Post category: Greenhouse Growing

Glass

Once the only available material, glass is still the best, letting in light and retaining heat better than any alternative. It is more expensive and not as safe as plastics, but it lasts much longer. Safety glass is available, however, and though it is more expensive again than ordinary glass, it might be considered for a conservatory.

Rigid Plastics

Not as dear as glass, and safer, but they do not let in as much light; nor do they look as well, except for perspex, but this is expensive, too.

Plastic Film

This is usually polythene which is cheap and safe and lets in light but does not last so well. PVC and Ultra-violet inhibited films are more expensive than polythene but last longer.

Other greenhouse vegetables

Early and late season supplies of carrots can be had from the greenhouse, frame or low tunnel. Sow ‘Early Nantes’ or ‘Amsterdam Forcing’ in December or January for supplies in late May and June. A late sowing of the same varieties in August gives a December crop.

Early carrots from a cold frame

White turnips, broccoli, radish, scallions and parsley can be grown under protection too. Sowing can start in January in a cold greenhouse or frame, December in a cool greenhouse. Delay sowing until early February under low tunnels but put up the tunnels about three weeks before sowing.

Florence fennel and chinese cabbage are two crops that are difficult to get right outdoors – they both have a tendency to bolt if the weather is not warm and moist. Sow florence fennel in late March and chinese cabbage in May. Keep both well-watered to prevent bolting.

Early and late potatoes can be produced under protection. Planted in December or January they will be ready in May.

In August, plant strong runners 30 centimetres apart outdoors in good soil. In October, lift the plants and carefully pot then in medium-sized pots. Leave the pots outdoors until the middle of January and then bring them into the greenhouse, or frame.

Early strawberries in flower

When the plants begin to grow, check them for greenflies and spray, if necessary. Depending on the level of heat available they will flower in March/ April and fruit in April/ May.

Alternatively, the plants can be covered, where they were planted, by a low tunnel to fruit in May/ June. Plants ‘forced’ under glass or plastic can be planted out and grown on outdoors, but are not worth forcing again.

Peaches, nectarines and apricots are closely related stone fruits that can be grown in a greenhouse as fan-trained trees on a wall. The trees can take up quite a lot of space, especially the apricot, but they can be hard-pruned too. ‘Peregrine’ is a common variety of peach and ‘Early Rivers’ is a nectarine variety.

Peaches can be raised from pips of shop-bought fruit and, unusually among garden fruits which are mostly grafted, will come true to type. Plant a young tree into the open soil and train it to a fan shape by tying in the branches to wires, set 30 centimetres apart, that run the length of the wall to be covered.

Pollinate the peach flowers by hand, using a child’s paintbrush, or by tapping the wires each day during flowering. If a lot of fruits set, thin them out to about 15 centimetres apart. When the fruit is picked in August, immediately prune out the shoots that have carried fruit and tie in the new green ones to flower the following year ones.

In the spring, when the buds break, rub away excess young growth, leaving only enough shoots to replace the fruiting shots and maintain the branch framework. These trees always produce too many shoots.

Never let the roots go completely dry. Use a mulch and water as necessary. Watch out for red spider mite and use the predatory mite for biological control, or spray with Sybol, Malathion or Liquid Derris, if necessary.

Planting

A grapevine can be grown successfully in quite a small greenhouse, but the bigger the better because a grapevine can fill quite a large greenhouse. ‘Black Hamburgh’ is the best variety for Irish conditions. Planting is usually done when the vine is dormant in December but the plants are pot-grown and can be planted at any time.

Grapes swelling on the vine

Grapevines are often planted with the root outside the greenhouse and the stem taken in through a hole in the wall or wall-base. It is not essential to plant outside, although it reduces the vine’s watering needs, but the roots will generally grow out under the wall in any case.

Training

Train the vine to grow up into the roof of the greenhouse. First, train it up to gutter height. Then train a single shoot along the wall at this height. When it reaches the other end, allow side-shoots to grow up the roof on wires, tied into place 30 centimetres apart.

From these side-shoots, which form permanent rods, come the flowering shoots. These arise every 30 centimetres or so. Allow only one shoot to develop at each station and pinch out its tip one or two leaves past the flower bunch. Tap the rods each day, during flowering, to ensure pollination. Pinch out subsequent side-shoots at one leaf.

