One method of growing under protection which has increased in popularity is growing in peat bags. This method is costly and high yields together with high prices are required to offset the extra costs involved.
The growing house
Strawberries in peat bags can be grown either in glasshouses or polytunnels. Glasshouses retain heat better than polytunnels and in cold winters this is an advantage.
However, in mild winters this is not so as strawberries require a period of cold during the winter to break dormancy and may achieve this more easily in a polytunnel.
As glasshouses normally have a more sophisticated ventilation system than polytunnels the atmosphere is less humid.
Polytunnels should preferably have netted sides to a height of approximately 1 metre to allow adequate air movement or alternatively be equipped with fans.
Polytunnels with vertical sides for at least first metre are preferred giving more efficient use of space and allowing easier access.
The growing system
Strawberries are planted into peat bags either in January to give a crop in May or in June/July to give a small crop in August/September/October followed by a larger crop in May.
- Growing in peat bags removes the risk of soil borne diseases, for example red core, and allows more effective control of watering and feeding.
- The bags are supported at shoulder level so the ripening fruit hangs down allowing easier, quicker picking and free air flow around the berries so reducing botrytis.
- Apart from plant establishment, watering and feeding is through drip nozzles into the bag. The foliage and berries therefore remain dry and the quality of the fruit produced is high.
- High cost of set-up associated with the system.
- More attention must be paid to nutrition and good crop management.
The peat in the grow bag should have an open, free draining texture. It is important that the compost does not "slump" as this will lead to a lack of aeration or waterlogging at the roots.
The formulation, the size of the bags and price can vary quite significantly between different supplies.
Water which is very hard ie has a high alkalinity can cause a rise in substrate pH.
The alkalinity is made up of the carbonate and bicarbonate content and water should be tested to determine the levels of these. If the water is very hard then acid should be added to reduce the alkalinity.
The bags need to be raised 1.4 m - 1.5 m (4½ - 5 feet) off the floor so some form of support is required. The essentials for such a support system are:-
- that it is strong enough to support the combined weight of the crops and bags;
- that it presents no impediment to the fruiting truss;
- that it allows drainage from the bag.
The support system can be made from box section steel, small diameter steel tubing or metal gutters. These can either be suspended on chains from the roof of the growing house structure, provided the load carrying capacity is sufficiently high, or mounted on steel posts securely fixed into the ground.
A high standard of design and construction is important with such support systems to minimise associated health and safety risks.
The most widely grown variety is Elsanta which produces firm well flavoured berries. Unfortunately the variety can produce misshapen fruit under low temperatures.
It can also fail to colour evenly giving rise to a white "nose". In spite of this it is the variety preferred by supermarkets.
The relatively new variety Eros has performed well in trials producing high yields of fruit. However, it tends to be softer than Elsanta and therefore less acceptable in the punnet trade.
The variety Honeoye comes into cropping a few days before Elsanta and in terms of yield both perform well under protection. However, the fruit is softer than Elsanta and fruit can quickly become very dark especially in hot temperatures.
The use of ever-bearers will allow harvesting into October/November. Currently available ever-bearers for protected cropping are Evita and Bolero.
These varieties have a very long fruiting season which gives high yields.
For bag culture, ever-bearers should be planted in March/April and the first flowers removed. They will crop in late summer and autumn.
For protected cropping the following plant material may be used:
These large multicrown plants are mother plants which have produced runners in the previous year. Because of their large crown size these plants have the potential to produce a high yield. Due to their maturity they do not cold store well so are normally used for a January planting to produce a crop in May and then discarded. Yields from these plants should be in the region of 560 g per plant.
Waiting bed plants
These are plants produced by planting out a small runner in the spring. The plant is deblossomed and grown on to produce a large plant by the end of the year which is then lifted and cold stored. Waiting bed plants should have a crown diameter of 17 mm plus and may be as large as 22 mm.
These plants can give a yield of around 400 g/plant from a January planting or 280-300 g/plant from a June planting.
15 mm + plant
Sometimes referred to as A+ plants, these are popular for summer planting because they are slightly easier to establish than waiting bed plants. They can yield up to 220 g/plant from a June planting.
