Although it has its limitations, the unheated greenhouse can be a valuable addition to any garden and offers an excellent introduction to the world of growing under glass. Inevitably the choice of plants which can be grown is restricted to kinds which need only modest frost protection over winter. However, with a little care in their selection, the unheated greenhouse can still provide almost year-round colour and interest – and in the summer months, almost anything can be successfully grown. The trick is to understand the limitations and the opportunities of this kind of greenhouse and how to work with them to get the best results.
Picking the Right Plants
There is a bit of a tendency to view the unheated greenhouse as something of a poor relation, looking on plants grown under these conditions as being a month or so “behind” those grown in a warmer greenhouse environment. It is much more useful to accentuate the positive and see them as being three weeks or a month ahead of plants grown in the garden – a simple distinction, but a significant one. If nothing else, it means that seeds can be sown and bulbs planted sooner. Half-hardy annuals, for instance, sown in April or early May in the unheated greenhouse should be ready to make a good selection of bedding plants for planting out in the garden in early June and early bulbs potted up in the autumn will provide flowers in the early spring.
There are plenty of suitable candidates for the unheated greenhouse, including Japanese azaleas, lilac and forsythia – all of which will flower noticeably earlier – and hardy annuals such as cornflowers and godetia too are ideal for a spring display. Camellias are perhaps the best choice of all and will often start flowering in late January and carry on until May and sometimes even later. The owner of an unheated greenhouse need never feel cheated of colourful displays!
A surprising range of fruit and vegetables can also be grown very successfully under these conditions, provided the right varieties are selected. Apples, pears, plums and cherries will do well in tubs and in a lean-to greenhouse, a peach or nectarine tree trained against the back wall should produce good fruit in August or September. Amongst the peaches, probably the best one to go for is “Peregrine” or possibly “Hale’s Early”, while “Humboldt” and “Pine Apple” are two excellent varieties of nectarine for unheated conditions.
Strawberries grown in pots will be producing fruit a month earlier than their outdoor relatives, while lettuces can be grown year-round if desired – though a good hardy variety such as “Imperial” will need to be chosen for the winter months. It is also possible to grow that old greenhouse favourite – the tomato – provided planting is delayed until early May.
The Greenhouse in Winter
Winter poses the greatest challenge to the grower, but most of the main problems can be avoided with good management. Ventilation is of course vital in a greenhouse throughout the year, but it becomes particularly so in the winter. It is important to ensure that there is free circulation of air throughout the colder months, though obviously a certain amount of common sense needs to be used when the weather is at its worst – icy winds blowing directly onto delicate shoots does little for them!
It is also useful – if not essential – to insulate the greenhouse at this time of year, bubble wrap or polythene sheeting offering a cheap and effective way of increasing the frost protection it offers. Finally, since the plants are hardly growing at all, watering should be done carefully and sparingly to avoid possible problems with moulds or mildew.
While the range of plants suited to the unheated greenhouse is limited to the more hardy varieties and it has little of the flexibility enjoyed by its warmer counterparts, it remains a very valuable asset to any gardener. Using it to its best and enjoying the potential opportunities it provides may take a little while to get right, but the benefits it offers make it time well spent.