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Pioneering the Future

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Crop Guide: Growing Strawberry

 

2.1 Optimal conditions for field-grown strawberries

2.1.1 Microclimate
A favorable microclimate for strawberries is an area with full sun at least six hours per day, uniform temperature, rainfall and drainage, and good protection from wind.
If the site lacks a good microclimate, the grower must improve it, or choose another. A plot with a slightly sloped land facing south (northern hemisphere) provides good light exposure and air- and water-drainage.
 
 
2.1.2 Permanent shelterbelts
If the site lacks wind protection, shelterbelts should be planted to reduce wind velocity. Winds will dry fruit and plants during the growing season, desiccate plants during the dormant winter period, remove the winter snow cover in northern countries, and cause soil drifting during the plant establishment year. A properly planted and maintained shelterbelt can assist in obtaining a uniform blanket of snow to insulate plants from winter temperatures.
 
A number of ornamental plants are suitable for shelterbelts, and each has its advantages and disadvantages. Two species that have performed well in northern countries are Green Ash and Swedish Aspen. Ideally, the hedge should grow quickly, take up minimal amounts of land and produce a minimal amount of shade. Competition for fertilizer and moisture is also an important consideration. When selecting shelterbelt plants, avoid these characteristics:
  • over-density of branches inhibiting snow infiltration
  • wide spreading top
  • high labor requirement for pruning
  • limbs and branches susceptible to breakage from snow, ice and wind suckering roots
  • high seedling production
 
2.1.3 Photoperiod
As mentioned earlier (in Chapter 1, paragraph 1.6.1) June-bearers will not produce flowers when the plants are continuously exposed to days shorter than 12 hours. Ever-bearers, however, will only flower when the plants are continuously exposed to days longer than 12 hours. It is necessary, therefore,. to plant the right cultivar in the appropriate season.
 
Photoperiods highly affect the growth and development of the strawberry plant. Long days (11 to 14 hours) encourage the production of stolons, longer petioles and larger leaf blades. Short days decrease plant metabolism and growth rhythm and they stop producing stolons, and produce less leaves with shorter petioles and smaller blades.
 
 
2.1.4 Temperature
Day-neutral strawberries will flower and set strawberries whenever the temperature is in the range of 200C to 290C (350F to 850F). 290C is considered the upper limit at which day-neutral strawberries will produce flowers. When temperatures descend gradually the plant can tolerate even temperatures as low as -60C (210F), but it will die when temperatures fall to -120C (100F).
 
When flowers are developing at sub-optimal temperatures they will develop in an irregular manner, with a much lower number of stamen, and those that are existent will produce markedly less pollen. When the temperature falls under 100C, the existent pollen will fail to germinate, so the productivity of these plants will be decimated in that season.
 
Because strawberries bloom very early in the spring, plantings should not be located in topographic frost pockets. Where cold air drainage is limited, the crop may be lost to late spring frosts, which can kill the flowers. Installation of frost control measures may need to be considered.
 
Descending temperatures are one of the stimuli (together with shortening day length) pushing the differentiation of the plant apices towards its reproductive phase (production of flowers), while ascending temperatures push the differentiation of the plant apices towards its vegetative phase (production of stolons).
 
2.1.5 Relative humidity (RH)
Development and spread of powdery mildew is favored by moderate to high relative humidity and temperatures of about 150C to 270C (600F to 800F). Unlike most other fungi that cause plant disease, powdery mildew does not require free water for spores to germinate and infect. In dry years, when most other diseases are not a problem, powdery mildew can represent a very serious danger.
 
Development of Angular leaf spot (bacterial blight) is favored by moderate to cool daytime temperatures around 200C (680F), a low night-time temperature (near or just below freezing), and high relative humidity. High RH also has a deleterious effect on the opening of the pollen sacs of the stamen. It is highly important, therefore, to enable good aeration of the plants growing in protected structures during the flowering season.
 
2.1.6 Soil type
Strawberry plants grow and produce satisfactorily in a wide range of soil types, from sandy to clay loams. The best soil for strawberry production is a deep, well drained sandy loam, well supplied with humus (over 2% organic matter). Heavy clay soils that are usually poorly drained encourage disease development and impede precisely timed field operations. Coarse textured sandy soils are often infertile and droughty, and require more frequent irrigation and greater attention to fertilization practices. Plants established in low-lying muck or organic soils are more vulnerable to frost injury.
Strawberries will respond positively to high organic matter content in the soil.
 
