Raising Seeds in a Heated Cold Frame
An unheated cold frame can be used to raise seeds of all types of plants which would normally be sown directly in the garden, such as many half-hardy annual and hardy biennial bedding plants and hardy perennials — even shrubs and trees. Cold frame protection means that the seeds can be sown a few weeks earlier than in the open garden so that young plants have more time to develop before being planted out in their permanent sites.
Seeds are best sown in pots or trays of a proprietary seedlingand set out on a bed of gravel in the frame to provide good drainage. Water them regularly — the compost will dry out very quickly in warm weather.
Hardening off greenhouse-raised plants before planting them outdoors is another valuable function of a cold frame — young plants must be acclimatized gradually to the lower outdoor temperature. By adjusting the ventilation in the frame, the day and night temperature can be modified as needed over the course of several days or weeks.
Growing vegetables in a cold frame produces earlier crops than outdoors. Ridge, , and melons are ideal for this type of cultivation and should be grown in the in the base of the frame rather than in containers. However, they are best raised from seeds sown in a greenhouse at about 21°C (70°F), then planted into the frame at the beginning of late spring.
Prepare the cold frame soil some weeks before planting by digging in well-rotted manure or compost at the rate of a bucketful per sq m/yd. Just before planting, rake into the top a general fertilizer at 75g/3oz per sq m/yd.
Grow on the vegetables in the frame, ventilating it as necessary during the day. Always close the lights at night until the plants are fully established. In early summer, remove the lights completely, or hinge them wide open, during the day and night.
Shrubcan be rooted successfully in a shaded cold frame —softwood cuttings in early summer; semi- in mid to late summer. Again, insert them directly into the soil in the bottom of the frame or into pots of a proprietary compost.
Overwintering tender plants
A cold frame provides just enough protection from winter frosts to sustain slightly tender plants such as border.
Cut back the plants after flowering, lift them and box the stools up in potting compost. Place the boxes in the cold frame and provide some ventilation — except in extreme weather — to reduce the risk of fungal infection.
Forcing bulbs Instead of forcing hyacinths, narcissi andin an airing cupboard, shed or cellar, try starting them off in a cold frame — the cooler conditions will promote more sturdy growth. Plant the bulbs in pots or bowls and bury them under a 15cm (6in) layer of soil or cold ashes. Eight weeks later they can be uncovered and brought indoors for flowering.
Using a heated frame
By providing supplementary heat you can expand the use of a garden frame considerably — it then becomes a mini hot-house. Electric soil-warming cables laid under the surface of the soil in the bottom of the frame will hasten early vegetable crops.
Air-warming cables or tubes can be fitted on the inside walls of the frame above soil level to provide extra protection against cold temperatures in winter. Follow the manufacturer’s instructions for the installation of heating cables or tubes.
Pre-warm the soil for early crops — such as carrots, lettuces, springand — and ornamental plants by switching on the heating two days before sowing. Hardy species can be sown as early as mid to late winter in a heated frame; or late winter to early spring for half-hardy types. Set the thermostat at around 18°C (64°F) for most vegetable crops.
Ventilate the heated frame during mild days. On cold nights cover the frame lights with sacking, sheets of newspaper or some other insulating material anchored with bricks — soil-heating won’t be able to compensate for extremes of weather. Open the lights progressively during the day as the weather warms up, but close them again at night and during spells of windy weather.
When all danger of frost is over, remove the lights or open them fully and switch off the heating.
As withstools, the more tender rootstocks of and can be overwintered in a frame provided it is heated. In early autumn, cut down the plants and lift them from the ground. Transfer them to boxes of compost and stand them in the bottom of the frame.