HOW TO GROW STRAWBERRIES

Commercial, Hydroponic Strawberry Production
By Dr. Lynette Morgan. Ph.D.
SUNTEC International Hydroponic Consultants, New Zealand


Strawberries are a popular hydroponic crop and using hydroponic methods can greatly increase yields and quality over soil based production systems. Hydroponics has a major advantage for strawberry production in that the system can be cleaned and sterilized between crops. Soil based strawberry crops rely heavily on the use of soil sterilization with chemicals to prevent plant losses from certain root rot pathogens. Strawberries are also small plants suited to a number of different hydroponic systems. They can be produced in nutrient film technique (NFT), bag, or bed systems and even in tiered or vertical systems.

There are a few essential requirements for growing strawberries. These include selection of the right cultivar, pretreatment of the runners before planting, and maintaining the right temperature levels for good fruit production. After harvest, the berries must also be handled and stored correctly to prevent many of the postharvest bruising and rot problems that downgrade the quality of the fruit.

Types and Cultivars of Strawberries Best Suited to Hydroponics and How These Are Propagated
The most widely grown type of strawberry are those called day-neutral cultivars. These will be induced to flower and fruit after a period of chilling or cool temperatures at 1-5 deg C. Other cultivars used for soil cropping are either "short day" or "long day" cultivars that are induced to fruit only after they have experienced short days (less than 12 hours of daylight) or long days (14 or more hours of daylight). In regions where the day length is fairly constant, day-neutral cultivars should be used.

Day-neutral types include many of the most commonly grown cultivars. These develop and initiate flowers irrespective of the day length, provided they have received a period of cold temperatures. This cold temperature is usually given to the plants or freshly dug runners for a period of around 6-12 weeks.

Plants or runners are cleaned, trimmed, and wrapped in plastic before storing under refrigeration for the 6-12 week period. After this time, they are taken out of cold storage and planted out into a hydroponic system under warm conditions. Leaves will grow and develop and then flowering will occur within a few weeks, provided the plants had a long enough time in cold storage.

Some of the most popular strawberry cultivars used in commercial production are:
 

However, each country and growing region often has its own locally developed cultivars.

Systems for Hydroponic Strawberry Production
NFT and media based systems are most commonly used for hydroponic strawberry production. NFT, where the nutrient is recirculated through small pipes, tubes, or gullies, is popular in countries such as New Zealand and Australia. In these types of systems the plants are usually supported in small pots or cups so that the crown of the plant doesn't slip down into the nutrient causing it to rot. Some growers also use small rockwool cubes to support the young plants in NFT systems.

The ideal size for NFT channels is 100-150 mm (6-inch) diameter rectangular gullies, although many successful crops are grown in smaller, round channels. Where smaller channels are used, a greater slope is required to prevent any ponding of the nutrient as the root systems grows and fills the channel. Adjustable slopes on NFT gullies are also a good idea for strawberry crops. NFT channels can also be arranged into tiers to make the most use of growing space.

One system in Australia used the inclined vertical pipe system. This has a continuous flow of recirculated nutrient and largely avoids any problems with nutrient ponding in the gullies. Plants in this system are supported by rockwool cubes. It also allows more plants to be grown in the same area, although its not well suited to growing conditions that have lower light levels as there can be some shading on the lower layers.

Flood and drain systems can also be used for strawberry production. Many of these systems use channels similar to those used for NFT. However, each plant is planted into a substrate or media in the base of the channels. Rockwool cubes are commonly used--as is gravel or coarse sand--to hold the plant in place when the nutrient is flooded into the channel and to retain moisture during the drain cycle. These types of systems are harder to manage once the root system has grown and filled the media in the channel. Facilitating good drainage in the system was difficult and root rot often occurred.

Small channels of media also heat up in the sun, causing a number of problems with growth and diseases. Where flood and drain systems are to be used for strawberries, a good depth of free draining media must be used and the flood cycles carefully managed.

