Plants for Unheated Greenhouses

Too many people deprive themselves of the pleasure of owning a greenhouse by thinking of the cost of heating it during the winter. But you can enjoy enormous benefits from protected cultivation without providing any additional heat. There is a wide range of plants that need only shelter from wind, excessive rain, frost or extreme cold, to give finer results and earlier crops or flowers than if grown in the open. Indeed, these plants will not do well in a greenhouse that is too warm — and so an unheated one is ideal.

This section on plants for unheated greenhouses looks at what types of greenhouses are suitable and suggests some plants that can be grown from seed and from bulbs to provide a display of colour all year round.

The degree of hardiness of the plants you can grow will vary according to the severity of winters normally experienced in the part of the country in which you live. In mild areas, for example, comparatively tender plants will survive quite easily. In colder regions you may have to be more cautious in your selection.

Suitable greenhouses

To make the most of free warmth from the sun the unheated greenhouse should let in as much light as possible. A glass house will trap more warmth than a plastic one, but if you propose to grow only hardy plants then this will not be so important. Remember, however, that any extra free warmth may be useful for gentle forcing and early growth, even if the plants are completely cold-resistant. An all-glass (glass-to-ground) greenhouse will capture much natural warmth, and so will a lean-to structure set against a south-facing wall. In some cases such a lean-to will remain frost-free overnight as the rear wall can store warmth during the day and radiate it at night.

Don’t choose a site shaded by buildings or trees if you are erecting a cold greenhouse. Nor should you put the house in a hollow or at the foot of a slope or hill; these sites are often frost pockets. Wet or waterlogged ground is also best avoided since plants in a cold greenhouse dislike an excessively humid atmosphere in winter, and are more prone to attack by fungus diseases.

Spring sowings

Probably very few home greenhouse owners use heating from about late spring (April) onwards. In spring a glass greenhouse will usually become sufficiently warm naturally to germinate most bedding plants, tomatoes and more exotic summer fruits like melons. These can be grown on to give useful summer and autumn cropping. Without warmth, or perhaps with just a small heated propagator, most of the more hardy and quick-growing bedding plants can be raised. Tagetes (African and French marigolds) are extremely easy plants, and you can leave the sowing until quite late. Avoid slow developers that prefer congenial warmth (such as fibrous-rooted bedding begonias). But the slow growers like antirrhinum will do well since they are perfectly hardy and can be started as early as late winter (January).

Summer and autumn displays

During the summer and autumn the greenhouse can become a wonderland of colour for only the price of a few packets of seeds germinated from late spring to early summer (April to May) without artificial warmth. Outstanding for its colourful blooms is salpiglossis; stop the plants at the seedling stage (when a few centimetres high) to promote bushy growth. Pot on into 13cm (5 in) pots.

The new compact strains of schizanthus are also easy to grow and showy, and need no stopping or training. Many of the more choice garden bedding plants, especially F1 hybrids, make splendid pot plants. F1 hybrids of zinnias are spectacular when given weather protection, and you can produce magnificent columns of blooms if you grow the large-flowered antirrhinum as single spikes by removing all sideshoots.

The cold greenhouse is ideal for protecting delicate or double flowers that are prone to weather damage and summer winds and rains. Double petunias do very well, provided there is maximum light, and some varieties will fill the house with a clove-like scent that you won’t notice outdoors. The recently-introduced variety of hibiscus Southern Belle, with its enormous, but papery-thin, flowers, is almost certain to be spoilt outdoors but will be seen in its full glory with greenhouse protection. Sow it in a warm place indoors and move the young plants to the greenhouse in about early summer (late April or May). From then on they make rapid growth and the plants will need 25cm (10 in) pots as a minimum size for their final homes.

Bulbs for summer and autumn

For summer and autumn display many bulbs or similar storage organs can be started in spring. These include begonia, gloxinia, achimene, polianthes (tuberose), hippeastrum, the glory lily, gloriosa (which, contrary to many warnings, can be started late and still flower well), canna (of which the best for the small greenhouse is the variety Lucifer — the earlier you start these the better), smithiantha and many lilies and nerines.


With nerines, use the named greenhouse varieties that are potted on in early autumn (August). Storage organs of most greenhouse plants that flower from summer to autumn can be stored dry over winter in a frost-proof place in the home, and started into growth again in the following year.

Winter and spring displays

Of special importance for the winter-tospring display are hardy bulbs. An unheated greenhouse is the ideal place for them; and you will be amazed at how much more beautiful many of them are than you had previously realized. They usually flower much earlier under protected conditions and the flowers will often be more noticeably scented and unblemished. Choose unusual varieties with double or more delicate flowers.

As well as the popular hyacinths, daffodils and narcissi, grow a selection of the smaller, dainty bulbs generously grouped in bowls or pans. Such collections could include allium, babiana, bulbocodium, chionodoxa, eranthis (winter aconite), erythronium, fritillaria, galanthus (snowdrop), leucocoryne, leucojum (snowflake), muscari (grape hyacinth, of which there are some especially fine forms), puschkinia (striped squill), scilla (squill), tecophilaea and urginea.

Crocus, alone, can be had in an astonishing range of varieties and species from which you can choose a selection to flower over a long period. And under glass the birds cannot tear them to shreds. Tulips grown in pots provide a glorious display, especially the double varieties.

There are numerous other spring-flowering plants notable for colour. Polyanthus make fine pot plants and are easy to grow from seed. Coloured (and completely hardy) strains of primrose are now available and are particularly good for pots.

12. July 2011 by Dave Pinkney
Categories: Bulbous Plants, Greenhouse Gardening | Tags: , | Comments Off on Plants for Unheated Greenhouses


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