It is important that planting material is of a high quality if adequate yields are to be obtained.
For planting in January using crown plants a plant density of 8-10 plants/m2 is recommended.
For planting in June/July using 15 mm plants use 12 plants/m2 and for waiting beds 10 plants/m2.
These are normally planted in April/May at a plant density of 8-10 plants/m2. Plants are established outdoors and deblossomed before moving into the growing house in mid-July.
In order to achieve maximum yields it is essential to have 100% plant establishment and good early growth.
The plants should be kept cool after receipt. When planting the roots should be as straight as possible and preferably left untrimmed. The crown should be level with the top of the bag - if too deep, plants will die, if too shallow, plants may fall out of the bag as fruit develops.
When planting ensure that the roots are moist and that the peat is sufficiently damp.
Always use frequent overhead irrigation until root growth is well established. The leaves begin to grow on cold store runners very quickly and before the root development takes place. The moisture that the plant looses through the leaves has to be replaced by irrigation until such times as the roots have grown sufficiently to take up moisture from the peat. For a January planting the irrigation needs to be used for up to three weeks but for a June planting 10-12 days is usually sufficient. Ideally the irrigation should be fitted to an automatic timer. For January planting irrigation two to three times daily may suffice but for a June/July planting four to five minutes every hour during daylight may be necessary.
The plants can be established outdoors for the first two or three weeks and then moved into the growing house. This allows establishment under cooler growing conditions which provides a less stressful environment for the plant but care must be taken when moving bags into the cropping house so as not to damage the fragile root hairs.
There will be sufficient nutrients in the bag to feed the plants for the initial few weeks after planting, after which liquid feeding will be required.
The feeding can be given by using the proprietary feeds or made up from straight fertilisers.
During spring growth nitrogen and phosphate demand by the plant is high which maximises shoot, root and flower development.
As growth continues potassium demand increases reaching a peak as harvesting commences.
Following harvest there is a period of regeneration, the plants require nitrogen and phosphate to sustain crown production and also initiate flowers for the next year.
Feeding therefore needs to reflect these different stages. From establishment to the start of flowering a feed giving approximately 120 mg/litre nitrogen, 50 mg/litre phosphate and 150 mg/litre potash should be given. During flowering the potassium content should be increased to 250-300 mg/litre.
For the January crop this is the only feeding required. For the autumn crop extra nitrogen and phosphate should be given during September. Extra feeding is likely to be necessary in February/March for the overwintered crop as nutrient levels in the bag at that time are generally very low.
Proprietary feeds which give nutrients in the correct proportions can be used ie 1:1:2 for early growth followed by 1:1:3 during flowering and 2:1:4 or 3:1:6 during the autumn.
The conductivity of the feed should be monitored regularly After planting conductivity should be 800 us/cm increasing to 1200 us/cm pre-flowering, and to 1500-1700 us/cm between flowering and fruiting. These figures should be added to water supply conductivity.
Periodic nutrient analysis of the feed run-off is recommended.
Leaf tissue samples are a useful means of checking nutritional levels in the plant.
As soon as the first flowers open bees should be introduced to assist in pollination.
Either bumble bees or honey bees can be used. Bumble bees are available from a number of biological control companies. While more expensive than honey bees they are more active in cool dull weather, which often prevails early in the year, and are also less likely to become aggressive if temperatures are high. The hive is placed inside the growing house.
Honey bees are best placed outside the structure with a covered entrance to allow access to the crop. If boxes are placed in the house the hives may overheat. Honey bees will not work under cool weather conditions and become aggressive if temperatures are high.
A stocking rate of 1 hive/1,000 m2 is suggested.
Elsanta is particularly affected by poor pollination and without the use of bees a high proportion of misshapen fruits can result.
For summer planted cold stored runners, the temperature should be kept as cool as possible prior to fruiting. Give as much ventilation as possible and if necessary damp down the floors and alley ways to try to keep the temperature below 24°C.
Temperatures above this will reduce plant size and result in smaller berries and reduced crop yield.