2.1.7 Soil pH and EC
If the soil pH is unknown, the grower should submit a soil test. Strawberries prefer slightly acidic soils with a pH of between 5.5 and 6.5. Too low pH values may require applications of ground limestone to increase the pH of more acid soils. Good vigor has also been obtained on soils with a pH slightly higher than neutral (7.5). Soil pH levels over 8 can adversely affect certain strawberry nutrients, especially the iron levels in certain cultivars. Yellowing in strawberries is also common where soil pH is high. The use of green manures and acidifying fertilizers can reduce the soil pH to some degree.
Optimal EC value is 1.5.
Source: Fennimore et al, UC Davis
 
 
2.1.8 Soil salinity and alkalinity
Strawberry plants are extremely sensitive to salinity, especially at the transplant stage. See the special paragraph devoted to this subject in Chapter 3, Special sensitivities of strawberries.
 
Highly alkaline soils can occur in bands or patches throughout a field. This can cause yellowing or chlorosis of the plants, a condition that can lead to significant yield losses or complete plant die-back.
 
2.1.9 Soil surface drainage
The surface drainage system must permit water to move away from the field quickly and completely. Water standing on the strawberry field for even a day or two will injure plants, especially during the intensive growth period.
 
2.1.10 Internal soil drainage 
Poorly drained sites should be avoided. On sites with certain contours, such as dips, ridges and slopes, poor drainage can cause "yellowing" in strawberries. This condition can significantly reduce yields. On sites where drainage is a problem, the use of clay- or plastic-drainage tile could be considered. Raised beds, 20 cm (8 inches) high and 30 cm to  60 cm (2 to 3 feet) wide are recommended and should be maintained over the life of the planting. Application of perlite and working it into the soil will enhance good drainage.
 
2.1.11 Terrain slope
Strawberries should ideally be planted on slightly inclined slopes, especially if they face east or south-east (in the northern hemisphere).
Strawberries require cultivation, so avoid plantingthem on steep slopes. Plantings on 10% to15% slopes are likely to erode, with some plants being buried and others washed out of the soil. If sloping sites must be used, run rows across the slope or on the contour and use a wide row width.
 
2.1.12 Soil tilling
Soil tilling and nutrient availability can be improved by using green manure crops such as field peas, buckwheat, canola or rye. These crops can be rotated with the strawberry crop.
 
2.1.13 Quality of irrigation water
Irrigation is essential for high-yield strawberry production. Since strawberry plants are shallow rooted, permanent moisture is necessary to maximize production. An average of 300 mm to  450 mm (12 to 18 inches) of irrigation water is required over the growing season. Water applications may also be needed for spring frost control and for summer crop cooling when temperatures are above average. Water quality should be adhered to, regardless of the water application purpose.
Irrigation water with significant salt content, combined with poor soil structure, may also cause soil to develop unacceptable salinity levels.
As water quality crucially determines salinity severity, the following water characteristics are considered best for avoiding salinity injury to the plants.
 
Table 2.1: Most important parameters of irrigation water for strawberry fields
Parameter
Ideal level
pH
6.5 – 8.5
Electrical conductivity (ECw)
< 1.0 dS / m
Total dissolved salts (TDS)
 <450 mg / L
Sodium absorption ratio (SAR)
< 30
Chloride contents
 < 130 mg / L
Boron contents
 <0.7 mg / L
Nitrate contents
<5 mg / L
Bicarbonate contents
<1.5 meq / L
 
 
2.1.14 Previous and neighboring crops
Strawberries should not be planted in soils where strawberries, raspberries, vine crops, alfalfa, potatoes, tomatoes, peppers, eggplants, beans, carrot, okra or sod have been grown in any of the previous four years, unless the soil has been fumigated. Such sites are likely to contain disease and insect pests that may attack plants. The fungus diseases black root rot and verticillium wilt, which have attacked the above crops, build up in the soil. These diseases reduce the productivity of strawberry plants. Likewise, sites that are heavily infested with sedge, nutgrass, quackgrass, Johnson grass, and thistles should be avoided or treated prior to planting to destroy these chronic weeds. Fumigation should be considered if a long-term rotation that excludes the host crops is not feasible.
 
