Growing blackcurrants for the commercial market is a precise science, and many factors must be considered to give an adequate return for the grower. As with most horticultural crops attention to detail is critical, as any omissions, however small, can cause loss in plant health and decrease in yields. The longevity of a plantation is also critical to the return on capital and so great care must be taken to ensure that the blackcurrant bush remains healthy. A frost in late April or early May can decimate the crop, losing the grower thousands of pounds value overnight. In the following sections you will find precise details on how we grow blackcurrants commercially, including siting, best soil types, biological details of plant development, risks in growing, nutritional requirements, weed control and how we cope with prevalent pests and diseases.
The initial siting of the blackcurrant plantation is one of the most important factors of growing a healthy plantation. Blackcurrant flowers are easily damaged by spring frosts, consequently it is important to have a sufficient downhill slope to permit cold air to drain to lower levels, and to shelter the plantation from the NW through to NE winds. Cold air behaves like water in finding the lowest level but is slower moving. Exposed sites are avoided as strong winds during spring and summer can easily damage the bushes and strip both flowers and fruit.
Many plantations are planted adjacent to existing woodlands or hedges, but where it is necessary to provide additional shelter, windbreaks are established. Historically, species suitable for windbreaks include Alder (Red, Italian or Grey but not Common Alder) for in-field planting, and Pines, Alder or Birch for perimeter protection. Poplars, once so common, are now not used as they are very water demanding. However where possible multi-species windbreaks are now encouraged to create a wide range of habitats for wildlife.
Not only should a plantation be planted on a suitable site, but the soil must also be appropriate for the blackcurrant bush to flourish. Blackcurrants produce both surface & deep roots requiring well-drained and aerated soils with a good capacity to retain moisture. Soils with a high water table or imperfect drainage lie wet in winter and this will lead to the death of the deep roots and reduction in growth and yield the following season. The pH of the soil should ideally be between 6.5 and 6.8.
Any soil having a low organic matter content (organic matter is so essential for worms) may not be satisfactory regarding fertility & moisture retention and would need to be improved by ploughing in farmyard manure or crop straw before planting. Blackcurrants are tolerant of a wide variety of sites and soil types, judging by the range of locations and latitudes chosen by growers for this crop.
Soil texture, depth, drainage and moisture holding capacity are limiting factors affecting bush growth. Improvement by deep cultivation, sub-soiling, mulching & incorporation of organic manure, and attention to drainage will promote root activity and help to sustain overall bush development.
Water availability is a function of soil depth & organic matter content, and any water shortage can be augmented by irrigation. Plantations on high ground tend to be cooler and receive more rainfall than those at sea level.
Having established the most favourable site with suitable soil, the grower is ready to begin planting. The grower must also choose a range of varieties, suitable for their location; for example, later flowering varieties can be planted in more frost susceptible areas, as they are less likely to be damaged by spring frosts during flowering. Most growers have at least 3 varieties in their plantations, as they want to spread their harvesting dates throughout the picking season.
Planting can take place from October to March. The soil must be reasonably dry and crumbly when cultivated for planting, so that the roots are not impeded when they start to grow. It is important to complete the planting work well before any buds begin to swell. Planting in autumn will lead to the development of some fibrous roots capable of supplying the moisture and nutrients required when growth commences in spring.
Blackcurrants are planted in rows as either rooted bushes or cuttings (literally 8" long strong twigs cut from healthy bushes after defoliation). The first year they are cut back to promote further healthy growth; the second year they grow a sparse crop of ½ tonne/acre and it is not until the third year, or 30 months after planting that they finally yield a crop that begins to pay back the initial planting investment. Most commercial growers average about 100 acres and each acre has around 4,000 cuttings per acre. The optimum yields of 4 – 6 tonnes/acre are usually in years 4 – 8 depending on bush health and season after which time the yield starts to drop away.
Many plantations are planted with grass between every row of bushes – these have to be mowed on a regular basis and on a 100 acre plantation there could be as much as 80 miles of mowing at one time!
Blackcurrants are best established by planting a rooted 1-year old bush which must be from virus tested clean stock, or more commonly, by planting cuttings direct into the cropping site at the spacing selected. This is cheaper and quicker than planting "roots". Planting cuttings can be done through black polythene mulch as this will help suppress weeds and conserve moisture. However, black polythene provides an ideal habitat for vine weevil, and therefore growers do not use this where this pest may be a problem.
Rooted bushes are normally planted in a furrow, deep enough to accommodate the roots, which is then covered with soil immediately to prevent drying out, and firmed prior to backfilling by tractor. After planting, the shoots are cut off just above the soil level. These shoots may be used to produce cuttings for further planting.