If heat is available, temperatures towards the end of September when the weather starts to cool should be maintained at 16°C (day) and 10°C (night). After cropping the temperature should be reduced to 12°C (day) and 8°C (night) for 3-4 weeks. This allows sufficient time for crown regeneration and for the plants to initiate next year's crop.
For the spring crop if heating is to be given temperature ranging of 12°C (day) and 8°C (night) should be used to start growth during early February. Once flower buds are visible, increase the day temperature to 14°C with a night temperature of 10°C and during flowering give good ventilation once the temperature rises above 18°C. In the fruiting phase the day temperature should be maintained at 15-16°C with the night temperature of 8-10°C.
Elsanta requires a period of chilling over the winter to break dormancy and a record of temperatures should be kept from mid-November to late January. If a sufficient number of cold units have not occurred by late January, night break lighting is required to break dormancy.
Managing the crop
Strawberry plants grow vigorously under protection and leaf growth can shade developing fruits.
The leaves should be held back from the fruiting trusses behind strings running along both sides of the row. A number of the fruit trusses will have to be eased out below the strings and as the fruit develops the trusses will hang clear of the leaf canopy. This allows good air movement around the fruit which gives improved disease control and makes picking easy as the fruit is clearly visible. Ripening is also more uniform.
All runners developing before harvest should be removed. If runners are left on the plants they will compete for water, nutrients and light and cause a reduction to yield.
Diseases and pests
This page provide information in relation to diseases and pests that can affect Strawberry Production.
Diseases of major importance
Botrytis or grey mould is a widespread and damaging disease of strawberries. It causes rotting of the green and ripening fruits, quickly reducing them to a soft mass covered with grey dusty spores of fungus.
Botrytis is an inhabitant of dead plant tissue so old strawberry leaves and leaf stalks provide extra material for the fungus to live on and to produce spores. Infection starts during the flowering period although the disease remains inactive until the fruit begins to develop. Infected fruit is a source of secondary infection.
Good control of botrytis can only be achieved thorough protectant cover of fungicide during the blossoming period. Because of the long flowering period apply up to four sprays beginning at first open flower and continuing through until petal fall.
Commence spraying early in the flowering period, even in dry weather as additional later sprays after blossom will not compensate for omission of early sprays, particularly if weather conditions favour development of the fungus.
Thorough wetting of all plant parts, especially the flowers, is required to give disease control. The use of a ruffler bar to aid penetration of the spray into the crown of the plant is recommended.
Powdery mildew thrives under hot dry conditions and is likely to be a problem in the late summer. The leaves of attacked plants show characteristic dark red coloured blotches while the margins curl inwards to expose the underside which can become covered with a white powdery mildew. The infected plants often appear to be suffering from drought. The fungus can also cover flowers and fruit in a white powdery mildew.
The variety Elsanta is very prone to mildew and preventive sprays should be applied frequently.
Red core is a serious disease of strawberries. The fungus lives in the soil and attacks the roots of strawberry plants causing them to rot.
The effects of the disease are normally first noticed in late spring or early summer when diseased plants look as though they are suffering from drought. The plants are dwarfed and they will die in dry weather. The leaves are often dull bluish-green and older leaves are sometimes tinged with red.
If the roots of an infected plant are examined in spring it will be found that the white feeding tips have rotted away. Main white roots may be brown and rotten for various distances from their tips. If these roots are cut lengthways a red discolouration of the centre tissue will be seen. This is the 'red core' from which the disease takes its name.
The disease can be spread on infected runners or by infected soil on implements carried from one field to another. Where drainage is poor or there has been a high winter rainfall, the fungus readily invades strawberries. The fungal spores are carried in drainage water so causing spread of the disease.
Before planting make sure that drainage is good. Growing plants in beds or ridges will help in heavy ground.
Some modern varieties, for example, Elsanta are particularly prone to crown rot. Symptoms of this disease are seen when the youngest leaves wilt and often turn bluish-green. This wilt spreads quickly to the entire plant which collapses normally within a few days. When diseased plants are lifted they often break at the upper end of the crown leaving the main part of the crown and the roots behind in the soil. If the crown is cut open lengthways, browning of the tissue will be seen. It appears that cold stored runners are susceptible to infection by the causal fungus.