Sites surrounded by natural bush stands may contain native strawberry plants. These plants can harbor insects, diseases and viral pathogens that may infect cultivated strawberry stands. To maintain good sanitation, it may be necessary to kill native strawberry stands within 400 m (1,300 feet) of commercial strawberry fields.
 

2.2 Growth practices for field-grown strawberries

2.2.1 First year, fiels preparation 
Day-neutral and June-bearing strawberries
Day neutral and June-bearing strawberries are easiest to manage on raised beds. The beds should be 15 cm (6 inches) high and 60 cm (24 inches) across on the top. The beds should be 120 cm (4 feet) apart at the center, leaving about 60 cm (2 feet) between beds for a walkway.
 
When grown on an annual basis, the center of the beds should be 120 cm (4 feet) apart in order to leave a 60 cm (2 foot) walkway between beds. The following spring / summer the plants will bear a heavy early crop followed by a smaller summer and fall crop.
 
An important part of field preparation before planting is the application of a base-dressing of fertilizer, which should be rich in phosphorus with an N-P-K ratio of 1-2-1 or 1-3-1, such as 5-10-5 or 8-24-8. It should be worked into the upper 8 cm to 20 cm (3 inches to 8 inches) of the soil. This issue is further elaborated upon in Chapter 4, Mineral nutrition of strawberries.
 

2.2.2 Planting

What
Use only certified disease-free plants from a reputable nursery. Plants should have large crowns with healthy, light-colored roots.
 
Strawberry plants ready for planting
When
Strawberries should be planted in the spring as soon as the soil is dry enough to be worked and prepared. Do not work the soil if it is wet; rather wait a few days until it dries. This is usually in March or April, allowing the plants to become well established before the hot weather arrives. Try to do the planting on a cloudy day or during the late afternoon.
 
How
Strawberries should be planted in holes large enough to slightly spread the roots out, and deep enough to bring the soil half way up the crown, making sure that the crown is above the soil level (do not cover the crown!) and that the uppermost roots are at least 7 mm (1/4 inch) below soil level. Pack the soil firmly around the plants and irrigate immediately after planting.
 
Figure 2.2: Proper planting method A, as compared to improper methods B, C and D for strawberry planting. At B the crown is too deep; at C the crown is too high; and at D the planting hole is too shallow, forcing the roots to bend and remain near the surface.
Source: Ellis et al, 2006
 
Water the plants slightly (5-10 mm) with a very diluted solution of water-soluble fertilizer, e.g., 0.2% (2 kg / m3) of 20-20-20 within a few hours after planting.
 
 
2.2.3 Plant arrangement
Matted row system
 
This is the best system for growing June-bearing cultivars. In the matted row system, all runners are allowed to grow and daughter plants are allowed to root freely to become a matted row, no wider than two feet. The strawberry plants should be set 45 cm to 75 cm (18 inches to 30 inches) apart, in rows 90 cm to 120 cm (3 feet to4 feet) apart.
 
Figure 2.3: Matted Row System. Straw mulching is seen in the aisles.
 
 
2.2.4 Management of runners
As strawberries grow they will produce runners that will spread out and root to produce additional plants. Position the first runners with approximately 15 cm (6 inches) spacing between them. Only allow a few runners per plant,then remove additional runners to promote crown growth. The width of each row should be limited to 60 cm (24 inches) to maintain easy access in the planting. Runner plants that grow outside the 60 cm row width should be pinned back into the row to root or be removed if the plants become too crowded (less than 15 cm between plants). The runners can be positioned into the desired row width before they root and held in place with small stones, clumps of soil or old-fashioned hairpins (see Figure 2.4).
 
Soon after planting, the crowns will produce a few leaves, and flower buds will emerge. During the planting year, all flowers should be pinched off. This encourages runner growth and plant vigor to fill out the bed, leading to better yields the following year. Runner plants will begin to emerge from the crowns in the early summer. These should be used to fill out the rows.
 
 
Figure 2.4: Runner positioning by active intervention
 
Spaced-row system
This system limits the number of daughter plants that grow from a mother plant. The mother plants are set 46 cm to 76 cm (18 inches to 30 inches) apart in rows 1 meter to 2 meters (3 feet to 7 feet) apart. The daughter plants are spaced to root no closer than 4 inches apart. All other runners are pulled or cut from the mother plants. Even though more care is needed under this system, advantages include higher yields, larger berries and fewer diseases.
 
