Growing Green Beans in the Greenhouse
If you have a heated greenhouse with an air temperature of 16°C (60°F) and a minimumtemperature of 13°C (55°F), you can grow good crops of out-of-season in the borders. Greenhouses which are glazed down to ground level are best, as the French beans need plenty of sunlight. You can also grow them in frames, but only if you live in a really warm and sunny area.
Sow the seeds in good quality seed, with enough heat, any time from late summer to late winter. Expect cropping from late autumn through to late spring or very early summer. For minimum root disturbance during trans- planting, use peat pots or sow the seeds directly onto soil blocks. Once the first pair of true leaves are showing, transplant them into the border or frame. The soil should not be too rich, or the plants will make excessively leafy growth at the expense of pod formation. Ideally, soil which has been manured for a previous crop is best.
Space the plants 22 cm (9”) apart, in single rows 30-37.5 cm (12-15”) apart in frames and in double rows about 30 cm (1’) apart in the borders. Climbing varieties grown in the borders will need support. Use strong garden twine, fixed vertically to two parallel, horizontal wires. The top wire can run under the roof, and the lower one should be about 15 cm (6”) from the ground. Two plants will climb up the same string.
Cultivation is the same as for outside growing, but make sure the plants are kept growing in a reasonably moist atmosphere, otherwise red spider mite could become a major problem, and ensure that the glass is as clean as possible, to allow maximum light to reach the plants.
The beans are ready for picking from ten to twelve weeks after sowing, depending on weather conditions. Once the pods have started to form, check them daily, as they mature quickly. Most varieties are best when about 10 cm (4”) long. Unless you are growing the crop specifically for the seeds (either green, as flageolets, or ripe and dried, as haricots) do not allow the ripe pods to remain on the plant. If you do, the seeds will grow larger, but the texture and flavour of the pod itself will deteriorate. Secondly, the plant will concentrate its energy on the swelling seeds, at the expense of the pod production, and your crop will diminish accordingly. Daily picking will ensure that cropping continues for five weeks, or more.
When tested, pods ready for eating will snap cleanly in half, without any stringy fibres. The beans inside will be visible, but will not have expanded to their full size. Cut the pods from the plant with scissors or secateurs. You can also sever them with thumb and fingernails. Never try to pull the pods off; the plants are very shallow-rooted and you may pull the whole plant out of the ground. French beans are best eaten on the day of picking, because, although they are excellent for deep freezing, they do not otherwise store well. If you plan to shell the half-ripe beans, and eat them as flageolets, leave the pods on the plants until they are just beginning to turn colour. At this stage, the beans should be pale green. They can either be cooked fresh or dried for later use.
The beans of some varieties, if left to ripen fully, can be dried and stored for winter use. In mid-autumn, when the pods are pale brown and beginning to split, the beans are ready for harvesting. Cut the plants down, shell the pods, and spread the brown or white beans out on clean paper or wooden trays to dry. The floor of a greenhouse is a suitable drying place, but any room which is light and airy will do.
Some autumns turn cold and rainy before the pods have fully ripened. If this happens, dig up the plants, and hang them upside down in a greenhouse or attic. The pods can then finish ripening under cover; once the pods are brittle, shell and dry the beans in the usual way.
When cropping is over, cut off any remaining growth above ground level. If it is healthy and free from insects, place it on the compost heap. Otherwise, burn all stems and foliage to minimize the spread of. As with all leguminous crops, French bean roots will increase the nitrogen content of the soil as they decay. This is particularly important if the following crops grown on the site are nitrogen-hungry, such as brassicas and .