Tips For Growing Plants Under Glass

Tips For Growing Plants Under Glass

Growing under glass offers many advantages: early, extended and out-of-season cropping, improved quality and quantity, and less dependence on the weather. Greenhouses, cold frames and cloches outdoors, and window sills and sun lounges indoors, provide basically similar growing conditions.

All such structures should be drip- and draught-proof. Ease of access, sufficient work space, ventilation and provisions for watering and heating are all essential. Greenhouses need a minimum height of 1.8 m (6 ft); frames and cloches need at least 30 cm (1 ft). A sunny, sheltered site is ideal, with a hard-paved access for wet-weather work.


Plants require different composts or rooting media according to their condition and stage of development. Seed, cutting and potting mixtures are all available ready-mixed, and there is little difference between the various proprietary brands. Potting composts are available with different concentrations of fertilizer; those with the heaviest concentration are used for strong-growing plants.

Containers can be pots, either clay or plastic, purpose-made seed trays, or shallow wooden trays, often available from greengrocers. All containers should be clean, free-draining and of a suitable size and depth for the intended purpose.


Cover the bottom of the container with a thin layer of pea gravel for drainage. Top up with damp seed compost to 6 mm (¼ in.) below the rim after levelling and light firming. Sow large seeds, like marrow, individually in small pots, cover with a sprinkling of compost and water in, using a fine rose.

For smaller seeds, water the compost 30 minutes before sowing, using a fine rose on the watering can. Scatter the seeds thinly on the surface, covering those of medium size with finely sifted seed compost to a depth equal to twice their diameter. Cover the containers with a pane of glass or some transparent plastic sheeting and place a sheet of folded paper on top. Keep warm and moist, removing the glass (or plastic) and paper when the seedlings appear. Protect the seedlings from strong sun.


When the seedlings are large enough to handle, prick them out, either singly into small pots, or 4-5 cm (1 ½-2 in.) apart into boxes containing potting compost with a low fertilizer content.

To harden plants off for outdoor cultivation, place in unheated frames about three weeks before setting out. Gradually increase the ventilation to acclimatize the plants. Tomatoes and cucumbers, for planting in unheated greenhouses and frames, are best stood in their new surroundings for two or three days before planting out.


Chrysanthemums and dahlias are best raised from cuttings rooted in warmth during spring. Start the stools and tubers into growth in slight heat, about a month before cuttings are required. Cut new growths, 5-10 cm (2- 4 in.) long, just below a leaf joint. Remove the bottom two or three leaves. Dip the cut end in hormone rooting powder and insert around the edges of pots or boxes filled with cutting compost. Keep moist, lightly shaded and in a minimum temperature of 10°C (50°F). When new growth is visible, pot up the cuttings.


Many popular flowering plants should be stored under cover during winter. Corms of gladioli and begonia tubers are lifted, dried, cleaned and stored loose in shallow trays, in an airy, frost-free place. Cut the tops of dahlias and chrysanthemums back to 15 cm (6 in.) above ground level and carefully lift the roots, shaking off the old soil. Store the roots and tubers in boxes filled with peat in a cool greenhouse or frame.


Many annuals and bedding plants can be started under glass and then moved outdoors, once all danger of frost is over. Alyssum, antirrhinums and pansies are examples of such plants.

Fibrous-rooted begonias are raised from seed each year. Tuberous-rooted varieties are started off in boxes of moist peat, in a minimum temperature of 10°C (50°F), and then potted on.

Chrysanthemums under glass are grown from cuttings and potted up into their final pots in late spring. Large-flowered chrysanthemums are stopped, by removing the growing tip when six to ten leaves have formed, to encourage a bushier plant. To disbud to develop larger blooms, remove all flower buds below the main bud as soon as they can be rubbed out.

