The unhealed greenhouse is a valuable asset to any gardener hut it is rather limited in its scope. The choice of plants must necessarily he restricted to the more hardy or those which need only slight protection during the winter months.
An unhealed greenhouse becomes more useful if it is lined with thin polythene sheeting to serve the same purpo*.- as double glazing. This certainly helps to keep out less severe frosts and maintain a higher temperature, thus making it possible to grow a w ider range of plants. It is very important with this type of house to use the ventilators to keep the air circulating freely in winter. Nothing is worse for plants than stagnant air. which, among other things, encourages attacks from botrytis and other fungus diseases. Ventilation must he given throughout the year hul obviously with discretion and intelligence in the coldest weather. Watering, too. must be done with especial can: in the winter months for plants in unhealed houses are barely growing at this time of year whereas their counterparts in heated greenhouses are in more active growth
One should think of plants grown in an unheated greenhouse as being three weeks to a month later than those in a greenhouse in which a minimum temperature of 4 to 7 C. <4() to 45 F.t is maintained. Such plants will also he roughly as much time ahead of those grown out of doors. Leading on from this. I would add that seed sowing in the unhealed greenhouse can be a grc.il success but it is wise to make sowing dates later than those which apply lo the heated greenhouse, For example, April and early May
is the best period to sow seeds of half-hardy annuals and these will provide good plants for planting oui in late May and early June.
Bui if there arc limitations, the unhealed greenhouse can still be great fun One of the best planis for such a house is. I think, the camellia. Camellias can withstand quite a bit of frost, but wiih the protection of a greenhouse they will start to flower in late January or February and continue into May and sometimes June. Evergreen azaleas will also flower much earlier lhan usual in such a structure as will liiac and forsvthia. Hydrangeas will over-winter in an unheated greenhouse as long as they are kept on the dry side, to start inio growth in the spring and flower in June. Jtil^ and August.
Hardy annuals like clarkia, cornflower, godetia and mignonette arc a good choice for spring flowering, Salpiglossis will also sometimes come through safely in an unheated house and in milder parts of the country so will calceolarias and schizan-thuses. Many times, even in greenhouses with some heat, I have seen these last two plants completely frozen after a hard night of frost. They have been sprayed over with cold water while m ihis condition and have come through unharmed to flower profusely in April. May and into June. The range of plants which can be grown in an unhealed greenhouse in summer is almost unlimited pelargoniums. gloxinias, fuchsias, begonias and so on. They are started off m frost-free conditions in April and May and then transferred to the unhealed greenhouse from May onwards when the sun's rays will provide sufficient warmth.
A w hole range of early-flowering bulhs can be planted in pots and pans to make a splendid display in early spring. For example, the yellow Hoop Peiticoal Daffodil. Narcissus bulbocodimt conspiatus. the lovely cream Angel's Tears. ,V. triandrux alhus. the rich yellow V. cyclamirwus, the yellow and orange Grand Soleil d'Or and Lrumpet varieties like the golden King Alfred, as well as hyacinths and dwarf bulbous irises such as the deep purple, golden-blotched Iris reticulata and /. histrioides, a lovely rich blue marked with whiteand gold. Lilies like ¡.Mian specioswn rubrum and L. auratum, the lirst for late summer and the latter for late summer and early autumn flowering, as well as the more modem varieties and hvbrids. bring an exotic flavour to the unheated greenhouse.
Lettuces can be grown in an unhealed greenhouse during the winter months if one of the hardy varieties such as Imperial is chosen. Tomatoes can be grown, too, if planting is delayed until late April or May. Tomatoes need a minimum soil temperature of 14 C*,(57 V,). Strawberries can be grown in pots and these will be three weeks to a month earlier in fruiting lhan those oui of doors. The Black Hamburgh grape does well in such conditions and if the house is a lean-to, a peach tree is ideal for growing up the wall It is also exciting to grow apples, pears, plums and cherries in large pots or tubs. The quality of the fruit produced under glass is much superior to that on trees grown out of doors.
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