Growing Fruit Under Glass

Many gardeners that I come across have a greenhouse of some type and generally it seems that it is used for growing plants such as tomatoes, peppers, cucumbers and maybe melons. Indeed, one of my two greenhouses does just that! But there are alternative, fruity uses to consider which, if simple rules are followed, can be very productive.

The first fruit to consider is grapes and yes, I really mean that! Unfortunately, many gardeners believe that growing grapes means a heated greenhouse which is a great shame as there are several good grape varieties which are absolutely perfect for growing in a cold (i.e. unheated) greenhouse. Black Hamburgh is probably the best known and the easiest to grow in an unheated greenhouse and the resultant grapes are every bit as good as you will buy in the shops. The famous grape vine at Hampton Court is a Black Hamburgh so it has Royal support! There are other grapes around which can be grown in a cold greenhouse and these include seedless varieties such as Flame, Lakemont, Perlette and Vanessa. If you are able keep your greenhouse heated so that the temperature never drops below 5℃, there are more exotic grapes you can grow and Muscat of Alexandria is the finest in my opinion.

‘Black Hamburgh’ is one of the easiest vines to grow in a cold greenhouse.

Grapes require open, free-draining soil to grow well and it is important to get this aspect of their cultivation right since water-logged soils can affect them quite badly. Equally grapes need to be kept well-watered and not allowed to dry out at the roots particularly when the bunches of grapes first appear. It is for this reason that grape vines are best grown with their roots outside the greenhouse and the stems fed in through a space created for example, when a lower brick is removed. You are going to need to support the growth well and this is done using the Rod and Spur System. Inside the greenhouse use fencing type galvanised wire stretched horizontally 30cm (12in) apart and away from the glass. For wooden greenhouses you can buy vine eyes to stretch the wire between or for aluminium greenhouses you can buy special vine eye clips to attach to the aluminium bars. Ventilation is going to be very important, so make sure that your greenhouse has at least one but preferably two opening windows.

Having set up your support structure you should plant your vine ensuring you leave a space of at least 15cm from the wall or greenhouse side. If planting is done during the dormant season (from late November to late April), after planting cut the main stem back by two thirds and any side shoots back to one bud.

In the first year of growth (or if planting is carried out during the growing season) when the main stem reaches either the top of the greenhouse or wall or when it reaches 3m in length, cut back any side branches to five leaves and any side shoots growing from these branches to one leaf. When this is done, tie the main stem and branches to the supporting wires. In December/January cut the main stem back by two thirds and the side branches back to one strong bud.

In the second year of growth let the main stem grow. Let one or two of the side branches produce a bunch of grapes and then pinch back the tips of these branches to two leaves past the bunch of grapes. Cut back any side branches not bearing any fruit to five leaves. In December/January cut the main stem back by half to a bud on old (brown) wood and cut back the side branches to 2.5cm (1in) or two strong buds.

In its third year untie the top two thirds of your vine and let it bend down and almost touch the ground. As with tying top fruit branches to produce fruit buds down this will encourage side branches to break along the vine stem.

As soon as buds begin to grow from where the side branches were cut back tie the main stem up again to its supporting structure. Pinch out the growing tips to two leaves from the flower cluster of any branches that have flowers on allowing only one flower cluster to develop per side branch for dessert grapes although you can allow more for wine grapes. Now tie in each flowering side branch to a wire. Pinch out non flowering side branches to five leaves and pinch out any side shoots growing from side branches to one leaf. In December cut back the side branches from the main stem to one or two large, plump buds.

All of this training and pruning may seem complicated but it is not I can assure you! The method is logical when thought about and quickly becomes second nature. Providing you follow the steps that I have outlined you will grow grapes successfully and it is most certainly worth all of the effort.

The vines will need some attention over the year to keep them in peak condition and must be watered thoroughly to ensure that sufficient water reaches all of the roots and fed with a good quality fertiliser. As the fruits develop, they need to be thinned to allow each grape the space it needs to grow to a good size – an operation usually carried out with long scissors with pointed ends, working upwards from the bottom of the bunch and removing the smaller ones.

Unfortunately, vines suffer from a number of pests, including aphids, red spider mites and scale insects, so a careful look out needs to be kept – and prompt action taken – at the first sign of trouble. The only serious pest is likely to be red spider mite which causes leaves to fall after mottling and turning white. The odd fine web might be seen as well. To give an almost 100% control you can introduce a predatory mite (Phytoseiulus persimilis), and this will be far more effective than any nasty chemicals. Be vigilant though and as soon as you see any damage introduce the predator – if you leave it too late control may become very difficult. Good ventilation and keeping floors damped down on hot days will help as these pests will not thrive in humidity. The fruits can also be prone to mildew so again be vigilant and remove fruits as soon as you see any mildew forming. With care and a little effort, however, even the smallest greenhouse can provide a few bunches of home-grown grapes. To help ensure good hygiene in the greenhouse remove all fallen leaves and fruits, bag them up and burn them.

So we move on now to peaches and nectarines and these will do well in either cold or warm greenhouses. The real benefit of growing these in a warmer greenhouse will be earlier ripening but the real bonus of growing these fruits under glass, heated or not, is that peach leaf curl – which affects them both – ceases to exist. Grown under glass both peaches and nectarines can be grown in pots as bushes or indeed in the ground but the most attractive, productive and sensible way is to grow them as fan trees. Grown this way it is easy to manage regular pruning (which will maximise the fruiting as it is on the previous year’s wood that the fruits will be produced) and thinning of fruitlets (which needs to be done to ensure the tree produces good size fruits and doesn’t over crop).

