Strawberries 101: a production guide

Departments - Hydroponic Production Primer: Strawberries

Cultivars have been primarily developed for field production but are starting to be perfected for controlled environments.

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March 26, 2018
Christopher J. Currey
Fig. 1. These strawberries are grown in containers filled with soilless growing substrate and placed on gutters to collect leachate.
Photo: Christopher J. Currey

Genetics: The cultivated strawberry (Fragaria × ananassa) is a domesticated crop resulting from hybridizing several wild strawberry species. The vast majority of cultivar development has been focused on varieties that are well-suited to field production in mineral soils. While there have been some efforts to develop cultivars specifically for controlled-environment production, this has taken place in other counties. For growers here in the U.S., they will need to trial cultivars to identify which are suitable for their geographic location, production environment and market.

Production systems: Strawberries are most commonly produced in systems using soilless substrate, and there are several systems suitable for production. There are some containers that are manufactured specifically for hydroponic strawberry production and may be suspended on pipes or rails, or placed on top of gutters (Fig. 1). Additionally, long, continuous troughs may be filled with substrate and used for strawberry production. In addition to these systems, which are designed specifically for strawberries, smaller-scale producers may utilize containers such as hanging baskets or one-gallon nursery containers. Regardless of which system you select, a growing substrate that is very free-draining should be used. Soilless substrates comprised of organic components like coconut coir and peat moss and inorganic components like perlite are most commonly used.

Propagation and young plant production: Strawberries can be propagated in several ways. Tissue cultured plantlets can be purchased and bulked up prior to transplanting. Runners can be taken off plants, planted into small containers or cell trays filled with substrate, and placed under mist to root. Once rooted, these plants are then grown until they are sufficient size. Plants can also be dug from fields and placed into storage at temperatures below freezing until transplanting. Growers will need to propagate their own plants for production or seek out reputable suppliers of young plants.

Nutrient solution: Strawberries should be irrigated with smaller volumes of water at a greater frequency to avoid water-logging the root zone. Nutrient solution electrical conductivity (EC) can be lower compared to other crops from around 1.0 to 1.5 mS/cm, and pH should be maintained between 5.5 and 6.0 to help manage substrate pH and micronutrient availability. Specifically, strawberry plants can be susceptible to iron deficiencies, and if cooler production temperatures are used, this can be exacerbated. Strawberries also require elevated levels of potassium for developing fruits and calcium for calyces.

Temperature: Day temperatures around 75° F and night temperatures around 60° F are commonly used. While strawberries grow well under cool temperatures, growing too cool can result in long crop times. Alternatively, excessively warm temperatures (>85° F) can damage plants and reduce productivity.

Light: Strawberries can be considered a high-light plant. Using supplemental light to increase light intensity in the late fall, winter, and early spring can improve yield and fruit quality. In addition to supplemental light to increase photosynthesis, low-intensity lighting can be used to alter the photoperiod (day length) or light quality. While June-bearing cultivars flower in response to short days and ever-bearing cultivars can flower under a range of day lengths, there are some cultivars that require or are most productive under long days and, as a result, lighting is required to promote flowering when the day length is too short. Some growers use low-intensity light rich in far-red light, as this can promote flower truss elongation and allow the fruits to hang down, out from the canopy.

Fig. 2. Flowering trusses do not produce fruits that ripen at the same time; the development of fruit is staggered.
Photo: Christopher J. Currey

CO2: Supplemental carbon dioxide (CO2) is useful for enhancing growth in the winter when ventilation is limited and CO2 concentrations in the greenhouse can drop and limit plant growth. Supplemental CO2 to maintain concentrations from 500 to 1,000 ppm can be provided by burners or liquid injection.

