) and continues through harvest, ending in late May or early June. In conjunction with a plant analysis program, drip fertigation can improve nitrogen and potassium efficiency and possibly enhance berry firmness and quality. Over about a 100-day period from late February through late May or early June, nitrogen and potassium maintenance drip applications are made daily or weekly at the equivalent of 1.0 pound per acre of nitrogen per day, along with I pound per acre of potassium oxide per day. Injecting a higher proportion of potassium than of nitrogen in this mainte- nance program has not been observed to improve yield or fruit quality. For shipping berries to market, a lower nitrogen application rate of 1/2 pound per acre per day, or 3-1/2 pounds per acre per week, should be considered.

Plant tissue should be analyzed biweekly in the late winter and early spring to determine more precisely the nutrients that may limit optimum plant growth and fruit production. Most recent, fully expanded leaves should be analyzed to identify nutritional problems. The following sufficiency ranges are used for Chandler strawberries in North Carolina (from the most recent fully expanded leaves): nitrogen, 3.0 to 4.0 percent; phosphorus, 0.2 to 0.4 percent; potassium, 1.1 to 2.5 percent; calcium, 0.5 to 1.5 percent; magnesium, 0.25 to 0.45 percent; iron, 50 to 150 ppm; manganese, 30 to 100 ppm; zinc, 15 to 50 ppm; copper, 4 to 15 ppm; and boron, 25 to 50 ppm. Petioles should be detached and analyzed for nitrate nitrogen concentration. Experience indicates that highest yields are obtained when petiole nitrate nitrogen ranges from 5,500 to 6,000 ppm early in the season and from 800 to 1,200 ppm at the end of the season.

Water Management and Operation

As the strawberry plant grows, larger quantities of water are needed because its root system becomes more extensive and more foliage is present. Relatively little water is needed in the fall, other than for plant establishment in the first two weeks after planting. In the late winter, begin a drip fertigation program in late February or early March; water will be furnished to the plants during this process. However, more water will be needed as foliage and blossoms begin to develop rapidly in March and early April. Peak water use occurs in May during harvest. Obviously, more water is needed under hot, dry conditions to replace evaporative loss.

Meastiring Water Loss

The rate of water loss by evaporation can be determined by measuring the water loss from an open pan over a given period in agricultural weather reports. This rate of loss is usually called pan evaporation. A far easier method of determining the daily water need for crops watered by a drip irrigation system is to use tensiometers. A tensi- ometer consists of a porous, porcelain-tipped tube containing water with a vacuum gauge at the top. The porous tip of the tensiometer is buried in the soil at root- zone depth. As water moves out of the tube through the porous tip, a vacuum is created. The vacuum gauge reading (in centibars of soil siiction) indicates the mois- ture status of the soil; the greater the vacuum, the drier the soil. Tensiometers work well for the sandy soils commonly found in many vegetable- and strawberry- growing areas. A value of zero means that the soil is completely saturated with water. A reading of 10 repre- sents a normal field capacity soil water status. The watering system should be turned on when a reading of 20 to 30 appears on the gauge.

Tensiorneters are usually installed in pairs, called a station,' with one at a 6-inch depth and one at a 12-inch depth. Turn on the watering system when the 12-inch- deep tensiometer reads 20 to 30, and turn off the system when the reading on the shallow (6-inch) tensiometer drops to 10 (field capacity). Tensiometers can be pur- chased with solenoid switches to automate the operation of the irrigation system.

Row Covers

Row covers should not be regarded as frost protection systems. For total frost protection, bonafide sprinkler irrigation systems are the answer. Row covers do pro- vide temperature modification, and crops may be slightly early if covers are applied in winter. But the practical value of having a winter row cover depends on obtaining a very substantial price advantage for the early crop.

Several trials have been conducted to investigate the degree of protection, if any, offered by overwintering row covers in southeastern North Carolina. In general, the covers were not found to provide any economic benefit for winter freeze protection. Overwintering covers (strips or floating) may be justified in western North Carolina and more northern climates.Floating covers provide economic benefits in late winter and early spring during advective or windborne freezes (Figure 12). During such freezes high winds prevent the use of sprinkler irrigation to protect the crop. The floating covers can be deployed for short durations. The covers can be returned to storage once field condi- tions have moderated. Removing and storing the covers will help to extend their life; two or three years is typical.