Hill system
This is the best system for growing day-neutral and everbearing strawberries as they do not send out many runners. In the hill systAn established strawberry bed grown in a double row hill system.em all the runners are removed so that only the original mother plant remains. Removing the runners causes the mother plant to develop more crowns and flower stalks. Multiple rows are arranged in groups of two, three or four plants with a 60 cm walkway between each group of rows. Plants are set about 30 cm (1 foot) apart in multiple rows. During the first two or three weeks of growth, the planting should be weeded; then the bed should be mulched.
 
Plant two rows of strawberries on each bed. The rows should be 30 cm (1 foot) apart, 15 cm (6 inches) from the left and right of the center of the bed. Plants within the row should be 20 cm to 25 cm (8 inches to 10 inches) apart. It is best to stagger the plants in the two rows on a bed such that a plant in one row corresponds to the space between plants in the other row.
 
 
 
2.2.5 Blossom removal
During the first growing season, remove flowers of June-bearing strawberries as soon as they appear. Removing the flowers promotes root and runner development, thereby insuring a large crop the following year.
 
For everbearing and day-neutral strawberries, remove the flowers until the end of June and then, after that date, allow the flowers to remain to set fruit for a summer / fall harvest.
 
2.2.6 Fertilization
After the first harvest, in the second season strawberries should be fertilized after renovation in July. Water the fertilizer in to get it down to the root zone. This application is made to keep the plants in a vigorous condition and to promote new growth and thus the development of more fruit buds. Do not over-fertilize because this will cause excessive vegetative growth, reduce yields, increase losses from frost and foliar disease, and result in winter injury. Read more about this issue in Chapter 4: Mineral nutrition of strawberries.
 
2.2.7 Cultivation
Cultivation is one of the most important practices in a new strawberry planting and it should be done frequently (once a week) for the first 6 to 8 weeks. Cultivation kills the weeds and loosens soil for better runner penetration. If necessary, herbicides can be an effective tool in controlling weeds in strawberries.
 
2.2.8 Mulching
Strawberries are very susceptible to frosts during their vegetative activity (as compared to their hibernation during cold winters). Such frosts may occur in the fall and in the spring. It is advocated, therefore, to mulch field-grown strawberries in the fall between mid-November and mid-December. Apply straw mulch after a few days of temperatures down to -60C (200F). Straw should be sifted loosely over the plants, just enough to cover them from view. After a week of settling, add additional straw up to a 7 cm to 10 cm (3 inch to 4 inch) deep layer over the rows. This mulch will protect the plants from cold temperatures that can kill the buds and injure roots and crowns. Where fall frosts are not likely, winter mulching is practiced to prevent damage to the roots caused by rapid freezing and thawing of the soil, and subsequent heaving of the plants out of the soil.
 
Mulches that have covered the plants during the winter months should be removed in the early spring, when the strawberry leaves show yellow, but should be left in the aisles to cover the blossoms in the spring when frost is predicted. Some of the mulch should be left around the plants to keep the fruit from soil contact and to conserve soil moisture.
 
Other benefits of mulching include help in controlling weeds and grass, protection from severe cold temperatures and help in keeping berries clean during harvest. Mulch the beds in the late fall, remove the mulch in the spring, and pinch blossoms for the first 4 to 6 weeks to improve later yields.
 
Many materials can be used for mulching. Straw is the most commonly used mulch. A typical straw mulching layer for winter protection will have about 9 tons / ha of wheat straw. But any loose material, which will provide cover without matting, can be used, such as pine needles or wood shavings. Do not use hay because it contains weed seeds that will start to grow among the strawberries the following spring. The mulch layer should be 15 cm to 20 cm (6 inches to 8 inches) deep over the plants.
If straw will be used, obtain straw shortly after wheat harvest. Loosen bales and soak in water. This, as well as subsequent rains, should germinate most of the grain before it is time to apply the straw.
Alternatively use aged sawdust. When sawdust is used, try to maintain a 2.5 cm (1 inch) mulch depth. No additional winter protection is needed.
“Floating” fabric row covers may also be placed over the plants to provide some winter and frost protection. These lightweight fabrics create a greenhouse effect that will make the plants bloom and fruit earlier in the spring and produce larger yields.
 