Cuttings are prepared from the basal portion of strong hardwood shoots, discarding the thin tip portion together with weak or mildewed growth. These are planted at a slight angle instead of bolt upright, to help promote rooting. The plantation sizes are generally between 5 and 10 acres. Cuttings, planted as 8" twigs, never cease to amaze even the most cynical of growers; as in spring these buds break and burst into life, putting down their roots. By the end of the first growing season they may have grown as many as six new branches and up to 2 foot high.
Each year blackcurrants make new growth consisting of shoots or side branches that range in length from about 5cm to lm. The strongest shoots usually grow from buds at or below soil level, providing valuable replacement for older branches and contributing to the increase in the overall size of the bush. The number of new basal shoots produced each season is small by comparison with the total number of side shoots developing from the branch system.
In order to sustain high cropping levels, production of high quality extension growth has to be maintained to ensure that the total number of flower buds shows a steady increase from one year to the next. It is also important to have a high percentage of young buds, on one-year and two-year old cropping wood as they produce more flowers, larger fruit and normally are more closely spaced compared with buds on older branches. The causes of irregular cropping may be traceable to some of the factors listed below which may adversely affect the performance of the plantation or undermine its yield potential.
Blackcurrants remain dormant from the time the leaves fall in October until the following March when the buds again swell and burst in response to rising temperatures and increasing day length. Exceptionally, after a very prolonged cold winter, bud break might be delayed until April. A very mild winter may lead to bud development during February, when the absence of the requisite minimum period of winter chill will be accompanied by uneven bud break on sensitive varieties.
There is increasing evidence that the amount of cold experienced by blackcurrant cultivars, in some regions, in some winters, is inadequate; leading to delayed and uneven bud break, with consequent adverse effects on yield and quality. The disruption of the normal pattern becomes apparent from the reluctance to leaf out by buds, apart from those at the tips of the shoots. Eventually a proportion of the delayed buds comes into leaf, but they might not flower or crop as well as normal. Other buds remain dormant throughout the entire season; although when dissected they are likely to contain a normal complement of leaf and flower initials.
The majority of buds on healthy shoots should develop flower initials with the exception of a small percentage of tip and basal buds that remain vegetative. The production of flower initials by the buds commences in June, provided the nitrogen status is sufficient to initiate the process. By the end of summer, flower formation should be nearing completion, and by October the embryo flower clusters can be identified within the interior of the buds when they are dissected.
Flower formation is liable to disruption when the bushes are under stress from drought, disease or insect attack. Leaf loss occasioned by disease infection causes premature bursting of buds and flowers in late summer/early autumn, and a generalised reduction in vigour and yields the following year.
The fruitlet develops after a certain number of ovules have been fertilised and viable seeds produced. Where an individual fruit has produced fewer than 35 - 40 fertile seeds, it might not be able to develop any further, and could drop prematurely some weeks later. This premature dropping is part of the process familiar to all growers called "run-off." Where the requirements for satisfactory cropping have not been met in full, the crop prospects could be adversely affected but for the majority of plantations, frost remains the biggest single limiting factor.
An important factor over which growers can exercise control is the availability of soil moisture as any deficits can be corrected by means of irrigation, and by maintaining a high level of organic matter in the soil. This helps to ensure the mobility of the nutrients that are necessary to ensure satisfactory development of the leaf canopy & the fruitlets.
Phosphates requires moisture for its uptake by the fine feeding roots, and is recognised as the most critical element affecting fruit setting.
During periods of cold dull weather accompanied by a drying wind, the rate of plant growth will be reduced, as also will be the availability of nutrients if the soil is dry. Growers frequently use foliar feeds to enable the leaf canopy to absorb nutrients in a soluble form when climatic conditions are unfavourable.
The interval in days between the dates of first open flower & start of harvest is remarkably constant for each variety, and is only marginally affected by seasonal variations.
Blackcurrant flowers are attached to a stem or "strig" with the oldest flower at the base nearest the bud and the youngest at the tip. Each "strig" carries from 10 to 20 flowers and some buds also produce 1 or 2 shorter secondary strigs. Following bud burst and the emergence of the first 2 leaves, the flower clusters begin to be visible and from this stage they become susceptible to frost. The flowers change colour from pink/green to red as the clusters emerge fully to hang from the bud cluster in the characteristic "grape" stage. The flowers open in succession, starting at the base of the strig. The basal flowers are less exposed and obtain better nutrition than the tip flowers which may be frosted or fail to set fruit. Basal fruits may be twice the weight of later pollinated tip fruits, set more seeds & have longer to grow.
