Growing Salad Crops in the Greenhouse

Anyone with a greenhouse, frame or a few cloches should give serious thought to growing salad crops. They are rich in vitamins and minerals, low in fat content and, usually, in carbohydrates. With weather protection a wide range of these vegetables can be cropped almost all year round

To make the best use of the type of protection you have, and to employ it economically (especially important where heating is employed), give proper consideration to its height. A greenhouse has plenty of headroom and you can use this to advantage by growing tall crops like tomato and cucumber, while low-growing crops can often be grown as catch crops (quick crops grown alongside a main crop) in the remaining space.

You can also adopt the catch-crop technique with frames and cloches; for example, along with lettuce grow even smaller vegetables like radishes, so that no space is wasted.


Undoubtedly the tomato is the most popular of all salad crops. It does, however, have its fair share of problems. Many of these arise from the practice of growing plants in the ground soil and using crude animal manures or garden compost that has not been properly fermented. These methods introduce innumerable pests and diseases to which tomatoes are very susceptible. Like most crops they also succumb to ‘soil sickness’ if grown in the same ground year after year. For this reason it is wise for the home grower, with the average-sized greenhouse, to grow the plants in good soil compost in pots at least 25cm (10 in) in diameter, or in a trough made from a frame of timber boards draped with polythene with slits cut for drainage. Growing bags, filled with special tomato compost are now also available. These are placed flat on the floor and holes are then cut in them to take the plants. You can, however, make your own simple and inexpensive compost. Mix three parts of peat with one part of washed grit and add (according to instructions) a proprietary, balanced, complete fertilizer containing magnesium, iron and trace elements.


Erratic watering of tomatoes and extremes of feeding lead to cracked and split fruit, and often to premature falling of flowers and young fruits. Ring-culture was designed to even out water uptake, but the method is still much misunderstood. The plants are grown in bottomless pots, and you can buy inexpensive fibre cylinders for the purpose. Set the ring on an aggregate consisting of ‘ballast’. This is a mixture of coarse, stony pieces (that help support the rings of compost) and fine particles that convey moisture to the rings by capillary action. Keep the aggregate thoroughly moist, but not waterlogged. Apply liquid feeds to the compost in the rings. Alternatively, instead of this aggregate, you can use peat. Discard it at the end of the year (dig it into the outdoor garden) and use fresh peat for the next tomato crop. This way you don’t have to dispose of heavy aggregate, nor do you have to clean and sterilize it for future use. You also avoid the risk of carrying over pests and diseases from one crop to the next.

It is important to put the aggregate down on plastic sheeting so as to prevent the tomato roots entering the ground soil. Do not, in place of aggregate, use ashes or other materials that may contain harmful chemicals, and do not overwater as this can cause root rot.

Although tomatoes can be cropped all the year round, this is an uneconomical practice for most people, owing to the fairly high degree of warmth needed for winter fruiting. The usual method is to sow from early spring (February) onwards, depending on the warmth available (a minimum of 10°C 50°F). If your greenhouse is sited in a sunny position, and warmth can be maintained, you can start sowing in early winter (November). Tomatoes need shading only when the weather is at its hottest, and to keep temperatures below 27°C (80°F) during ripening — otherwise there may be ripening troubles.


Similar advice applies to cucumber, which most people find convenient to sow in late spring (April). This is another crop apt to be frequently overwatered; remember, cucumbers are not bog plants! If the roots get too wet they decay, and the young fruits rot and fall. Growing methods can be much the same as for tomatoes, but the plants must be properly trained for the best results. For a small greenhouse, stretch about five wires along the roof some 15-20cm (6-8 in) apart and stand the pots on the staging so that the plants can be grown up underneath the wires.


Stop the plants (by taking out the growing tip) when they reach the top wires; then train any lateral shoots along the wires and tie them in. When a flower bearing a little fruit has formed stop (by cutting off) the shoot two leaves further on. Remove all flowers not bearing fruits. These are male flowers and if they are allowed to pollinate the females, they will cause the fruits to become seedy, club-shaped and often bitter. Varieties bearing only female flowers are now obtainable and well worth consideration. Cucumbers need more shade than tomatoes, and this is ideally provided by a coating of white Coolglass on the exterior of the greenhouse.