Pollination: Strawberry flowers may be self-pollinated. However, complete pollination is important for producing well-formed fruits. Strawberry flowers may be pollinated by hand using a vibrating wand; while this is useful for small plantings, the labor-intensive nature of hand pollination may not be practical or feasible for large operations. In outdoor production, strawberries are pollinated by wind, and this can be mimicked in the greenhouse. Hair driers on the “cool” setting or hand-held leaf blowers can be used to pollinate flowers, and this is less labor-intensive than hand-pollinating with a vibrating wand. Bumblebees can be used to pollinate strawberries and are most effective for large-scale plantings, as they require minimal labor compared to any strategy reliant on labor for execution.

Pruning and training: Older, yellowing leaves are removed since they may consume more energy than they produce; the removal of these leaves also promotes air flow throughout the canopy and crowns of plants. Healthy leaves may also be removed if there is too much foliage and the plant is too vegetative. Flowers may be removed from strawberry plants that are undersized when they are first planted to promote vegetative growth and increase crown size; plants that are too small and allowed to flower will produce fruits that are too small to market, as well as slow down vegetative growth. For strawberry plants that are of sufficient size, flowers may be removed to promote fruit development. Flowers that are lower on the truss moving farther from the terminal flower, referred to as the primary or king flower, may be removed to ensure that the other developing fruits on the truss will reach a marketable size. Misshapen fruits that are not marketable should be removed to promote the development of other fruits that can develop into marketable product (Fig. 2).

Pests: Strawberries are susceptible to several insect and mite pests. Aphid feeding can distort new growth, and the honeydew they excrete can promote development of sooty mold. Spider mites can diminish the vigor of plants by damaging foliage and reducing photosynthesis. Thrips feeding can also distort foliage and fruit, and they can also transmit some diseases.

Diseases: Gray mold, or Botrytis, is the most common disease in greenhouse strawberry production and can render fruits unmarketable. The Botrytis that affects fruits can infect the plants while they are flowering and will go undetected until fruits start to develop. Avoid cool, humid conditions and maintain strict sanitation to keep crops clean. If you are recirculating nutrient solution, treat the solution to suppress pathogens such as Pythium and Phytopthora.

Physiological disorders: Deformed fruits are the primary physiological strawberry disorder. When ovaries are fertilized and seeds are produced, they produce auxin and the fruits expand. However, a lack of seed development can occur as a result of several different causes. First, the pollination of the flower may not be thorough enough. Second, pollination may have occurred, but non-viable pollen or cool temperature may have inhibited fertilization of ovules and development of seeds. Additionally, if there is not sufficient pollen for a bee population, they can become aggressive in their search for pollen and damage flowers.

Another common physiological disorder is tip burn, which can occur both on the foliage, as well as on the calyx on fruits. While tip burn on leaves is undesirable, tip burn on calyces is more problematic because it can reduce the marketability of fruit. Nutrient solution with insufficient calcium concentrations can lead to tip burn. However, conditions that inhibit transpiration (high humidity, low light) or cause excessive transpiration (low humidity) can also lead to deficiencies even when there is sufficient calcium available to plants.

Harvesting: Strawberries should be harvested every day and, during periods of warmer temperatures and high light, twice per day. At a minimum, fruits should be at least 50 percent ripe; depending on the market and supply chain, fruits can get up to 75 percent ripe. While one advantage of controlled-environment production allowing fruits to ripen more on the plant before harvest, avoid letting fruits get over-ripe on the plant since these fruits will be of lower quality once they reach the consumer. When harvesting fruits, be sure to leave the calyx attached to the fruit. Some markets may also pay a premium to have a short section of the stem or pedicle attached to the fruits, but take care when packaging so that the stem doesn’t damage other berries.

Postharvest care: Once fruits are harvested, they should be cooled to between 36 to 41° F as soon as possible after harvesting and stored in 90 to 95 percent relative humidity. While many hydroponically grown crops may be hydrocooled by submersing them in chilled water, this is not a suitable practice for strawberries; fruits can be easily damaged, and the moisture can promote disease and speed the rate of spoilage. Rather, forced-air cooling should be used to reduce fruit temperature. Clamshells with holes for ventilation or punnets are both popular packages for strawberries.

Christopher (ccurrey@iastate.edu) is an assistant professor of horticulture in the Department of Horticulture at Iowa State University.