Row covers can be placed over the plants in the early fall or the early spring. Plants covered in the fall will have greater yield benefits from the covers, but additional mulch such as straw should be applied in mid-November for extra winter protection.
 
Figure 2.6: Strawberry flowers with frost damage (bottom), next to undamaged  flowers (top).
Source: Ellis et.al, 2006
 
 
2.2.9 Plasticulture
The bulk of modern commercial production uses the plasticulture system. In this method, raised beds are formed each year, are fumigated, and are covered with large plastic sheets, under which the irrigation tubing is installed. The main advantages of this mulch are:
  • A considerable increase in soil temperature of the active root zone of the plants. This enhances plant physiology and enables much earlier flowering and fruit bearing.
  • Prevention against weed growth, thereby saving on herbicides and work.
  • Protection of the beds from erosion by strong rainfall.
Plants are planted through holes punched in the covering. Runners are removed from the plants as they appear, to encourage the plants to put most of their energy into fruit development.
 
At the end of the harvest season, the plastic is removed and the plants are plowed into the ground.
 
 
Figure 2.7: Open-field grown strawberries on black plastic mulches
 
However, because this method requires a longer growing season to allow for establishment of the plants each year, and because of the increased costs in terms of forming and covering the mounds and purchasing plants each year, it is not always practical in all areas. In these cases the plants are not destroyed after harvest, but are kept from year to year, growing in rows or on raised beds. This system is most common in colder climates and where growers have less capital. It carries lower investment costs, and lower overall maintenance requirements. Yields are typically lower than in annual plasticulture.
 
2.2.10 Irrigation
On the one hand, strawberries are sensitive to periods of severe drought; dry soil can literally kill them or stop fruit production. On the other hand, they are sensitive to waterlogging that causes the roots to suffer oxygen deficiency, and is an optimal condition for soil-borne microbial- and fungal- diseases. Continuous optimal water status helps to get the highest yields of large berries. Strawberries should have a minimum of 25 mm (1 inch) of water per week. Up to 50 mm (2 inches) of water should be given weekly while the fruit is forming, from early bloom until the end of harvest. Watering should continue during dry periods in August and September. This later water helps reduce stress on the strawberry plants, which helps fruit bud formation in the following year.
Considerably higher rates are required during prolonged hot, dry periods. But, when it rains, a rain gauge should determine weekly rainfall. Irrigate to make up for rainfall deficiencies. Mulching helps keep the moisture level of the soil more consistent.
 
The irrigation system may also be used for frost protection, wherever this is valid. It should be turned on when the temperature drops to 0.50C (330F), and left to run until all the ice formed on the plants has completely melted. To take advantage of the irrigation system, several factors should be considered before installing an irrigation system:
  • Water supply. Water may come from wells, ponds, lakes, and municipal lines. An irrigation pond would need to hold about 230 m3 / ha (150,000 gallons / acre) of water for plasticulture production to provide protection on three consecutive frost or freeze nights.
  • Pumping capacity. A pumping capacity of as much as 5.1 mm / h (0.2 inches / hour) or 51 m3 / h is recommended for severe frost and freeze conditions.
  • Pump. An electric pump is recommended for reliability if a reliable electric power service is available.
  • Sprinkler type. Low-impact sprinklers are preferred. Special frost nozzles can be installed for some types of sprinklers, which will emit only enough water to protect the flowers and not flood the beds.
  • Sprinkler spacing. A 12 meter x 12 meter (40 foot x 40 foot) triangular spacing will greatly improve the sprinkling distribution pattern under higher winds as compared to conventional 18 meter x 18 meter (60 foot x 60 foot) spacing. Sprinklers should be set up to provide complete coverage of the planting.
 
Figure 2.8: Mulching and laying drip line
 
Figure 2.9: Drip lines under plastic mulched four-row bed
 
Figure 2.10: Sprinkle-irrigated / fertigated strawberry field
 
 
2.2.11 Pollination
The strawberry is mostly self-pollinated and, under field conditions, self-pollination is satisfactorily supplemented with natural agents like wind, honeybees, and other insects. Based on studies of 11 cultivars, self-pollination accounts for 53% of fertile flowers, wind increases development to 67%, and insect pollination increases it to 91% (Source: Ellis et al, 2006). If wild bees are not plentiful, then honeybees can be used, but they are not strongly attracted to strawberries and may be attracted to competing flowers. Honeybee visits are limited to good weather. However, honeybees in sufficient numbers should be effective pollinators.
 
