Advice
RHS Help & Advice
Grape care
How to prune, thin and ensure good fruit set on grape vines in summer
Although the main vine pruning season is autumn to early winter, vines require maintenance pruning in the spring and summer to prevent them getting too crowded. Dessert grapes also benefit from thinning of the fruit to help ensure the remaining grapes are large, sweet and free of pest or disease problems.
Selecting and tying in new growth, removing unwanted growth
Identifying spurs
This shows a spur on a vine cordon (or main stem, also known as a rod). The spurs are the growth points from which new shoots arise. They form when the vine is pruned hard regularly in winter. You can see the green points on this spur where new shoots will appear.
Selecting strong shoots
Only the best and strongest new shoots are selected to grow into laterals - usually one or two per spur - in order that the framework be well spaced for light and air penetration.
Removing weaker shoots
Weaker and unnecessary shoots are rubbed off while still tiny, or pinched off between thumb and forefinger when a little larger.
Keeping an extra ‘reserve’ shoot
Sometimes, a third ‘reserve’ shoot is kept as insurance in case a shoot is needed to fill a gap, replace an unproductive lateral, or to extend the framework. Those being kept for next year can be allowed to remain, tucking them out of the way between the support wires. Those being kept for insurance can be removed in about six weeks, when you are confident that the two selected shoots will not need replacement.
The Guyot system
In the Guyot system for training outdoor grapes, a third shoot is reserved to provide growing points for the two main laterals next year.
A double Guyot system has two main laterals trained along the bottom wire in opposite directions, and fruiting sub-laterals plus the third ‘reserve’ shoot tucked between the higher tier of double wires.
Tying in new growth to the framework
With a glasshouse vine grown as multiple cordons (multiple main stems arising from a short trunk or ‘leg’), the green arising from the cordons, which are the ones that will produce fruit, need to be tied in regularly to make a well-spaced framework:
A well-spaced framework has cordons about 1m (3ft) apart, and laterals about 30cm (1ft) apart.
Shoots being tied in must be bent down to the horizontal wires. This can be tricky, as young vine shoots are brittle and snap easily. Wait until the new shoots have toughened up and gained some flexibility before bringing them down to the wires, and tie them in during the morning when they are at their most turgid. Shoots that have grown limp after a day’s sun, only to be tied in during the evening, can break the following morning once they have regained their full turgidity and then strain against too-tight ties.
Make sure that the vine training wires are about 30cm (1ft) away from the glass. Leaves that touch the glass are vulnerable to scorch.
Stopping laterals
Laterals are stopped, by pinching out the tips, when they have produced two leaves beyond the first fruit cluster. Laterals failing to produce a bunch of grapes should be pinched at five leaves, unless extra green framework is needed to cover bare patches or provide shade. Only allow non-fruiting laterals of vigorous vines to carry on growing like this, so that yield and crop quality do not suffer.
Standard grapes
With standard grapes, often grown in small gardens or in pots, four or five shoots arising from a stump at the top of the trunk are allowed to grow into a head. These shoots should be stopped at five or six leaves by pinching out the tips, or at two leaves beyond the fruit cluster on shoots that have developed grape bunches. Shoots can be tied in ‘maypole-style’ to a supporting bamboo cane stuck in the soil.
Removing tendrils
Tendrils should be removed as soon as they start to form, as not only will they will sap energy from the vine, but they will also scramble across your training system and entangle with the fruits.
Removing excess bunches of grapes
If all embryo bunches of grapes were allowed to develop, crop quality would suffer, and the vigour of the vine would be sapped.
As a general rule, 450g (1lb) of fruit should be allowed per 30cm (1ft) of main lateral, rod or cordon.
On a glasshouse vine grown as multiple cordons, only one bunch of grapes should be allowed to develop on each fruiting lateral branching from the cordons.
Thinning dessert grapes
Dessert grapes need extra thinning, as individual fruit size, sweetness and quality is particularly important.
The grapes must be thinned by hand, using scissors, to increase the spacing between them. This allows each grape to develop to a good size, and allows sufficient air and light penetration between them to encourage even ripening and discourage problems with fungal diseases.
This crowded bunch of grapes needs thinning. If all these fruits were allowed to develop, they would remain small, lacking the room to swell, and would be vulnerable to fungal diseases because of the lack of air flow between them.
Special vine scissors, with long, finely tapered blades are available from good garden centres or mail-order suppliers. It is worth shopping around to find a design you feel comfortable with. A small forked stick can be used to steady the bunch while cutting out the excess berries. Don’t use your fingers, as this damages the bloom on the grape skins, and makes them more vulnerable to fungal disease.
Show growers sometimes shape grape bunches while thinning, to achieve a perfectly proportioned conical bunch.
A primary thinning can be done while the grapes are still tiny, and a secondary thinning can be done, if necessary, once the grapes have increased in size.
Assisting pollination and fruit set
Cover the conservatory or glasshouse beds with straw, to maintain a dry atmosphere inside. Humidity discourages pollination and fruit set. The straw mulch will also conserve moisture in the soil for the vine roots.
Shaking the rods briskly helps to distribute the pollen, and may be all that is necessary for cultivars setting fruit readily. Some cultivars, Muscat types for example, can be more temperamental in their fruit set. In this case, a gentle downward stroke with a cupped hand over each bunch can help to transfer pollen between the flowers. Do this at noon on a sunny day, when the atmosphere is driest, and ensure there is sufficient ventilation to keep humidity levels down.
Maya Albert