Media Beds/Bags
Most hydroponic strawberries are actually grown in media based systems and these seem to be easier to manage than NFT for less experienced growers. There is a wide range of substrates that can be used to grow strawberries, including rockwool, expanded clay, perlite, vermiculite, gravel, coconut fire, pumice, and untreated sawdust, to name a few. Any substrate for strawberry production needs to be sterile and reasonably free draining. Common media systems for strawberries include bag or bed systems. These contain the media and plants and nutrient applied may be recollected or is often used drained to waste.

Rockwool slab systems are common in some areas of the world for greenhouse strawberry production. Coconut fiber slabs can be used in much the same way. Usually, the nutrient is applied to a 10 percent runoff, which is drained to waste, although many growers are attempting to collect the waste nutrient and reuse it. Where rockwool or coconut fiber is used, the nutrient application has to be well managed and monitored since these media can become overly wet resulting in plant losses from crown rot.

With any media based system, planting depth is essential. The crown of the plant, which is where all the leaves and flower stems originate, must not become too damp. Only the base of the crown should be in contact with the media, with most of the plant sitting well up above the media surface. Plants that are planted too deep tend to rot very quickly. Airflow around the base of the plant and under the leaves is also important in preventing many fungal diseases, so having an elevated system up on a bench or support structure helps with air moment around the plants.

Vertical or Hanging Systems
Vertical systems are commonly used for strawberry production in some countries. Vertical systems can consist of stacked pots or buckets, such as in the Verti-Gro system, or as hanging bags of media. Vertical or hanging systems work best where there are good light levels year round, as the plants on the lower layers of these systems may not receive enough light for good production where light is low.

Hanging bag systems can be difficult to manage as the media contained within the bag can become overly wet at the lower layers causing plant losses. The greenhouse structure must also be able to support the weight of many bags of damp media. Salt buildup in the lower layers of the bag has also been found to be a problem in many commercial operations.

Aeroponics
Aeroponics is being successfully used to grow strawberries in trails in New Zealand. This system uses a continual fine misting of nutrient onto the root system and avoids many of the root rot problems associated with plant losses. The capital investment and maintenance required in aeroponics, however, means it may not be a good commercial option for most growers.

Propagation
Strawberries can be propagated by runners, which are small plantlets formed at the end of a long thin stem or stolon produced by the mother plant. They can also be produced by division of larger crowns in mature plants, by tissue culture, and from seed. However, for commercial production, strawberry plants are produced from runners. Runner production means the new plant will be the same as the parent plant so that good yields and fruit quality can be continually obtained from the cultivar chosen.

Hydroponic strawberry growers can propagate their own runners. This is usually a good idea since most prechilled runners that can be purchased are from soil raised plants and need to be washed to remove this soil before planting into a hydroponic system. Having soil grown runners or plants also increases the risk of introducing pests and root diseases into the hydroponic system, so such runners should be treated with a fungicide dip before planting out.

Obtaining new plants from runners is relatively simple. After the parent plant flowers and fruits, runners will begin to be produced. Look for a long stem or stolon where a new plantlet will form at the tip. Each stolon will produce a number of plantlets that can then be pinned down into pots of media. Roots will form on the underside of the young plant. Once the roots are well established, the new plant can be cut from the parent plant. Each parent plant can usually produce at least 15-20 new plantlets over a 3 month period, although this depends on the cultivar being grown. Some cultivars produce more runners and plantlets than others. The rooted runner can now be prechilled under refrigeration for a number of weeks and then planted out to produce a new crop of strawberry fruit.

Strawberry plants can be cropped for a number of seasons. However, yield is usually highest in the first and sometimes the second season so hydroponic strawberries are often grown as an annual crop only.

Temperatures and Training
Before planting out into the NFT system, the strawberry roots may need to be trimmed. Once the chilled runners or plants have been planted out into the NFT system, plant growth and establishment should be rapid. In NFT, some form of plant support, such as small pots or tubes, should be used to hold the plants upright while the develop. Plants put into media need to planted at the correct depth so the base of the crown is in contact with the media but not buried.