The anthers are the male organs producing the pollen. They burst open when they are ripe, releasing the powdery pollen that is distributed by the wind when it is dry, as well as by bumble bees and other insects, mainly flies. The pollen grains may drift or be carried some distance before alighting on a stigma. These become sticky when receptive, helping to retain the pollen. The stigma is sited in the centre of the flower at the termination of the style, down which the pollen tubes must grow to effect the fertilisation of the ovules or embryo seeds. When a pollen grain succeeds in landing on a style, it will germinate by sending out a pollen tube that will grow at a rate governed by the temperature. In warm weather it could reach the ovule within 48 hours of germinating; conversely low temperatures slow down the growth rate so that the tube might not reach its objective until 7 - 9 days later. This would be beyond the period when the ovule can be fertilised.
Most blackcurrant varieties flower soon after mid-April, when they are still at risk from damaging frosts and cold winds. A few varieties flower later, notably Ben Alder & Ben Tirran, which begin flowering at the beginning of May.
Blackcurrant flower tissue is susceptible to frost damage as soon as the "grape" clusters can be detected within the expanding bud, normally at any time after mid-March, and when the temperature falls to -1.9°C (28.5°F) or below. During frosty spells, the temperature on a still night may drop slowly to give only a short time below the critical temperature. Provided the flowers remain dry, it is unlikely that damage will ensue from a period of up to one hour at a temperature of -1.9°C. Protection will be gained from the "canopy" effect of expanding leaves as well as from miniscule pockets of still air acting as an insulator within the flowers.
The symptoms of frost damage will show up within 24hr - 48hr following a severe frost. The entire flower may be blackened or only the stigma and anthers change colour. Alternatively, the ovary (the shiny green "receptacle") might appear wrinkled or blanched. When the flowers have suffered chilling or the effects of a succession of minor frosts, the symptoms take longer to appear. Either the flower withers, becomes papery or then drops off, or the sepals and ovaries gradually become deep red in colour, before they eventually drop off.
Wind frosts are usually a more serious threat to the grower than radiation frosts because heat will be removed from the tissues more rapidly. With a wind frost, the external cold air movement rapidly cancels the insulating effect of air space inside the flower buds. Some protection may be gained from shelter belts and areas of nearby woodland, where they have been positioned so as to deflect any cold air currents away from the plantation.
The technique of protecting plant tissue from frost damage by continuous water sprinkling was devised at East Malling Research Station by Rodgers and Modibowska. Water is sprayed onto the planations using hundreds of irrigation sprinklers, and by ensuring that the sprinkler nozzles rotate at least once per minute, the temperature of the ice that forms around the flower buds is prevented from falling below freezing point even though the surrounding air may be at a lower temperature.
Frost protection is mostly used in the east of England, but with present day later flowering varieties growers are not generally investing in this type of system.
To successfully produce blackcurrants, growers implement a nutrient programme designed to promote growth and yield: nitrogen is directly related to strong plant growth; phosphates for growth, fruit setting and crop yield; potassium promotes growth in individual shoots and increases the weight of individual fruits; magnesium, being a constituent of chlorophyll in leaves, helps increase yields through interaction with potassium; calcium is required for cell division and enlargement, therefore essential to young plants and buds.
Soil samples and leaf analysis throughout the season determine the levels of nutrients available to the plant and where there is insufficient, they can be added either as foliar feed or to the soil during the winter.
Traditionally, blackcurrants have been looked upon as gross feeders requiring large quantities of bulky manure's rich in nitrogen. Present day practice is based on a more restricted use of nitrogen, often applied in the form of split dressings to provide a sustained level of nutrition. The availability of nitrogen is the principal factor governing the amount of vegetative growth produced. Nitrogen also plays an important role by increasing flower numbers in proportion to its availability within certain defined limits.
In the soil, the chief source of nitrogen is from the breakdown of organic matter. Maintaining the soil organic matter content therefore contributes equally to the availability of nitrogen & to the soil moisture retaining capacity of the plantation. To help this, growers plant grass or clover mixes in-between rows of bushes. Not only does this practice improve the soil organic content and structure, but it also helps prevent soil erosion, and encourages beetles and other insects.
Recommendations on the amount of nitrogen to be applied to plantations are not easy to formulate, as many factors have to be taken into account. Perennial fruit crops utilise nutrients in developing a system of roots and branches, which increases every year. For blackcurrants, the parts being harvested comprise only the fruit, which contains a moderate level of nitrogen. All other plant material including leaves & prunings remain on site to be recycled, thus returning nitrogen to the crop.