Daffodils, narcissi, hyacinths and tulips can be grown under glass. Place the bulbs in bowls or pots of moist bulb fibre in early autumn. Keep in dark conditions, at about 7°C (45°F), until growth appears. Then move the bulbs into a light, airy room or greenhouse. The warmer the temperature, the quicker the flowering. Avoid temperatures above 16°C.


Many outdoor crops will benefit from being sown under glass and then transplanted outdoors. French and runner beans, cabbages, cauliflowers, leeks and onions are examples. Although the earliest lettuces are raised entirely under glass, the outdoor-grown crops will give quicker results if sown under glass.

Marrows, courgettes and ridge cucumbers crop well in frames and under cloches. Male flowers must be retained.


Tomatoes are probably the most popular greenhouse crop. Bush varieties can be grown indoors or out, while standard varieties are grown under glass, trained as single-stem plants. Sow seeds of both sorts thinly and cover with a 3 mm (v8 in.) layer of sifted seed compost. A minimum temperature of 16°C (60°F) is necessary for germination.

Prick out the seedlings singly, into 9 cm (3 ½ in.) pots, and grow on at a minimum temperature of 14°C (57°F). Shade from strong sun and ventilate freely in warm weather. Harden off outdoor varieties before planting out in June. Set plants about 40 cm (16 in.) apart, with 60 cm (2 ft) between rows.

Train greenhouse standard varieties up canes or other supports. Remove all side shoots, and stop each plant at two leaves above the fourth or fifth truss. Start feeding plants weekly with dilute liquid feed as soon as fruits on the bottom truss start to swell.

Raise fuchsias from soft-stem cuttings taken in spring. Pot on and harden off those for outdoor use. Mature plants can be over-wintered successfully at a minimum temperature of 5°C (41°F); keep fairly dry while dormant. In spring, cut the previous year’s growth back to the main stem, increase the temperature to 10°C (50°F), syringe daily, and new growth will start.

Pelargoniums are raised from 8-10 cm (3-4 in.) long stem cuttings, taken in summer. Pot up rooted cuttings singly, into small pots, and provide a minimum winter temperature of 10°C (50°F).


Sow cucumber seeds on edge, 2 cm (4/5 in.) deep, in pots of moist seed compost. A minimum temperature of 16°C (60°F) is needed for germination. Once the seed leaves have started to grow, pot the seedlings into 12.5 cm (5 in.) pots filled with moist potting compost.

When four or five leaves have developed, set the plants out in their final positions. In the greenhouse, set the plants 60 cm (2 ft) apart on raised beds consisting of equal parts of well-rotted manure and John Innes No. 2 compost. This mixture is also suitable for frames.

Train and tie the main stem of each greenhouse plant up a single cane. Tie in the side shoots at 15 cm (6 in.) intervals to horizontal.

1 Lift gladioli after flowering in September. Dry and clean the corms when the foliage is dry. Store in shallow layers in an airy frost-free place. Replant in April.

2 Cut down dahlia stems and lift tubers in September or October. Dry and clean. Dust with flowers of sulphur and store in dry peat in a frost-free place.

3 Cut chrysanthemum stems back to 12 cm (5 in.), lift and clean. Store in clean soil or potting compost over the winter. Keep slightly moist. Start growth in spring.

Remove the growing point as soon as the top wire is reached. Stop side shoots at the second leaf, and sublaterals at the first. Pick off any male flowers, which are those without embryonic fruits attached, to prevent pollination and inferior fruits.

In warm weather, damp down the greenhouse. Once the fruits begin swelling, give weekly feeds of dilute fertilizer.

Greenhouse-type cucumbers grown in frames can be treated as above, except that the training procedure differs slightly. Stop the young plants after the fifth leaf and train out four shoots, one into each corner of the frame. Stop these at the ninth leaf. The laterals and sublaterals are then stopped as for greenhouse culture.

Ridge cucumbers are grown in frames and cloches, and should not have the male flowers removed. Training depends on the variety chosen; follow instructions on the packet.

Sorry, comments are closed for this post.