Pollination of all the pistils of a flower is necessary for maximum berry size. If few are fertilized, an irregularly shaped berry or “nubbin” of only one fifth the size of well-fertilized berries will develop.
Flowers pollinated at the most receptive time produced 13% to 58 % heavier fruit than those before or after the best time. The best time is 1 to 4 days after the flower is open. Generally, 2½ strong hives per hectare (= one per acre) is recommended. Bee hives should be placed in the sun on a dry surface (see Figure 2.11) Source: Ellis et al, 2006.
 
If a grower has a very large acreage of berries, few active beekeepers in the area and little or no natural shelter-belt areas, then it may be very good insurance to have honeybees brought into strawberry fields as the first flower opens. Growers will be in competition with apple growers in some regions, and beekeepers will prefer other crops to strawberries. For some cultivars, where primary flowers have been damaged, there may not be an economic advantage for bees. Also, certain insects can cause poor pollination. Therefore, frost control and insect control measures may need to be adequate before bees are considered.
 
Figure 2.11: Placement of beehives in the sun, on a dry surface to secure strawberry pollination
Source: Ellis et.al, 2006
 
2.2.12 Harvesting 
Generally, berries ripen within 28 to 30 days (as few as 20 days under optimum conditions) after first bloom. The time between first bloom and full bloom can be 11 to 12 days. A great increase in the number of ripe fruit occurs over the first 4 to 6 days of harvest. Berries are harvested every other day under normal temperatures for about 6 to 7 pickings. Avoid picking the fruit when plants are wet. Keep harvested berries out of the sun and place them under refrigeration as soon as possible. Pick berries when they are fully colored for optimal size and flavor. Berries do not improve in quality after picking.
 
For hand harvest, it is wise to employ enough pickers to harvest the berries by noon, during the cool part of the day when pickers are most efficient. Harvested berries should be delivered and sold within 24 hours of harvest to reduce spoilage. About six pickers can harvest an acre of berries, or about 10,000 pounds, over the season.
The average picker can harvest 10 quarts (12 to 15 pounds) per hour over the entire season. Under excellent conditions, up to 175 quarts in a 10-hour day may be harvested by the average picker.
 
2.2.13 Second and third year care - mulch removal
Remove straw mulch in the spring as the weather begins to warm (before bloom). Check closely after each warm period (late February through March); if the plant foliage begins to show yellowing, remove the mulch. Rake mulch toward row aisles. This creates a clean walkway and will keep the fruit dry and clean. If a frost is predicted after the mulch has been removed, it may be raked back over the plants for the night to protect the flower buds.
Leave a light sprinkling of straw mulch on the plants; they will push through. If spring frosts threaten, rake the mulch back over the plants, but be sure to rake off the mulch during the day.
“Floating” fabric row covers can also be applied in the early spring. Leave the row covers on until the plants begin to bloom. This may occur two to three weeks earlier than plants without row covers, so be prepared to protect the flower buds from frost. Although the row covers will provide some frost protection, it is best to mulch or use irrigation over the row covers if a hard frost is predicted during early spring.
 
2.2.14 Harvest
Hand-pick berries daily if possible and pick all berries that are ripe. Toss out all moldy berries. This will help prevent rots from spreading. Grading and packing often takes place in the field, rather than in a processing facility. In large operations, strawberries are cleaned by means of water streams and shaking conveyor belts.
 
2.2.15 Renewing the planting
Renovation is an important part of strawberry care. Strawberry beds can usually be carried over for 3 to 5 years or more if the plants are vigorous, the bed is kept weed-free, and the planting is properly renewed or renovated every year. In order to insure good fruit production, June-bearing strawberries grown in the matted row system should be renovated every year right after harvest. As strawberry plants tend to get infected with leaf diseases, which may result in declining yields if not treated timely, plants infected with diseases should be removed.
 