Successful establishment requires the bushes to be grown to a harvestable size as quickly as possible, in order that it is able to produce consistent, and good quality, crops. One of the grower's objectives should be to ensure that the nitrogen will be available as and when required by the crop.
The requirements of young bushes under establishment are best dealt with by a manure or green organic crop, if available, ploughed under to provide slow release nitrogen, potash & phosphate. Failing this, a good compound fertiliser can be substituted. A suitable analysis would be 15:15:20.
Plantations in their second or third growing years, require nitrogen to build up big strong bushes to provide the type of framework required to sustain future cropping. Nitrogen is essential from mid-April until mid-July when the growth rate diminishes in line with the decreasing daylight hours.
For the mature plantation, the bush framework is fully developed and applied nitrogen requirements are less than in the formative years due to the substantial quantities returned in the form of fallen leaves and prunings. The nitrogen content of the year's crop of leaves and prunings could be up to double that removed in the harvested fruit. Much of this nitrogen will be returned following the breakdown of organic material within the plantation - the resultant recycling of nutrients enables bush vigour to be maintained by the use of a reduced input of fertiliser compared with younger bush requirements.
The availability of phosphorus is essential in establishing young bushes and it is of critical importance in respect of shoot growth & overall length, fruit setting and crop yield. Phosphates require adequate soil moisture for mobility and this encourages root growth and uptake of nutrients. Soils with a high organic content and those to which animal manures have been applied, will benefit from the increase in water holding capacity as well as the phosphates liberated following the mineralisation of the soil organic material.
Fertilisers containing water-soluble phosphates are quickly available to plants, whereas ground mineral (rock) phosphates are not water-soluble and whilst cheaper, are of value only on fairly acid soils where phosphate requirements are limited.
Phosphate levels are maintained in plantations by having soil and leaves analysed on a regular basis, and applying phosphate in line with current recommendations.
Potassium, or Potash as it is sometimes known, has a very significant effect on shoot length, which is independent of the availability of nitrogen & magnesium. This nutrient also plays a critical part in increasing the weight of individual fruits and therefore of overall yields. Any deficiency is likely to lead to reduced flower numbers.
Potassium is also of very great importance in the water status of plants which have a lower transpiration rate when they are adequately supplied, leading to increased cell size & water content. High applications of potassium fertilisers are required to support continued intensive cropping.
Due to its importance in metabolism & the fact that its content in blackcurrant fruit is around 2%, the availability of potash to the bushes must be ensured otherwise crop yields will suffer.
Clay soils are generally rich in potassium, whereas organic soils, peat's and free draining acid sandy soils will be deficient due to their lack of potash bearing minerals. Potash may be lost due to fixation in certain soils containing mica, and very large amounts of potassium fertilisers would then be required to rectify the shortage.
As with phosphates, levels are maintained by using leaf and soil analysis.
Magnesium is a constituent of chlorophyll in the leaves of plants and it interacts closely with potash in nutrition. In blackcurrants the response to magnesium increases as the availability of potash increases. By increasing the availability of potash, increased yields can be obtained if the level of magnesium is sufficiently high. Like potash, the amount of magnesium in the soil is highest in clays, whilst on sandy soils leaching may result in low availability.
Magnesium appears to be an important nutrient for fruit of all kinds, although blackcurrants can tolerate small deficits without serious effects. Deficiencies are most likely to arise on acid organic soils subject to leaching as well as on very free draining soils, which have been well limed. Excessive potassium levels may give rise to a magnesium deficiency.
Calcium & Lime
Soils which are well supplied with calcium have a higher pH level and contain more carbonates, nutrients and nitrifying bacteria compared with peats & sands which may be deficient. In these calcareous soils, heavy metals are less soluble so that iron deficiency - chlorosis - is more likely to occur.
In plants, calcium is required in cell division and enlargement, being essential to the growth of root tips & the growing points of young shoots. Pollen tube growth also depends on the availability of calcium. This nutrient is not easily transported in plant tissues - the youngest leaves & shoots may show deficiency symptoms even though older tissues contain adequate levels of calcium.
Applications of lime will increase the pH & improve the structure of non-calcareous soils. Liming is important on these soils to replace calcium, which might be lost as a result of using acid fertilisers, and through nitrification & leaching. Lime leaches out more rapidly in high rainfall areas.