Sweet Peppers

These peppers, which you may also encounter under their Spanish name of pimiento or generic name of capsicum, are an excellent crop to grow with tomatoes, and they take up less height. You can eat the fruits when green, yellow or red, and you can cook them or use them raw. Modern hybrids are easy to grow and give splendid yields. If possible, sow early — at the same time as your tomatoes, in a similar compost and in 18-20cm (7-8 in) pots. Again, if there is sufficient warmth, sowing can be done in early winter (November) to give pickings by late the following spring (April). Well-grown plants produce as many as 30 fruits on each, but thinning is usually necessary to prevent overcrowding and distortion of the fruit that may press together when swelling. Otherwise they need little attention apart from watering and feeding as required. Green fruits will ripen to a full red colour after picking if put in a warm place indoors. You can also grow plants outside, keeping them under cloches until summer. Do not uncover them before all danger of frost is past.


Lettuce is most conveniently grown in frames or under cloches, but greenhouses are often devoted to this valuable year-round crop. A cold greenhouse or, for out-of-season production, a frost-free greenhouse, is all that is necessary. Frames can be equipped with electric soil-warming cables or special outdoor cables can be run in the soil under rows of cloches. These are operated from a transformer (that lowers the voltage) so there is no danger if the cables are accidentally cut when working the soil with tools.

It is vital when growing lettuce to choose your varieties with care. Not all are suited to winter culture, and winter types may bolt in summer. Examine the nursery seed catalogues carefully. The descriptions of the various types explain which are suited to which seasons


Like tomatoes (and for similar reasons) lettuce are prone to pests and diseases if grown in the ground soil. The most deadly enemy is grey mould (botrytis) a common fungus that attacks most plants living or dead. It is especially serious for lettuce and can wipe out the contents of frames or greenhouses in a few days unless promptly checked. Poor ventilation, allowing an excessive humidity, encourages the fungus that forms a greying, furry covering over the plants and causes them to rot. If you have added decaying manure or vegetable matter to the soil, this will certainly encourage an outbreak. So as a wise precaution, fumigate with TCNB smokes or spray with a fungicide such as Benlate, according to maker’s instructions. In winter, when conditions in the greenhouse are best kept drier, it is not advisable to use a spray, so use TCNB smokes. It is not practical to fumigate frames as it is difficult to measure the correct dosage and to disperse the smoke evenly.

You can produce fine crops of lettuce by growing them in pots or troughs of a prepared sterile compost as described for tomatoes. You can also use this compost in frames, taking care to line them first with polythene to avoid contact with the ground soil. Under cloches it is usually convenient to sow lettuce directly into the soil and thin out, but you can, if preferred, raise the plants in seed trays in the greenhouse and transplant them.

Radish and Beetroot

There are several different varieties of radish that are easy to grow as catch crops. Study the catalogues carefully to select suitable types. Some force more easily than others. Radishes are remarkably free from pests and disease, but active surface pests like slugs and cutworms will eat them however ‘hot’ they may be to our taste. The variety Red Forcing is particularly good for under cover cultivation. Always thin out radish as soon as possible, otherwise the roots never get a chance to grow to their proper size, and their flavour never fully develops.

Globe beetroot make a useful frame crop, and there will usually be room for radish, too, while the beetroot is developing. Sowing can be made in a heated frame from early to mid spring (February to March). Thin to one plant at each sowing point, since the ‘seeds’ sown are really capsules containing several seeds.

Spring Onions and Carrots

Spring onions, either sown in spring or summer, need only cloche protection. White Lisbon is still a favourite variety.

Carrots are often overlooked as salad vegetables, but young, raw carrots are delicious when grated and seasoned, and are extremely nutritious. Choose the stump-rooted types for frames and cloches; read the catalogue descriptions carefully since not all varieties are happy under cover. Sow in a warmed frame in mid autumn (September) and late winter (January); in a cold frame sow from early spring (February) onwards.

Other Vegetables

Mustard and cress and several other sprouting vegetables like fenugreek, alfalfa, Mung bean and Adzuki bean can be fitted in as catch crops to most heated greenhouses and frames all the year round. They are easy to grow and need no elaborate preparations or compost making, and they are probably the quickest-yielding of all crops. It is only recently that their important food value has been appreciated, and they are a valuable source of protein.

12. July 2011 by admin
Categories: Greenhouse Cultivation, Salad Crops | Tags: , , , , | Comments Off on Growing Salad Crops in the Greenhouse


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