The bed should be renovated shortly after the harvest is complete, usually late July. First, mow the old foliage with a mower, cutting off the leaves, about 3 cm (1.5 inches) above the crowns. Rake the leaves and, if disease-free, compost or incorporate into the soil. Then use a rototiller or spade to cut each plant row to a 15 cm (6 inch) width. (Runner plants from the 6 inch strip of "mother" plants will form a new matted row of plants.) To prevent overcrowding of plants and reduce the incidence of leaf diseases, thin plants to 5 to 7 plants per 30 cm x 30 cm (1 square foot). Next, spread a light, 1 cm to 2 cm (1/2 inch to 1 inch) layer of soil over the remaining plants, but do not bury the crowns.
 
Irrigate the planting well, wetting the soil to a depth of 15 cm (6 inches). During the rest of the growing season, irrigate to provide 25 mm (1 inch) of water per week, and continue to control weeds. During the summer, runner plants will emerge and should be placed to fill out the row to the desired 60 cm (2 foot) width, similar to the planting year.
Keep the planting healthy and vigorous throughout the season by controlling weeds, maintaining the proper plant density and row width, and watering regularly.
 

2.3 Optimal conditions and growth practices for protected strawberries

An ever-increasing hectarage of strawberries is produced under different protection conditions that range from low- and high-plastic tunnels, to plastic- and glass-greenhouses, equipped with the most modern production inputs, functioning during off seasons.
 
2.3.1 Protective structures
Temperatures below -0.5°C can cause severe damage to full blooms. Therefore, in Holland and Belgium, strawberries are grown in glasshouses during winter. In Spain, Italy, France, the UK and Germany, strawberries are grown in winter under polyethylene tunnels. In Israel, strawberries are cultivated under polyethylene tunnels and in greenhouses. When grown under protection the plants can be planted in soil or on in soilless containers of various types.
 
Figure2.12: Mini-tunnels, Israel
 
Figure2.13: A maxi-tunnel, Israel
 
2.3.2 Soil type
The best soil for strawberry production is deep, well drained sandy loam, well supplied with humus (over 2% organic matter). Heavy clay soils that are usually poorly drained, encourage disease development, and impede precisely timed field operations. Coarse textured sandy soils are often infertile and droughty, and require more frequent irrigation and greater attention to fertilization practices.
 
Strawberries will respond positively to high contents of organic matter in the soil, but application of fresh manure should be avoided.
 
2.3.3. The use of zeolytes
Zeolytes are negatively charged crystal alumina silicate, balanced by +1 to +2 valence cations. Zeolites also have high absorption level, water retaining and releasing, high cation exchange capacity (CEC) and high pH buffering capacity has been shown to be helpful for growing strawberries, for the following advantages:
  • Holds minerals tight during complete cultivation, so few minerals are washed away
  • Easy to use
  • Good development of the leaf
  • Good quality of the strawberry fruits
Source: Strawberrys, Loomans, 04.doc
Perlite / zeolite at 3:1 and 1:1 ratio (v/v) were found very efficient as soilless growth substrates, by Fotouhi Ghazvini et al, 2007
 
2.3.4 Soliess media
The ideal characteristics of growth media are summarized as follows:
Source: Chen & Inbar (1985)
  • Physical characteristics
  • High water retention
  • High hydraulic conductivity to allow efficient drainage
  • High porosity to allow aeration
  • High air content even when water tension is low
  • Particle size distribution that allows the aforementioned characteristics
  • Low bulk density
  • Provide the plant with high physical anchoring
  • Stable volume, to minimize changes due to shrinking and compacting
  • Chemical characteristics:
  • High cation exchange capacity
  • Reasonable level of nutrients and ability to supply these to the plant
  • Buffer capacity to maintain stable pH level
  • Low soluble salt content
  • In the case of organic media it should have a low C/N ratio, with a very low decomposition rate
Some of these characteristics cannot be satisfied by all growth media types, so the actual growth medium is usually composed of two or more components that complement each other, optimizing the growth characteristics.
Several generally accepted compositions of soilless media for strawberries consist of various blends of peat-moss, rockwool, coconut fibers and perlite, such as:
  • 60% peat + 40% coconut fibers
  • 50% black peat + 50% medium peat
  • 70% peat + 30% perlite
The most common type of soilless media used for strawberry cultivation is a peat grow-bag. Some growers use the cheapest peat bag available. However, this is false economy since strawberry roots are sensitive to waterlogging, and grow best in an open structured, free draining substrate. Strawberry plants grown in poor quality peat often turn yellow (particularly in the second crop), as a result of compost slumping and a lack of aeration, or waterlogging at the roots. Improved strawberry grow-bags feature free draining texture and will not slump, even after 9 to 12 months’ use. The peat is augmented with 10% polystyrene or perlite, which further increases the aeration of the compost. With this mix, it is also possible to carry out a second planting in the same bag, and take a further two crops with only a small (10% to 12%) loss of yield.
 