Grapevines are normally vigorous but if not, mulching the root area in spring will encourage better growth. Use some general fertiliser as well, if growth is weak or after a heavy crop. Watch for red spider mite.

Diseases

Powdery mildew is a serious problem disease. It is important to allow air movement in the greenhouse. Rose and fruit fungicides containing the active ingredient myclobutanil are highly effective, and one or two thorough applications in late spring on the new foliage before flowering will clear the problem.

Grey mould disease can be prevented by ventilation and maintaining a dry atmosphere in late summer. Prune out the fruit-carrying shoots in autumn when the fruit is picked, leaving the framework of permanent rods.

Shrubs and climbers

A wide range of ornamental plants can be grown in a greenhouse or conservatory. Some of these must have the protection of a greenhouse and cannot be grown successfully in the open garden. Other plants, though they can be grown outdoors, grow better with the extra heat provided inside a protective structure.

Tibouchina urvilleana

Flowers

Freesias need greenhouse protection

A wide range of herbaceous plants, such as busy lizzie, streptocarpus, pelargonium, cymbidium orchid, canna and peruvian lily, can be grown in the greenhouse. Some of the indoor plants are bulbs, corms or tubers such as gloxinia, tuberous begonia, amaryllis and lily that are potted up in March or April.

Freesia, cyclamen and anemone can be potted up in August or September. Hyacinth, tulip grape hyacinth, crocus and dwarf iris can be potted up in September or October and brought inside in January. Most of these plants come into flower four or five months after potting up.

Many greenhouse flowers can be raised from seed. Cineraria, primula, calceolaria and butterfly flower are sown in June, July or August to flower in March, April or May of the following year. Cyclamen is sown in August to flower 15 months later.

Quite a few bedding plants, both spring and summer, can be used as flowering pot plants. Petunia, everflowering begonia, ageratum, tobacco flower and french marigolds can be sown in March to flower in summer.

Polyanthus and double daisy are sown in early summer, and stock in late summer, to flower in spring. Two very useful greenhouse plants also raised from seed in spring are morning glory and blackeyed susan. These are climbers, but they only last one season.

Many foliage plants, such as spider plant, tradescantia, coleus and ferns can also be grown in a greenhouse. Shading might be necessary to avoid sun scorching and, in winter, some must have frost protection, or be taken indoors.

Alpine plants

Greenhouse alpine: Primula allionii

Because they need very little space, alpines make excellent greenhouse plants. They enjoy the dry air, but they mostly dislike excessive heat, and are best placed near the door or the ventilator. In winter, it is important to keep air humidity as low as possible.

Special alpine houses with a lot of ventilation are sometimes set up by alpine enthusiasts. Many kinds of alpines, such as primula, raoulia, lewisia, gentiana, rhodohypoxis and many small alpine bulbs, can be grown indoors.

Watering

Plants in a greenhouse need extra care because of the high temperatures generated and the exclusion of natural rainfall.

Christmas cherry with other greenhouse plants

Cooling

Ventilation on hot days is necessary to keep plants cool. About 25° Celsius is an ideal maximum for most plants. Beyond that, growth slows down and stops. High temperature levels can be reached from late spring onwards.

In summer, ventilation alone might not be enough, so shading can be necessary. Apply Coolglass, or Summer Cloud, in June or early July and remove it at the end of August. A simple way of cooling the house on really hot days is to damp down the floor. Ventilation in winter and spring, on dry, bree days, dries the greenhouse atmosphere, helping disease control.

Feeding

Greenhouse plants must be fed much more often than plants outdoors. The frequent watering that greenhouse plants receive tends to wash the soluble plant nutrients from the restricted reserve of a pot.

Feed little and often – even as often as once a week for large, quick-growing plants early in the growing season. Liquid feeding is simplest to use and most effective. Take care not to feed a dry pot, for fear of scorching the roots.

Hygiene

Pest and disease damage is usually more severe in a greenhouse, where the warm conditions are ideal for insects and fungi, and predators are absent. Remove old and diseased plants, or pest-ridden plants, to break the cycle of infection. Control pests and diseases when they appear.

Wash down the glass, pots, trays and benches in the winter with household disinfectant. There are no products approved for domestic use in disinfection of greenhouse soil. If the soil is ‘tired’, replace it with fresh soil. If there are root disease problems, grow non-susceptible crops. Flood the greenhouse soil in early spring by watering heavily to leach out excessive salts left over from frequent feeding the previous year.

