The strawberry plant
The strawberry plant grows rapidly in the spring after winter chilling. Flowers are produced in May and June, depending on variety, or earlier if protected by cloches or tunnels. The flower trusses are initiated in the crown of each plant during the previous autumn and to a great extent this determines the yield in the following year. Flower initiation depends on both day length and temperature.
The trusses emerge in sequence on each crown and the flowers open in sequence on each truss, so giving a flowering period of approximately four weeks. One year old plants usually flower before older plants with only a small number of trusses with fewer flowers on each truss, so producing earlier fruit and generally bigger berries. Older plants produce heavier crops but berries tend to be smaller.
The fruit
Strawberries are self-pollinating. Pollination is achieved mainly by insects but wind movement and moisture may also help.
To produce a well-shaped berry it is necessary for good pollination to be achieved otherwise the fruit will be malformed. After the flower is fertilised it is generally 4-6 weeks before the berry is ripe.
Choice of site
Strawberries are susceptible to spring frosts so it is important to make sure that the site is not a frost pocket. Frost pockets may be caused by anything which prevents the slow flow of cold air away to lower ground. Avoid low lying areas of ground which may collect cold air.
Apart from this the strawberry grows and crops best under sheltered conditions. Wind can seriously reduce growth so a sheltered site with a gentle south-westerly slope is ideal.
Strawberries will succeed on a wide range of soils but a deep medium loam is preferred. Strawberry plants will not tolerate impeded drainage or waterlogging as this will lead to root diseases and death of the plant.
Light soils will have higher temperatures in the spring so growth will start earlier. However, these soils will dry out more quickly in the summer and without irrigation plants may suffer from stress. Heavy soils, while holding moisture and nutrients, may not be sufficiently well drained for modern day strawberry varieties.
Pre-planting treatment
It is essential that the soil is free of perennial weeds before planting. Grasses can be killed using glyphosate (Roundup) but where broad-leaved weeds such as buttercup, daisy, dandelion, docks, thistles or nettles are present, it is necessary to use a specific broad-leafed killer (examples include Banlene, Broadshot or Garlon) prior to spraying with glyphosate.
Ideally allow 10 days between sprays. If weed cover is very dense it may be necessary to give more than one treatment - for this reason it is as well to allow as much time as possible between killing off weeds and preparing the ground, otherwise there may be regrowth of these perennial weeds.
Strawberries benefit from the addition of farmyard manure to the soil before planting. Dressings of 50 tonnes per hectare are generally recommended. Ploughing should be deep and if the ground is likely to be compacted, sub-soiling before ploughing will help break up any soil pans and make sure drainage is not impeded.
Before preparing the soil for planting, soil analysis should be taken and base dressing applied accordingly.
Thorough soil preparation is required; avoid cultivating or working soil for strawberries if conditions are too wet. Strawberry roots are very intolerant of soil that has poor structure.
After ploughing, the soil should be cultivated to produce a rough tilth and ensure good germination of annual weed seeds which can be killed off either by light cultivations or by a contact herbicide used a few days before planting.
Growing systems
There are two types of growing systems for Field Strawberry Production, both are detailed below.
Open ground system
In this traditional system the plants are set at 90 cm (36") apart in rows with 42.5 cm (17") between plants giving a density of 25,317 plants/hectare (10,250 plants/ acre). Planting can take place from the end of August until the end of March when ground conditions are suitable.
From an early planting, that is, before the third week in September it should be possible to achieve an economic crop in the next summer but for later plantings it is advisable to remove the flowers and not take a crop in the first year. This will allow the crowns to build up for the following year.
When planting take care not to let the roots dry out. Do not leave plants exposed at the head of the field - always keep the roots covered with damp sacking. Runners should be planted so that the crown is level with the soil.
If planted too deeply the crown will be covered with soil and leaves have difficulty emerging and the plant may die. If the plant is too high, the roots are partially exposed and the new roots emerging from the base of the crown cannot establish readily. Plants with exposed roots are liable to herbicide damage and frost damage. Make sure the plant is firm in the ground.
