Advice
RHS Help & Advice
Blueberries
Blueberries, cranberries, bilberries and lingonberries all come under the Latin genus of Vaccinium.
North American highbush blueberries (cultivars of Vaccinium corymbosum) are mainly deciduous shrubs 1-2m (3-6 ft) in height. Grown mainly for their fruits, they also have ornamental value, with autumn colour, red winter stems, and small, pretty bell-like flowers in spring. Some cultivars are evergreen in mild areas.
Half-high blueberries are hybrids between Vaccinium corymbosum and Vaccinium angustifolium. They are good patio and container plants, reaching only 45cm-1m (18-38in) in height.
Preparation, planting and position
An open, sunny, frost-free, sheltered position is best. Partial sun may do if full sun cannot be provided.
An acid soil is essential (pH 4.5-5.5). Acid sand and peat soils are the natural habitat of wild Vaccinium species. Soils that support rhododendrons, heathers and camellias without effort are usually suitable for blueberries and related fruits.
Rich clays or loams are less suitable, and a chalky or alkaline soil is definitely unsuitable. Neutral soils can be suitably acidified with acidic mulches and fertilisers. On alkaline or heavy clay soils, it is best to grow blueberries in tubs of John Innes Ericaceous compost.
Blueberries and cranberries will not grow in waterlogged soil. Good drainage is essential.
Prepare the ground by digging in composted sawdust, composted pine bark. If none of these are available, you can use peat. Plant bushes after leaf fall (November - March). Tip back the branches and remove any flower buds before planting, so that the plant can concentrate on establishing its roots in the first year of growth. Addition of manure to the planting hole is not advisable. Mulch newly planted blueberries with composted sawdust, sand, leaf mould, chipped pine bark, or peat (if no alternative is available).
A 1.5m (5ft) spacing between plants is suitable for most blueberries, although some of the most compact cultivars can be planted at 1m (3ft) spacings. Container-grown plants usually establish better in the garden than bare-root plants. For pot growing, start with a plant in a 2-litre pot, and check the rootball every two years to see if it needs re-potting into the next size up of container. A 50-litre pot may be needed eventually.
Aftercare
Water blueberries with rainwater rather than with mains water. In dry spells, it may be necessary to supplement with mains water. Feeding with an ericaceous fertiliser, and mulching with acidic organic matter is necessary to counteract the alkalinity of mains water.
It is important to water little and often. Blueberries, and in particular cranberries, must not be allowed to dry out. Water the soil under the branches, and apply at the rate of 50-litres per sq m (11 gallons per sq yd), adjusted according to the area taken up by the bushes.
Feed after pruning with 50g per sq m of Growmore (or another balanced fertiliser having equal proportions of nitrogen, phosphorous and potassium), plus 15g per sq m of sulphate of ammonia.
Pelleted poultry manure is a suitable organic alternative (apply at 150g per sq m), but as it is usually alkaline, it should be applied with flowers of sulphur (at 15g per sq m) to counteract this alkalinity.
Mature bushes benefit from an additional 50g per sq m of sulphate of ammonia in June, if cropping is heavy or growth is weak. Alternatively, a liquid ericaceous plant fertiliser can be given fortnightly during the growing season.
Mulching with acidic organic matter in spring and autumn helps to keep the pH low, as well as to conserve moisture and keep down weeds. Suitable materials include composted sawdust, composted or freshly chipped pine bark, peat, leafmould and pine needle leaf mould. Avoid manure, mushroom compost, ordinary John Innes compost and wood ash, as these all contain lime and are alkaline in pH. Cranberries respond particularly well to mulching with sharp sand.
Pruning
Blueberries fruit best on wood that is one to three years old, although older branches will also produce a little fruit. New plants therefore need no pruning for the first three years after planting. Subsequently, bushes are pruned to encourage vigour and to maintain an open centred shape.