To trap as much heat as possible, a greenhouse should be situated in an open, south-facing position without shading from buildings or trees. The ridge of the roof should point east-west, the broadest part of the glass exposed to the sun. This does have the drawback of heating more on one side, and this can be avoided by running the ridge north south. For warmth, the next best side-on aspect is west because it heats up late in the day, and stays warm longer into the night.

Strong gales can cause severe damage

An east-facing side-on site goes cool even before sunset and the morning sun can cause damage to plants by too-quick thawing. A north-facing greenhouse, to be of any use, will need artificial heating – even for the usual range of plants.

Do not site a greenhouse in a hollow where cold air might seep in, nor in a windy position. Do not forget the strong eddies of wind around a house, though the site might seem sheltered.

Heat conservation

A ‘cold’ greenhouse means that there is no artificial heat provided. A ‘cool’ greenhouse has a heating system that will provide some artificial heat – usually just enough to protect against frost. A ‘warm’ greenhouse has a heating system capable of providing an air temperature of at least 10°Celsius.

Greenhouse flowers can be very rewarding

Heating systems

If basic frost protection is all that is required, there are two options – paraffin or electricity. A simple paraffin heater is relatively cheap to buy, and to run, but needs to be lit and refuelled, and can give off damaging fumes if not set up correctly.

An electric fan heater is more expensive to buy, but very easy to operate, usually featuring a built-in thermostat. Seek professional advice when installing an electric heater and be sure to set it up so that it does not get wet.

Running costs for a fan-heater are low, if it is used only for frost protection. A two kilowatt fan-heater will keep a 3.6 metres by 2.4 metres greenhouse free of frost on a night when it is minus 8° Celsius outside.

Heating a warm greenhouse to about 10° Celsius is a more difficult proposition. An extra radiator can be taken off the domestic heating system if a conservatory or greenhouse is attached to, or very close, to the house. Most central heating systems are on a time switch that shuts down the system at night – just when the greenhouse needs it most!

However, usually enough heat will have built up earlier to protect plants adequately. A thermostat over-riding the time switch can be installed to prevent very low temperatures.

Electric storage heaters can be used in a conservatory too. These use night-rate electricity. There must be no danger of them getting wet, for safety reasons. They tend to be bulky too. Free-standing greenhouses are difficult to heat economically beyond basic frost protection. A separate hot-water boiler is ideal, but few people would consider this necessary or affordable.

Vents

On a sunny day in summer, greenhouse temperatures could rise above 40° Celsius. Plants dry out quickly and can be scorched or killed at these temperatures. It will be necessary to allow the hot, dry air to escape.

Vents in the roof and sides should be provided. Opening the door helps too. Roof vents are important not only because hot air rises, but also, because it may not be possible to leave side vents or doors open in a conservatory for security reasons.

Polythene tunnels are difficult to ventilate properly, usually relying on leaving the ends open. This is adequate, once the tunnel is not too long.

Greenhouses

Glass was once the only available glazing material, and it is still the best, letting in light and retaining heat better than any alternative. It is more expensive and not as safe as plastics, but it lasts much longer. Safety glass is available and, though it is more expensive than ordinary glass, it should be considered for a conservatory.

Low polythene tunnel with early potatoes

Rigid plastics are not as expensive as glass, and they are safer, but they do not let in as much light; nor do they look as well, except for perspex, but this is expensive, too.

A new greenhouse, well stocked

Polythene is cheap and safe, and allows in light well but does not last so well as the other materials. PVC and U-V inhibited films are more expensive than polythene but last longer.

Frames

A garden frame is just a large, low bottomless box with a translucent lid. The sides of the box can be made of wood, concrete, galvanised iron or any other building material – very often salvaged scrap materials will do.

The frame can be any length but should not be wider than 120 centimetres for comfort and safety. It should be 25 centimetres high in front, and about 50 centimetres high at the back. The ‘lid’ is a number of ‘lights’, each consisting of a wooden frame with glass or plastic – on it. Each ‘light’ matches the width of the garden frame in its own length and should be about 75 centimetres wide, and can be made of suitable timber lengths.