When planting use a dibber or planting tool which will make a hole deep enough to avoid doubling up of the roots. Where roots are very long some trimming may take place but it is essential that the roots go straight down into the soil.
Strawberry rows should be no longer than about 65 metres. This reduces to a minimum the time pickers spend walking with their fruit to a collection point. Longer rows can be divided by a cross alley so allowing picking from each end.
Immediately after planting, provided the soil is moist, a suitable residual herbicide should be applied and further applications of residuals will be required on a regular basis. A spray programme can be worked out depending on dominant weed species present.
Runners will be produced in July and in open ground systems runners are allowed to root alongside the original plants forming what is known as a matted row. Excess runners are either cut off or destroyed by sprays of contact herbicide.
It is also possible to use cold stored runners planted in May/June. For treatment of these, refer to the section on raised bed systems.
Raised bed system
Where crops are to be planted through polythene it is advisable to lay two lines of T-tape along the top of the bed before the polythene is laid. This will allow fertigation during the growing season. While this may not be of as much benefit in our damp climate as in a drier climate, there will usually be some time during the season when fertigation will be beneficial and in certain years it will greatly enhance yield. A moisture meter can be used to determine when irrigation should be given.
Generally black polythene is used and this causes the soil to warm up early in the spring so advancing the crop. However, it can be a disadvantage in hot summers as the surface of the polythene can become very hot and fruit lying on it can 'cook'. Blue polythene can be used - this does not warm the soil as much as the black but conversely it does not get as hot in the summer.
The beds can be made up and T-tape laid and covered with black polythene any time when soil is easily workable. Do not burn the holes in the polythene until near the time of planting. There are then two options:
- Burn or cut holes about a fortnight before planting to allow annual weed growth and then use a contact herbicide, for example, Gramoxone or Challenge over the beds to kill this off immediately before planting.
- Burn or cut holes just before planting and either use a low dose of residual herbicide such as Dacthal to prevent germination or low dose Betanal E at a later time to kill off weeds that have germinated. Do bear in mind, however, that when using herbicides over polythene beds there is a chance of the chemical concentrating in the planting hole and causing crop damage.
As with the open ground system planting can take place from the end of August until the end of March but in recent years it has become popular to use cold-stored runners planted in May to give a small crop in late July/August and come into main crop in the following year.
When planting cold stored runners aim to plant only as much in a day as you can irrigate. For the first 10-14 days this irrigation should be in the form of a low volume overhead sprinkler system connected to a timer. The aim of this irrigation is never to let the crown dry out as it takes the roots some time to develop. After approximately 10 days, roots should become established and overhead irrigation after this should be given perhaps twice a week for the next 10 days and after this can be dispensed with. By this time there will be sufficient roots and if further irrigation is needed this can be given through the T-tape.
Early strawberries
Early strawberries are produced by protecting the plants with polythene covers either as low tunnels, floating films or portable 'French' tunnels.
Low polythene tunnels
Low tunnels are made of polythene (38 micron thickness) suspended over wire hoops and kept in place by tying twine over the polythene and securing to either side of the hoop.
Crops are usually covered at the end of January/early February.
Floating film
This is made of clear whole polythene (200-500 holes/m2) and available in 10-12 m rolls.
The film is laid over the rolls and anchored at the edges by using polythene bags weighed down with soil or sand or some similar heavy objects.
Portable 'French' tunnels
These are walk-in tunnels normally covering three raised beds. The hoops, placed at two metres apart, drop into T-pieces which are driven into the ground and covered by 600 g polythene in a similar manner to low tunnels. Rope or baler twine is used to hold the polythene in place by passing it over the hoops and round the base of the T-pieces. In addition, extra twine between the hoops secured to posts driven into the ground along the sides of the tunnel are recommended for extra anchorage as these tunnels can be blown away in high winds if not well enough secured. The polythene cover needs to be pulled tight initially, usually done by using a tractor, and the polythene either buried in the ground or tied and secured to a stake at either end.