Blueberries have two flushes of growth. In the spring they bear flowers on the tips of the previous season’s growth. These flowers become the first crop of berries. New sideshoots usually develop just below these first berries. Later in the season (usually July), strong new shoots grow up from the base of the plant, and develop flower and fruit buds at their tips. This second, later crop of buds and berries are usually plumper than the first, producing larger fruits. Production of this second crop of renewal growth and fruit is a sign that the blueberry plant is in good health.
Pruning is best carried out in February or early March, when the fruit buds are visible, but may be done at any time during the winter. Remove any crossing or horizontal shoots, any weak or twiggy stems and branches, any diseased or damaged growth, and any stems that are dragging on the ground. Remember to remove the twiggy growth at the ends of the branches that fruited last season, effectively tipping them back to a strong, upward-facing bud or branch. Then cut out one in six of the oldest stems from the base of the plant, removing the woodiest, thickest and most unproductive examples. From one sixth to one quarter of the total growth can be removed from the bush each year.
Hard pruning is recommended, as it tends to result in larger, earlier fruits, and to encourage earlier and more vigorous renewal growth.
After pruning, mulch the bushes with suitable acidic mulch from the list of choices above.
In Scotland and colder parts of Northern England, the second flush of renewal growth produced in the summer may fail to ripen. Dieback can occur, with hollow wood below the dead tips. In this case, the affected shoots should be cut back to sound wood, 15cm (6in) from the base.
Flowering and pollination
Blueberries, cranberries and related berries flower in spring, usually from March to May.
Even blueberries that are partially self-fertile benefit from cross-pollination by other cultivars. Three cultivars are recommended for optimal pollination and yield.
Different cultivars flower at slightly different times, but sufficient overlap usually occurs for pollinations to be successful. Some very late season blueberries may not cross-pollinate an early flowering cultivar.
In very cold areas, the earliest flowers can be at risk of frost damage. A double layer of horticultural fleece thrown over the plant will usually provide sufficient extra protection if frost is forecast. Some of the early-fruiting cultivars actually flower quite late but then ripen quickly, so cultivar selection is important in frost-prone areas.
Harvesting
Blueberries should be picked in stages, as they ripen, from late summer to early autumn. Fruit in the same cluster may ripen at different times, and four or five pickings may be necessary over the course of the harvest. Each plant generally provides 2-5kg (4.5-11lb) of fruit.
Cranberries and lingonberries ripen later than blueberries, being a mid- and late autumn crop, rather than a late summer and early autumn crop.
Blueberries are picked by rolling the berries between the forefinger and thumb to remove them from their stalks. They should come off easily, and feel soft to the touch. Fruit generally turns blue and then develops a white surface bloom over a period of a few days. It is only then that they are ready for picking.
Blueberries have excellent keeping qualities, keeping fresh in the fridge for at least a week.
Pests and diseases
Birds are the main pests, and bushes will need the protection of a fruit cage, or nylon netting stretched over a wooden frame.
Weeds can be kept at bay with regular hoeing or by application of a contact weedkiller such as Weedol.
Botrytis (grey mould) can be a problem in damp and poorly ventilated sites. We recommend increasing plant spacing, pruning for openness, and improving ventilation.
Phytphothora fungi can affect the roots of Vaccinium plants grown in insufficiently drained soils.
Blueberries are usually trouble free in the UK. Vine weevil can be a serious pest of containerised plants, and winter and tortrix moth larvae can occasionally decimate leaves and young fruits.
Propagation
Blueberries can be propagated from cuttings. Evergreen species tend to grow better from semi-ripe cuttings (taken in late June - early July). Deciduous species grow well either from semi-ripe or from softwood cuttings (taken in late spring). Select shoots that are 10-15cm (4-6in) long, and tear them off the main stem with a ‘heel’ of older wood attached at the base. Remove the lower leaves, and dip the base and heel in a hormone rooting powder or liquid. Insert the cuttings into a mix of 25% soil-free ericaceous compost, and 75% sharp sand. Place in a heated propagator with a bottom heat of 20°C, in a partially shaded position. Rooting should take about four weeks.
Maya Albert