The nutrient can then be applied at a low EC (around 0.5) for the first few days. Misting the newly planted strawberries with water during the warmest part of the day will help prevent too much transplant stress. After a week, the EC can be gradually increased to a full strength of around 1.8-2.4. Any plants that fail to develop should be removed and disposed of so they don't introduce rot disease into the system.

Ideal temperatures for strawberry production are in the range of 16-28 deg. C (61-82 deg. F). Low temperatures will delay the development of flowers and fruit and slow plant growth. Temperatures above 28 deg. C can stress the plants and prevent flowering and any fruit formed will be soft with poor keeping quality. Plants growing under high temperatures and radiation levels will require shading to reduce temperature levels and plant stress. Nutrient chilling can be used in tropical climates to grow strawberry crops, although this may not be economical.

Within a few weeks, the new plants will have formed a number of leaves. Any flowers that form within these first few weeks when the plants are establishing foliage should be removed. The plant is unable to support the development of good sized berries before it has developed sufficient leaf area so flowers should only be left to develop once the plant has reached a good size. Strawberries produce flowers singly and on trusses--often with a number of small and large flowers on the same stem. Smaller flowers can be removed if necessary to increase fruit size on the rest of the stem.

Strawberries benefit from some assistance with pollination. Outside, bees and other insects carry out much of the pollination. In a greenhouse, commercial strawberry growers often use large air blowers to pollinate the plants. Small handheld air blowers also work. These release the pollen into the flower surface and pollination will then occur. Good pollination is required for evenly shaped, large sized berries. Where pollination is inadequate, misshapen or undersized berries often occur. These should be removed from the plant as soon as they are seen.

Each of the small seedlike structures on the outer surface of a strawberry produce growth hormones that act to pull in sugars and assimilates from the leaves for berry growth. Where pollination has not been successful, some of these seeds on the outer surface don't develop and, as a result, uneven berry development shape occurs.

Once the fruit have set, they grow rapidly under good growing conditions. Strawberry plants produce a high fruit yield for their relatively small size and will often crop in flushes, producing many large fruit and then resting for a few weeks before flowering and fruiting again. Yields from day-neutral strawberry plants vary considerably between growers. Anywhere from 300 grams to 1.5 kilograms per plant per season has been found, with planting densities of 10-16 plants per square meter being maintained.

Nutrition and Water Quality
Water supplies for hydroponic strawberries needs to be of good quality. Strawberries are intolerant of high levels of sodium in the water. Levels above 50 ppm sodium will reduce yields and plant health. Where sodium is present in the water supply, nonrecirculating systems should be used. Adding soluble silica to the nutrient solution will also assist the plants to grow and thrive in water that contains some sodium.

Nutrient solutions for strawberries should have good levels of potassium to support fruit growth and quality. Potassium is important for developing good flavor, firmness, and a long shelf life in the berries. Nitrogen levels also need to be maintained for continuing leaf growth and the development of runners and new plants. Strawberries can be prone to iron deficiency symptoms under conditions of low temperature or where the root system has been damaged by transplanting or root rot diseases. pH in the range of 5.5-5.8 is ideal for strawberries in hydroponics

Crop Maintenance
Any older, browned, or discolored leaves should be removed. Regular crop maintenance is important as any old berries or foliage can host fungal and bacterial disease and reduce airflow around and under the plant. Deleafing old leaves should be carried out every week when the plants are actively growing, especially where conditions are warm, humid, and wet. This will reduce the occurrence of diseases.

Pests
Hydroponic strawberries can be affected by a number of pests, including mites, whitefly, thrips, aphids, caterpillars, and slugs--but the major pest of outdoor strawberry crops is birds. Birds will completely destroy crops of ripe berries and bird netting needs to be used to prevent this from occurring. Birds will also come into greenhouses where ripe berries are visible, so screens over vents become important to prevent this.