Spring Frost or Freeze Control

Overhead sprinkler irrigation is the best and most reliable means for protecting strawberry blossoms from cold injury in March and April. In North Carolina, it is not uncommon to pump water 10 or more nights each spring for as many as 8 to 10 hours each night. If, for example, the pump is used on four consecutive nights for an average of 10 hours each night at the rate of 1/8 inch per hour, approximately 136,000 gallons of water will be needed to protect just 1 acre.

The irrigation water provides heat to the plant as the temperature of the water drops to 320F, and especially as it freezes. As long as the temperature of the flower or fruit stays above 30"F, no damage will occur. The lower the air temperature, the more water is needed to main- tain the temperature of the flowers and fruit above damaging levels. Open strawberry flowers are more prone to freeze damage than green fruit which, in turn, are more sensitive than ripe fruit. If wind speed is 10 to 15 miles per hour or greater, water application with this technique is erratic, and plants as well as flowers and fruit can be severely damaged. With little or no wind, about 0.15 inch of irrigation per hour is required for frost protection down to a temperature of 22"F, and about 0.25 inch per hour from 22'F to 18'F.

Turn sprinklers on at 34 F and do not turn them off until the ice begins to melt and continues to melt when no additional irrigation is applied. Thermometers should be calibrated in an ice bath and placed in the lowest spots in the field, fully exposed to the sky, and just above the mulch. (Be sure they are not protected by nearby plants.) Digital, hand-held thermometers with thermocouple sensors that insert into the strawberry blossom are also recommended.

Plant Size Management

Several practices can be used to help control Chandler plant size to create a more open canopy for easier picking and higher quality fruit (Figure 13). Possibly the most critical factor influencing plant size is planting date. In each new region where Chandler plasticulture is at- tempted, it is important to establish the optimum plant- ing date with field trials. Time-of-planting trials at the Central Crops Research Station, Clayton, North Carolina, (lat. 35degrees40degreesN, long. 78degrees30degreesW) have consistently demon- strated that the optimum planting period is during the first week in October (Figure 14).

Earlier planting dates can result in larger plants with as many as 8 to 10 branch crowns. Thick canopies hinder picking and result in greater disease incidence as a result of reduced air circulation; air movement is also needed to enhance pollination and fruit set. Controlling Chandler plant size also entails decisions regarding plant spacing, irrigation, and fertilizer practices (Figure 15). In North Carolina, the influence of Chandler in-row plant spacing on marketable yield and fruit quality has not been investigated adequately.

A research team is currently studying the relationhips between plant spacing and fertilization practices on fruit quality, size, and shelf life of Chandler strawberries grown by means of plasticulture. As a convenient means of assessing Chandler plant size, simply remove the above-ground portion of the plant at the end of the harvest and count the number of branch crowns.

Optimum plant size characteristics and fruit size and quality are attained when the plant produces five to six branch crowns (Figure 16).

For more infon-nation on postharvest handling for strawberries, see AG-413-2, Postharvest Cooling and Handling of Strawberries, and AG-414-3, Maintaining the Quality of North Carolina Fresh Produce: Forced Air Cooling. Both are available from you county Cooperative Exten- sion Center.

Weed Management

One key to a successful crop is weed control. In plasticulture strawberries, this involves following a program of preplanned control measures, weed-inhibit-ing production practices, and careful attention to condi-tions within the field. You should select one of the three programs in Table 2 to help keep weeds under control while at the same time ensuring good crop production.

Table 2. Three Weed Management Programs

Conclusion

Strawberry plasticulture has replaced bare-ground matted-row culture in much of North Carolina. The major advantages of this system are

• uniform plant stands every year, unaffected by summer disease, drought,
or weed competition
• earlier picking, about 2 weeks
• larger berry size
• easier picking
• shorter turnaround time from planting to harvest (7 to 8 months).

Against these advantages must be weighed the higher initial plant costs, higher costs for hand transplanting, and the fact that this system does not lend itself to renovation for a second year of fruiting.

Also, the total yield with a plastic mulch system may be no better than the yields from a matted row system, yet establishment costs can be higher. There is considerably less margin for error with this production system. Even seasoned strawberry growers should consider only a 1/2 to 1-acre initial planting to 'learn the ropes' about strawberry plasticulture before making a large financial commitment.