The trees can be planted directly in the greenhouse soil although it is vital that the soil has been well dug over and plenty of organic matter incorporated. If growing in a lean-to greenhouse you can grow the fan against the wall or as a freestanding fan. Both of these methods will need a support structure of wires which the fan can be grown up and along. Keep the trees well-watered during the growing season and after feeding with a good handful of Fruit Feed or blood, fish and bone mulch with good organic compost in March. As temperatures rise, keep the greenhouse well ventilated to maintain a good flow of air and if the greenhouse is south facing it may be best to shade the southerly side with paint on shading or a mechanical shade. On really hot days damp the floor down with a good slosh of water.

Generally, peaches and nectarines are self-fertile but growing them under glass they will benefit considerably from hand pollination. This is not a difficult task as all you need is a fine paintbrush (traditionally a rabbit’s tail!) and when the flowers are open carefully transfer pollen from one flower to another. This will need to be undertaken on a daily basis every day the flowers are open as pollen doesn’t all ripen at the same time. Although there is a good choice of peaches and nectarines there are a number of varieties particularly suitable for growing under glass. Peregrine and Rochester are both good peaches and Lord Napier is a good nectarine.

A well pollinated peach or nectarine will set a very large number of fruits and it is vital to thin these out if good size fruits are required – which, of course, they will be! The final spacing between fruits should be around 22cm (9in) but I suggest that you achieve this in a couple of stages. Firstly, the fruits should be thinned out in late May/early June when the fruits are about the size of a twenty pence piece. At this point all clusters should be thinned to single fruits about 4 inches apart. At the same time remove all fruits growing into the wall or where they are in positions unable to expand. The second thinning should take place around four to six weeks later when they are roughly the size of a fifty pence piece. At this stage peach fruits should be thinned to 22cm (9in) apart and nectarines around 15cm (6in) apart.

Growing these fruits under glass makes infection by spores of peach leaf curl very unlikely but if you do find the odd curling leaves remove them as soon as possible, bag up and burn. The only serious pest is likely to be red spider mite and as with grapes this pest is best kept under control by using a natural predator. And again, good ventilation and keeping floors damped down on hot days will help as these pests will not thrive in humidity.

Red spider mite damage on a peach tree grown under glass.

You might choose to grow strawberries and raspberries in the greenhouse and this is perfectly possible but the only benefit is to bring the fruiting season forward, so this is not a route I particularly recommend unless you feel that these are the only crops worth growing. If this is the case I recommend you grow the strawberries in growbags – and on table tops (i.e. raised platforms to make picking easy and to keep slugs and snails away) – and raspberries in large pots filled with John Innes No. 3 and kept well-watered during the growing season. Red spider mite is likely to be the main predator and as noted earlier this can be kept under control.

Figs are very easy to grow and perhaps need the least attention of any fruiting crop other than to keep them well watered if growing in a pot, giving them protection in cold weather and eating the resulting fruits! However, if you want to ensure that you have a regular and successful crop it is important to understand the fruiting pattern of figs in this country. The first and usually most successful crop begins its life as baby fruits produced in late summer at or around the tips of the young shoots. These baby fruits (called embryo figs) are very small – often smaller than peas – but provided that they are not affected by cold weather they will develop and ripen the following summer. If the embryo figs are not protected against cold weather they will simply fall off. If you can protect the pots from cold weather by moving them into a greenhouse during the colder months you stand a very good chance that the embryo figs will hang on and expand into fully ripe and luscious fruits during the following summer. Generally speaking, I suggest that you move the pots under cover at the end of September and move them out again as soon as there is no further chance of a frost. You will need to ensure that the greenhouse temperature drops no lower than about 2℃. In really penetratingly cold winters you can loosely wrap the pots and trees in fleece as well.

Another way of growing figs is to grow them in a heated greenhouse in which the temperature never drops below 10℃ (which can be quite an expensive exercise) as this will enable you to produce two crops in one season. It is probably not worth doing this just for figs but possibly in combination with something exotic like citrus fruits. If you are going to give up valuable greenhouse space to a fig, I would suggest planting a more exotic variety such as Rouge de Bordeaux rather than the normal Brown Turkey.

‘Rouge de Bordeaux’ is a delicious fig that does best in a greenhouse in cooler locations.

Finally, it is worth contemplating growing citrus fruits under glass although it is not necessarily the easiest of tasks! It is possible to grow oranges, limes, lemons kumquats and tangerines in the greenhouse providing you can keep the temperatures above 10℃ during the winter and you buy your plants from reputable citrus nurseries. Grow plants in relatively good pots with plenty of crocks at the bottom (to give really good drainage) using John Innes No. 3 and another 10% of grit added. Feeding is only necessary during the growing season of March to September and use a high nitrogen fertiliser. Collected rainwater is best used for watering the plants. When daytime temperatures start rising keep the floor of the greenhouse damp to increase humidity, which will deter red spider mite.

As soon as the weather warms up – usually June – and night temperatures do not fall below 10℃ – take the pots outside the greenhouse and keep them there in a sunny warm position until temperatures start falling again. This is absolutely perfect for growing and fruiting these plants. Be vigilant for red spider mite and mealy bugs when the plants are under glass, using predators for the former and handpicking for the latter.

If you are successful in growing citrus, and many gardeners are, there is nothing better than growing, picking and eating your own citrus fruits!

Gerry Edwards
29th May 2016

Gerry is an experienced amateur fruit grower who is Chairman of the RHS Fruit Group, a member of the Royal Horticultural Society’s Fruit, Vegetable and Herb Committee and also their Fruit Trials Panel. Gerry judges fruit nationally for the Royal Horticultural Society and is also a qualified National Vegetable Society judge.