These tunnels provide better ventilation than low tunnels and the polythene does not need to be removed for picking. The sides can be pushed up to give ventilation in warm weather and every effort should be made to avoid temperatures over 25°C.
Points to Note
- It is best to cover young vigorous plantations.
- If plants are grown in open ground, residual herbicide must be applied before covering.
- If there is a lot of leaf trash, this should either be removed or alternatively a drenching spray of Elvaron applied before covering.
- Crops should be free of pests before covering
- In warm conditions under polythene, pest populations can build up quickly so after covering crops should be inspected regularly and if pests appear, remedial action taken.
- A good botrytis spray programme must be undertaken.
- With low tunnels, the sides should be raised up once flowering commences to allow pollination.
- With floating film, once flowering commences, the film should be removed during the day and replaced at night to allow pollination.
- In French tunnels, the sides can either be pushed up to allow pollinating insects access or pollinators can be placed in the tunnel.
Harvesting
The most important operation in strawberry production is picking. Fruit becomes quickly over-ripe and once picking gets behind, subsequent harvesting is made more difficult as rotting and over-ripe berries slow down picking of the rest of the crop.
Supervision is important, particularly where children are used to pick the crop. This supervisor should allocate pickers to particular rows, ensure that the rows are picked cleanly before allowing the picker to move on to another row. They must also ensure that the picker does not damage either the plants or the unripe fruit.
The berries should be picked by nipping off the stalk and not holding the fruit. As the fruit is soft, it bruises very readily and the bruising shows up within a few hours of picking, spoils the appearance and reduces shelf life of the fruit.
Pickers should grade the fruit as they pick, ensuring that all fruit in the punnets is of an acceptable grade for the market for which it is intended. Unmarketable fruit should be picked into separate containers and removed from the beds.
Cooling and storage
When strawberries are picked on a sunny day in July, field heat of the berry can be up to 30°C. To reduce the temperature, the berries should be picked early in the morning and immediately placed in cool shade or a cold store.
Where fruit has to be picked during the heat of the day, it should be moved as soon as possible into cool conditions. Where a cold store is available the fruit should be put in as quickly as possible and cooled to 4°C within six hours of picking.
Post harvest care
After harvest strawberry plantations are often weedy, particularly if the herbicide programme has been unsuccessful and the season has been wet. It is important to clean the beds immediately after picking since plants rapidly make new roots. Any delay in the removal of weed competition can reduce the potential of the crowns for next year's crops.
Where plants are vigorous, removal of the foliage by mowing off all the leaves is recommended. However, it is important that this is done before the end of July or the crop the following year may be reduced. It is important after mowing off that the plants receive sufficient moisture to allow for rapid regrowth. Weak plants or 60-day cropping plants should not be mown.
The leaves should be mown approximately 10 cm (4") from the ground. This mowing will also remove weeds and the trash should be removed from the bed.
In open ground systems the bed must be cleaned sufficiently to allow use of a residual herbicide - normally simazine - and a potash fertiliser applied. The alleyways should be sprayed using either residual herbicides or if weeds are present, using a contact herbicide either tank mixed with a residual or used some weeks prior to application of the residual.
Diseases and pests
This page provide information in relation to diseases and pests that can affect Strawberry Production.
Diseases of major importance
Botrytis
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.
Mildew
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
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.
Crown rot
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
Aphids
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
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.
Essential machinery
A good sprayer is essential. Most products recommended for botrytis control are non-systemic and therefore good control can only be achieved where there is good coverage of the flowers.
Air assisted sprayers are probably top of the range, giving both good coverage and use of lower chemical rates. However, a special strawberry boom fitted with ruffle bar to a field sprayer will also give good results.
Also have an inter-row sprayer so that weeds in the alleyways and, after harvest, any runners remaining in the alleyways can be burnt off. This machine can also be equipped with small disks to sever the stolon from the mother plant. This is an advantage.
Runners and trash lying on the surface of the polythene also need to be removed. There are machines available for this although these are expensive. Alternatively these can be removed by hand.