Some growers use other soilless media. For example, in Scotland, several growers use 100% perlite successfully, whilst on the south coast of England growers use rockwool slabs.
 
2.3.5 Soilless pH and EC 
When grown on soilless media, optimal pH value is around 5.7, and optimal EC values of the drip water in soilless cultivation systems are 1.5 to 1.7 dS / m. Hence, the optimal EC values of the drain in soilless cultivation systems are: 1.6 to 1.8 dS / m.
Source: Fruit & Veg Tech, issue of March, 2009, Abdal-Razak, Israel, 2004
 
Whatever media is chosen, it is best to raise the plants off the floor, ideally to a height of 120 cm to150 cm (4 feet to 5 feet). Where the load carrying capacity of the structure exceeds (10 kg / sq.m), metal gutters or a strained wire support system may be hung on chains from the roof. Alternatively, the support system may be mounted on steel poles or wooden posts driven into the floor. On a small scale, plants may also be grown on straw bales covered with plastic or wooden crates. However, as the crop gets closer to the floor, yields are lower and picking costs tend to increase.
 
 
Figure 2.14: Soilless cultivation of strawberries in the UK
Source: Atwood et al, 2005
 
2.3.6 Irrigation and nutrigation
Plants are best irrigated in protected cultivation with drip tapes or dripping systems. It is advisable for plants to receive nutrients with every irrigation session, with each plant receiving about 140 ml nutrient solution per day by multiple irrigation sessions, one to several minutes long, with ~90-minute intervals between the sessions.
 
2.3.7 Fertilization setup
Fertilizer solution should be discharged by 2 to 3 injectors (e.g., Dosatron), assembled in series. Two to three separate stock tanks should be used accordingly for fertilizer application.
 
Figure 2.15: Dosatron injectors assembled in series inject fertilizer solutions from two separate stock tanks in greenhouse-grown strawberries.
 
Figure 2.16: The nutrigation control unit of the soilless cultivation plot in the UK
Source: Atwood et al, 2005
 
2.3.8 Pollination
Under greenhouse conditions, the activity of natural agents like wind, honeybees, and other insects is highly restricted by the protective structure. Bumblebees provide good pollination for strawberry plants and they perform much better than honeybees or hand pollination. Therefore, the use of bumble bees is absolutely essential to ensure good pollination. One beehive (e.g., Koppert Biological Systems Inc.) containing approximately 50 bumblebees is sufficient for pollinating about 4,000 strawberry plants (500 m2 greenhouse area).
 
Figure 2.17: A bumblebee pollinating a strawberry flower
Figure 2.18: Evaluation of pollinators for high tunnel strawberry production
 
 
2.3.9 CO2  enrichment in greenhouses
CO2 enrichment of the greenhouse atmosphere markedly increases strawberry yield. Artificial CO2 concentrations of 400 ppm to 900 ppm (as compared to natural concentrations of ~350 ppm), accompanied by increased light intensity, increased total yield by 8.7% to 31%, due to both higher individual fruit size, and increased number of fruits.
 
Fruit quality is enhanced too. Elevating the CO2 concentration to 650 ppm and to 950 ppm resulted in higher fruit dry matter, fructose, glucose and total sugar contents, and low citric- and malic-acid contents. High CO2 growing conditions significantly enhanced the fruit content of the following aroma compounds: ethyl hexanoate, ethyl butanoate, methyl hexanoate, methyl butanonate, hexyl acetate, hexylppm hexanoate, furaneol, linalool and methyl octanoate. Thus, the total amounts of these compounds were higher in berries grown in CO2-enriched conditions than those grown in ambient conditions.
Sources: Lieten, Acta Hort., 1996; Wang & J. Bunce, in Science of Food and Agriculture, 2004
 
 
Need more information about growing strawberries? You can always return to the strawberry fertilizer & strawberry crop guide table of contents