Cloches

A ‘cloche’ originally was a bell-shaped glass jar placed individually over tender plants. The term was extended to include continuous structures of glass sheets supported by iron brackets. These continuous cloches were normally about 45 centimetres high but are now unusual, because low plastic tunnels and cloches have taken over.

Walk-in tunnels

A ‘walk-in’ tunnel, or polytunnel, can substitute for a glasshouse. Being relatively cheap to put up, it is possible to cover a larger area economically. A polytunnel can be of any length – using a greater number of the tubular steel supporting hoops.

Tunnels are generally sold as kits and the standard widths commercially available are 4.2 metres and 5.1 metres. Walk-in tunnels can be difficult to ventilate properly, and the plastic will have to be replaced. It usually lasts two or three years, although there are more durable kinds of plastic film that last for five years.

Low tunnels

A low tunnel is a row of wire hoops – 45 centimetres high, 75 centimetres wide – supporting a 1.8 metre wide polythene film. Length of row is variable. At each end of the low tunnel the film is tied firmly to a short stake. The wire hoops are 1.8 metre lengths of strong wire with an ‘eye’ twisted into them about 20 centimetres from each end.

Strings tied into each ‘eye’ and stretched across the polythene hold the tunnel film in place. Although both garden frames and low tunnels are limited in use by their size, they are very successful for low crops such as early and late vegetables, early strawberries, and cuttings.

Varieties

Greenhouse-grown lettuce is of better quality than outdoor crops and it is available from November to May when they are not. Sow the variety ‘Kwiek’ in late August for winter lettuce, and ‘Emerald’ and ‘Kloek’ in September/ October for late winter and spring supplies.

These varieties tolerate cold weather, but without frost protection at least, there can be some losses and the plants will not ‘heart’ up. Watch for greenflies. Do not splash water about or grey mould will attack some of the plants at soil level. Lettuce can also be grown in frames or in low tunnels.

The egg plant or aubergine is a member of the potato family, related to tomato, sweet pepper, chilli pepper and cape gooseberry. It has the typical potato family flowers, purple in colour and flat, star-shaped with a central pointed yellow pistil. Unlike the other potato family crops, the aubergine originated in India, where wild types are still found. Other species occur in various parts of the Far East and are used for food, for instance, in Thailand. It has been used as food for over two thousand years in Asia and several hundred in southern Europe.

It got the name ‘egg plant’ from the very egg-like fruits on some varieties; these are white or creamy and egg-sized and shaped, unlike the fruits on modern egg plant varieties which are usually purple. However, white varieties still exist and varieties with small fruits and even narrow pointed fruits are still grown in some parts of the world. Aubergine is the french name for egg plant, derived from the Arabic name, the Arabs having introduced it to Spain.

To grow aubergine, it can be treated it much like the tomato, best grown in a greenhouse or tunnel, although it can be grown outdoors reasonably successfully in a warm, sheltered setting. Results will be quite good in a good summer, not so good in a dull, cool year.

Cooking aubergine

Aubergine is a very versatile fruit/vegetable, like the related tomato. It is not a juicy fruit and not as versatile as the tomato but it can be used in many ways – in stews or casseroles, baked and grilled. It is a ‘meaty’ vegetable that is filling and used for this purpose in vegetarian cookery. It has almost no calories, not much in terms of minerals or vitamins, and its main value is its ability to absorb the flavours of other ingredients. It is an important ingredient of some dishes. It can be a little bitter and some recipes call for salting it before use.

Growing aubergine

Site and soil: Ideally, aubergine is grown in a greenhouse as it benefits from the extra warmth. But it can be grown outdoors in a warm spot. Indoors it is best grown in the open soil rather than in pots or grow bags, and it needs rich, fertile soft soil with lots of well-rotted organic material.

Varieties

‘Moneymaker’ is a well-known purple skinned variety. ‘Balck Enorma’ is another. White, egg-sized fruits are carried on ‘Mohican’ and ‘Clara’. Some companies offer mixed varieties.

Sowing

The seeds are sown, much like tomato, in late winter or early spring, starting them in a little warmth and growing them on with protection from frost on cold nights. The plants can be sown several to a pot and thinned to one good seedling.

Growing on

It is important to grow on aubergines without a set-back at any stage, potted on as they grow. If they are checked they often flower early and make poor growth afterwards. About five or six fruits per plant is a reasonable target for the large-fruited varieties, more for the smaller ones.