As Growers have become more environmentally aware, they have largely replaced chemical weed control between the rows with plantings of grass or clover mixes, controlled by mowing. This has removed the need for wide-spread herbicide use. In addition, the headlands of plantations are grassed down, and mown. For the area directly under the bushes, there are two main control methods available to growers. Either small amounts of specific herbicides can be used to control weeds, which would otherwise severely restrict the growth of the bush, or polythene mulch can be used at planting time to prevent weed growth. More environmentally beneficial practices are being investigated by using weed burners, to replace the use of herbicides under the bush.
Pest & Disease
Control Insect pests do occasionally attack blackcurrants, like any other crop, but growers have learned that some infestations can be tolerated and will clear up naturally, as the pests' natural enemies and parasites eventually bring populations back into balance. Growers have put an enormous amount of research into this area in the past decade, and the war against pests is an interesting one. Some of our successes are described below.
Vine Weevil is known to many gardeners to be a problematic nocturnal pest. The first signs of attack are the 'notches' around the edges of the leaves caused by feeding damage. They are not the real problem though as this is mainly cosmetic damage. The real problem is the larval stage that lives underground and feeds on roots – they are not fussy either, as they feed on more than 150 species of plants, but they do like to live in light soils or peat. When numbers build up, Vine Weevil infestations will seriously weaken bushes, even causing bush death in hot weather as bushes struggle with a much-reduced root system.
The main Vine Weevil predators in blackcurrant plantations are predatory beetles – mostly from the Carabid family. The commonest ones are Harpalus rufipes (strawberry seed beetle), and Nebria brevicollis (no common name), but the most obvious is Carabus violaceus (Violet ground beetle). It's obvious because at 20-30 mm long, they are the one of the largest ground beetles in the UK, and they will consume large numbers of Vine Weevil adults and larvae. Grass alleys between the rows, and more specificly grass "beetle banks" do a great deal to encourage the developments of these beetles, and growers work hard to maximise their numbers. Aphids are also familiar to gardeners, who will know that there are many different species, but there are just 3 that occur commonly on blackcurrants.
Aphids have enormous reproductive potential, and their numbers can build up exponentially under favourable conditions. They damage plants both directly, by stunting through feeding on the sugar rich sap, and sometimes causing distorted growth, but also indirectly. Their food source is plant sap, which is sugar rich but very low in protein. In order to obtain adequate protein, aphids suck up great volumes of sap, extracting the protein and excreting excess syrup through siphunculi (a small pair of tubes on their abdomen that look like 'twin exhausts'). It is this syrup (honeydew) that causes stickiness on leaves and eventually sooty moulds to grow.
The most common aphid predators in blackcurrants are Ladybirds and their larvae, and Orius predatory bugs, mostly Anthocoris nemorum, which hold small insects motionless in their forelegs and suck them empty with their needle-like mouthparts! Parasitic wasps such as Aphidius matricariae are numerous, and lay their eggs in aphids, and the eggs go through 4 larval stages inside the aphid, which swells and stiffens into an enlarged golden brown 'mummy', before the wasp leaves through a small hole in the abdomen.
Two Spot Spider Mite numbers can build up in warm weather. The colonies establish themselves on the underside of leaves that show telltale speckles at first, before going brown and falling off. Damage is from sap feeding, and eventually loss of leaves.
Spider mite populations are normally held in balance by predatory mites – they are only really visible with a good hand-lens, so have no common names. The most important ones are Typhlodromus pyri and Amblyseus cucumerus.
Managing plantations to take advantage of natural predators and parasites takes great knowledge, skill and careful monitoring, but is increasingly used as part of a package of integrated fruit production.
However, the biggest challenge that has faced, and continues to face, our industry is the Reversion Virus. This disease is carried by the Gall Mite (or Big Bud Mite) which lives in large and overgrown buds, and has proven to be almost impossible to kill. If reversion takes hold of a plantation it can spread very quickly and eventually all reverted bushes will carry no crop. Fortunately through our Breeding Programme (see Breeding) varieties have, and are being developed, that are resistant either to Gall Mite or to reversion. This has been done over the past 30 years by crossing the gooseberry, which is resistant, with the blackcurrant, and then using conventional plant breeding to breed out the gooseberry element! Resistant varieties are now being widely used.
Other Pests and Diseases include Leaf Curling Midge, Capsids, Red Spider Mite, Midlew and Leaf Spot. Different varieties are more or less susceptible to certain Pests and Diseases, and again conventional plant breeding is being used to select varieties that are resistant, together with the search for natural predators.
With increased resistance to Pest and Diseases on new varieties, there are now many opportunities to reduced chemical applications. With careful crop monitoring and Growers' commitment to biodiversity, the industry as a whole is increasingly committed to pesticide reduction.