Pests of major importance
Several species of aphids (greenfly) can occur on strawberries.
Aphids may cause severe damage especially after mild winters when high populations produce leaf distortion and stunting of plants. In addition, the flowers can be malformed and little fruit develops. Some species of aphids are carriers of virus diseases which can give various symptoms from marginal yellowing of the leaves, reduced leaf size, stunting of the plant, leaves crinkled and distorted, yield and vigour appreciably reduced.
There are a number of approved aphicides for use in strawberries and sprays are normally required in the spring after some growth has taken place but before flowering commences.
Red spider mite
Adult female mites overwinter and become active in early spring. The mites feed on the underside of the leaves causing mottled yellowing. If plants are heavily infested growth becomes poor and stunted with the whole plant appearing bronzed and covered with webbing.
Control can be difficult. Different chemicals act differently on certain stages of mite so it is important to examine the leaves to determine what stages of mite are present.
Where mites are detected, spray programmes should be commenced early in the year to eliminate populations before they have time to build up. A further spray may be required after harvest and this can reduce the risk of serious infestations in the following spring.
Vine weevil is a serious pest of strawberries. The creamy-white legless grubs feed on the roots in the winter and spring, finally excavating deep tunnels in the base of the crown and so causing complete collapse of the plant.
The adult vine weevil feeds on foliage at night during June causing a distinctive marginal notching. Although leaf damage is unlikely to be important, it indicates that damage by grubs may occur later.
Control of this pest is difficult. Adult weevils feed only at night and sprays intended to control adults have to be applied during the hours of darkness. In addition, not all adults feed every night so one spray will only eliminate a proportion of the adults.
Baits can be used to attract adults and these can then be trapped using a sticky base or a funnel trap.
Plants which are especially attractive to adults can be planted around the edge of the crop to encourage adults to feed. These plants can then be sprayed and adults killed.
Chemical or biological control measures may be used depending on conditions and timing.
Post harvest treatment and frost damage
Once the leaf canopy begins to die off, usually by early to mid-December, the plants should be trimmed off and given a thorough fungicide drench against botrytis.
The trimming should be delayed until the leaf canopy has begun to die down as earlier removal can cause reduction of crop yield in the following year. Remove all dead and dying leaves leaving the young short stemmed dark green leaves.
While strawberry plants require a certain amount of cold to break dormancy, too low a temperature during the winter can cause damage to the crown.
In peat bags the plants will still withstand temperatures as low as -5°C without serious damage. Where temperatures less than this are expected the plants should be given some protection - either by heating or by covering the plants with fleece.
Further uses of bags
After cropping in the house, the bags can be used to take an outdoor crop in the following year. While berry quality and size will not be as good a significant return can be gained.
Bags can be placed on a ridge or bed covered with a permeable plastic or on a support system. Feeding and irrigation is required as with the protected crop.
Alternatively the bottom of the bag could be removed and the plants placed on well prepared soil so that roots can grow into the soil. In this case feeding and irrigation is less critical as the plants will be able to draw water and nutrients from the soil. The ability to do this will depend on the development of new roots.
A summary of the information in relation to Protected Strawberry Production
- Ensure the support system is adequate.
- Use peat which is open and free draining.
- Choose a variety suited to your market.
- Use high quality planting material.
- For January planting use 10 crown plants/m2..
- For June planting use 10-12 (15 mm+)/m2 or 8-10 waiting bed plants/m2.
- For everbearers use 6-8 plants/m2.
- Use overhead irrigation to establish plants.
- Feed at each watering once the plants are established.
- Use 1:1:2 for early growth, 1:1:3 during flowering and 2:1:4 or 3:1:6 during autumn.
- Ensure sufficient run-off and monitor conductivity.
- Introduce bees in the spring to aid pollination.
- Avoid high temperatures during the summer, give heat if possible from the end of September, if still cropping.
- Monitor winter temperature to determine if sufficient cold units have been accumulated.
- Maintain an effective disease and pest control programme.