Planting out

Plant out the young plants from pots, usually thirty centimetres or more tall. Generally they have to wait untill mid-May when other plants are put outside and space is created.

Training

Allow the plants to grow to about fifty centimetres, pinching out the main growing points if they grow too tall. The plants might need the support of a light stake, especially as the fruits begin to swell.

Picking

Pick the fruits when they have made good size.

Troubles

Aubergines are very soft plants and attractive for a range of pests, notably greenflies and white flies. It may be necessary to spray with derris to control these.

The sweet peppers, also called bell peppers and capsicums, are varieties of Capsicum annuum, a species to which the hot chilli peppers also belong. The only differences are the size of the sweet peppers which are many times larger than the chillis and the fac that the sweet peppers have no capsaicin, the agent that causes the hot sensation. The capsicums are native to Mexico and Central America where there are records of cultivation for seven thousand years and perhaps longer. The peppers arrived in Europe about 1500, the hot kinds probably first as a substitute for the true pepper. Their cultivation spread to Asia subsequently.

Although the sweet pepper has been in Europe for over five hundred years, it only found its way on the kitchen table in this country in recent decades, and into greenhouses and gardens even more recently. The capsicum is a warm climate plant and really needs to be grown indoors at this latitude but the newer more vigorous varieties can give good results outdoors in a warm sunny, sheltered place. It is related to tomato and not all that difficult to grow, if the conditions are right. The main colour is red – the green ones are not yet ripe but can be used – and there are yellow, orange and purple varieties too.

Cooking sweet peppers

The sweet pepper is a very versatile fruit. Like the tomato, it can be used in many ways, both cooked and fresh. It can be used as an ingredient in many dishes and it is good at absorbing the flavour of other vegetables and herbs, while imparting its own distinctive taste. It is a very good source of vitamin C and vitamin A, good in fibre with little fat and low in calories.

While the tomato is most closely associated with the cuisine of Italy, it first arrived in Europe to Spain from Mexico, not long after Cortes took control of Mexico in 1523. However, it is perhaps fitting that the earliest documentary mention of the tomato is thought to be an Italian reference to a yellow-skinned form in 1544, hence pomodoro, ‘golden apple’. Although tomatoes had been widely cultivated in Mexico before the arrival of the Europeans, there was great suspicion about the tomato when it first arrived in Europe. Botanists of the time knew that many members of the Solanum family are poisonous and they recognised its family features, especially of its flowers.

Tomatoes acquired a reputation as an aphrodisiac and were known as ‘love apples’ and ‘pomme d’amour’. This reputation has faded but it was perhaps instrumental in encouraging people to taste the fruit. While mostly used as a ‘vegetable’, the tomato is, of course, a fruit and a berry at that. This is more obvious with the small-sized cherry tomatoes that with the usual sizes and the large ‘beef’ tomatoes. These latter types are grown for their mealy flesh rather than their juicy seed pulp. A wide variety of fruit shapes is grown, and colours mostly red though yellow and striped too. The tomato plant is a short lived perennial and it is not hardy, though some variety are more tolerant of cool conditions than others.

Cooking Tomatoes

Tomatoes are an incredibly versatile food, being used both cooked and fresh in myriad ways. The flavour of tomatoes complements many other foods, balanced as it is between acid and sweet flavours. The colour of the fruit is valued in many dishes and the tomato has good nutritional value, being a good source of Vitamins A and C, with some fibre and smaller amounts of many other vitamins and minerals. It contains the valuable anitoxidant, lycopene, the concentration of which is actually increased by cooking although some Vitamin C is lost.

Growing tomatoes

Site and soil: Tomatoes can be grown in a greenhouse or outdoors. Greenhouse tomatoes can be grown in pots or in the open ground. Outdoor tomatoes are successful in most years.

Sowing: Plants can be raised from seeds or purchased. The earliest seeds can be sown in the first weeks of the year for early greenhouse planting, and later planting in April can be made form sowings in February or early March. Choose greenhouse varieties for indoor use. Outdoor varieties are raised from seeds sown in the first half of March to make plants of good size for planting out.

Varieties

The seed catalogues list a very wide range of varieties to which new kinds are added all the time. Classic varieties include ‘Alicante’, ‘Gardener’s Delight’, the beef variety ‘Marmande’ and the cherry tomato ‘Tumbler’.