Diseases
Strawberries are very prone to a number of root rot disease and these are often the factor that results in many plant losses. Red stele or red core root rot caused by Phytophthora is a worldwide problem with strawberries and can cause significant losses. Infected plants become stunted, the leaves become a dull green color, and older leaves turn yellow or red. The plants can also rapidly wilt and die. Plants infected with red stele don't produce many roots and the stele or inside of the root turns a pink or red color. This disease comes from infected plants or soil contamination. Another root rot disease is black root rot caused by Pythium and Rhizoctonia species. Affected roots become black with much smaller roots than is normal. Infected plants become weak and fail to produce fruit and runners. There are a few other root rot fungi. Verticillium wilt and Fusarium can also occur. However, like most of the fungal diseases, these tend to attack weakened plants or plants that are waterlogged.

Healthy plants can usually resist attack. Addition of microbes, such as Trichoderma species and products containing other beneficial microbes, to the growing media or nutrient solution can help suppress any fungal pathogens that might be present.

Leaf or foliage diseases that can affect strawberries include leaf spot, Botrytis (or gray mold) of the leaves and fruit, leaf scorch, leaf blight, and powdery mildew. Leaf spots and other bacterial diseases can be largely prevented be not allowing the foliage and fruit to become wet. Some form of overhead crop shelter is vital where this can occur. Botrytis or gray mold is a very damaging problem where humidity is high as it can infect the leaves and fruit. Botrytis can be controlled with fungicide applications and with use of products contenting Thrichodera spores. Powdery mildew can be prevented with applications of sodium bicarbonate sprays or silica based sprays. Adding silica into the nutrition solution may also help with disease prevention.

There are also a number of fruit rot diseases. Generally, these occur where conditions are warm and humid and there is moisture on the fruit surface. Keeping the environment dry and the berries up out of any moisture will prevent many of these. Having good air movement around the base of the plants also assists with prevention of many of the fruit rot diseases.

Harvesting
Hydroponic strawberries should be harvested when they are light-red to orange-red. Fruit must be picked when they are at least 75 percent ripe or pink for good color and flavor development after harvest. If overripe fruit are picked, these will darken and soften in storage and will not be of acceptable eating quality. If picked too green, color, flavor, and aroma will not fully develop and the fruit will be tasteless. Any fruit that are uneven in shape, undersized, or have defects or disease damage should be graded out as soon as possible after harvest. Harvesting should be carried out during the early morning before temperatures warm up too much. Warm fruit do not store as well as those picked under cooler temperature conditions.

Once picked, the berries need to be cooled as soon as possible to retain quality. This is called removing the "field heat." Berries need to be cooled down to 5 deg. C at a high humidity (greater than 90 percent) to prevent too much moisture loss. This is usually carried out by refrigeration or with forced air cooling. High quality strawberries should store for at least 5-10 days under refrigeration.

Postharvest Handling
Strawberry fruit bruise easily and the stalks and calyxes can puncture the fruit flesh when berries are being harvested and packed. All fruit need to be handled carefully to prevent this type of damage. Any physical damage will increase the chances of postharvest rots and will result in poor keeping quality.

Growing High Flavor Berries
Many hydroponic strawberry growers aim to produce not only higher berry yields from hydroponics but also better tasting fruit. The taste or flavor of strawberries is a combination of sugars, acids, and volatile compounds. Sugars can be increased by ensuring each plant has sufficient foliage to produce assimilate and that light levels are not limiting production.

Maintaining a nutrient solution at a slightly higher EC (around 2.5) with good levels of potassium will help with sugar accumulation in the fruit. Cultivar selection is important where the flavor of the fruit is concerned. Many cultivars have fruit with naturally higher sugar levels and flavor than others, so trailing and testing different cultivars is always recommended.

Conclusion
In conclusion, hydroponic strawberries can be a challenging but profitable crop. Careful attention to propagation, chilling requirements, and planting are vital to ensure that the crop establishes well and does not develop root rots in overly damp conditions.

However, strawberries are a rewarding crop when grown well and will begin to produce fruit within a few months after planting out. Probably the greatest potential for hydroponics strawberry production is in supplying the off season market when outdoor soil grown berries are not available. Use of hydroponic technology allows this to be carried out in a cost effective way and produces high yields of good quality fruit.

 

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