Grapes grow best in areas where the spring is dry, the summer is hot and the winter is cold. In our recent past spring is damp, the summer is flooded and the winter is mild, but with a little shelter there’s no reason why this vigorous climbing plant can’t be grown with great success.
Keys to success
Fruit is produced on one-year-old growth
The plant will bleed if you cut it in the growing period
The fruit will rot if the atmosphere is too moist
Plants like a good feed once a year
Vines prefer to be cold in the winter
You need to remember that whatever pruning regime you use, you need to have some shoots that, this year, will grow leaves but no fruit, so that next year they can produce fruit. All pruning takes place at the end of the season when the plant isn’t actively growing. If your tunnel or greenhouse is heated you are best to grow grapes as standards in large tubs so they can be placed in the cold.
There are a number of culture systems based on making either a single stem (cordon) or a double one. The vines are either trained on a vertical wire system, so that the wires are at heights of 30cm to support the cordons, or a single wire in the top of the tunnel.
Single cordon vines
Plant your vine outside the tunnel and train the vine through the plastic (or a hole in the greenhouse glass) to the inside. Allow it to grow unhindered.
The soil should be well dug and mixed with plenty of well-rotted manure and compost. Traditionally vines were planted above a rotting dead sheep.
Stake the vine outside and attach a wire to a frame indoors, along which you will train the plant.
In the summer of the first year allow the plant to grow, and pinch back any lateral shoots to around five leaves.
When the leaves have fallen off in the winter, cut back the main shoot by just a little more than half and cut the laterals to a single bud each.
The following summer treat the plant like you did the previous summer; tie in the main shoot and build your frame of wires. Take out any flowers that form.
The following winter cut the main shoot back to old wood and the laterals to a strong bud each.
So in effect you have strengthened and prepared your plant ready for producing laterals that you will now tie in the following spring and summer.
The buds will then grow out, and the resultant growth is trained along the wires.
The third summer allow one bunch of grapes per lateral shoot to form, and any sub-laterals that form keep to a single leaf. In the winter, when the grapes are taken and the leaves have fallen, cut the laterals to two buds. It is these buds you will use next year and so on.
Indoor vines do well if they are fed with tomato fertiliser each month from a couple of weeks after they have burst into life in the spring until the grapes are ready for picking. Since the bark is fibrous, all kinds of pests over-winter and so scrape the bark away inside the tunnel or greenhouse.
If you are growing grapes in Scotland then it’s a good idea to heat the greenhouse to 4oC from mid-winter onwards. This will give them a good head start.
The big problem with grapes is that they are full of sugar and unless you have good ventilation between the berries, penicillium fungi will infect the bunches. You can use scissors (some growers have special scissors just for the purpose) to thin out the berries so that the others can grow unencumbered and a good air-flow around the grapes is achieved.
The cardinal rule on harvesting is to cut off the piece of lateral they are growing from so that you do not have to touch the grapes and either contaminate or damage them.
We have already spoken of fungal infection. It comes in three forms:
Botrytis occurs in wet conditions and is kept at bay by good pruning.
Downy mildew occurs where the temperature is really hot and the greenhouse or tunnel is very humid.
Powdery mildew forms on the leaves and fruit. You can prune it away, keep the tunnel or greenhouse really clean and spray with Bordeaux Mixture, developed by French monks just for this purpose three hundred years ago and is still considered to be an organic cure by many.
Some common varieties
These varieties are easily available and grown in the UK. You can grow almost any variety under cover, although it is best to choose the late-ripening ones. There are scores more!
A German white variety that gives excellent juice.
A very early maturing variety, bred from Gewurtztraminer, producing delicious brownish grapes.
Grown particularly in Kent, a good flavoured grape with lots of sugar.
The commonest German grape, very flowery aroma, no longer recommended.
Grown a lot in the USA, small white fruit, heavy cropping, now used mainly for Sparkling Wine.
A citrus flavour, ideal for cool climates, early ripening and a very reliable cropper.
The original gardeners grape – fantastic for lots of good fruit, if grown under cover.
Like blackcurrants, good sweetness and very reliable, slightly foxy flavour.
Just to show you can grow one of the classic varieties. Susceptibility to Botrytis is the problem.
There are a number of traditions about growing grapes indoors. Firstly, the root is planted outside a greenhouse and the vine is trained through a window inside. The other tradition is to grow a vine is in a hole, half filled with a dead sheep.
However, the extra space in a polytunnel makes it possible to plant the vines in bush form, as though they were outside in a vineyard. We shall concentrate on the old-fashioned greenhouse method, growing the plant as a cordon outside the tunnel/greenhouse.
Should the necessary dead animal not be available, dig a hole 60cm (2 ft) deep and at least twice as wide as the root ball. Keep the dug soil and mix half of it with an equal amount of compost.
Tease the roots and place onto the manure, filling in with the soil / compost mix. Firm well, using as much soil as you can fit into the space. Cover the base of the plant with a good layer of compost as mulch.
Initially it will be important to stake the vine (for the first year), allowing it to grow and become established. It can be supported inside the tunnel on an end brace or a specially erected pole
Vines create a lot of shade and are best in full sun. Tomatoes do not make good bedfellows with vines because of their height and the evaporation from the tomato creates an atmosphere that tends to promote fungal infections.
There are dozens of systems for growing grapes. Perhaps the commonest is the cordon system. There are some rather complicated books on the subject of how to grow grapes in various different ways, beyond the scope of the space we have available here. Readers should try Successful Grape Growing for Eating and Wine-making: By Alan Rowe.
Pruning as a cordon
Training your vine at a little higher than head height, tied to firm supports will produce a strong vine. You will need to consider erecting something more than just the hoops of the polytunnel, which are usually too far apart as an effective support. You will have something like a system of wires at 30 cm intervals. Each of these wires will support a lateral, which will bear fruit. The Leader will run at 90o to these wires.
Leader (also known as a Rod) is the main stem of the plant.
Water shoots come off the Leader low down the stock. Remove these.
Laterals grow from the leader’s nodes.
Sub laterals grow from the laterals.
In the first two years do not allow any fruit to form on the plant. You are simply building the structure of the plant.
In year 1 choose your leader branch and tie/support it until it reaches your wires.
Any sub laterals should be plucked out entirely.
In the winter, collect the fallen leaves, and cut the leader back to where it is at least pencil thickness, cutting just after a bud.
Repeat this in the second year, from the third year you can train a lateral for each of your wires. Remember, grapes appear only on new wood, so each year you prune the lateral ‘cordon’ canes back to about 10 buds (or you can use more short 3-bud spurs).
People allow too much fruit to develop and then wonder at why they are all covered with botrytis. The keyword is ventilation. Cut out bunches of grapes that fall near each other, leaving plenty of space for air to remove humidity. Fungal infections thrive when the humidity and temperature is high.
Mildew and Botrytis are helped with copper based fungicide. Bordeaux Mixture was invented 150 years ago by monks to treat vines in France and has been on the organic list for many years. Not now strictly thought of as organic, many gardeners still use it.
Your alternative is to grow one of the less susceptible varieties and keep the humidity low and the grapes widely spaced – at least no more than 18 bunches to a mature plant.