A small greenhouse, even if heated, can be an extremely valuable ‘tool’ in the cultivation of the outdoor garden. Indeed, it can have a quite dramatic effect in its improvement.
1) Bedding Plants
A small greenhouse, or even some frames, can very quickly return the initial expenditure if they are used for raising bedding plants alone, although many other year-round uses can be found too.
Most bedding plants can be sown and grown as described here*. If there is no heating sowing will have to be a few weeks later and more care taken over the hardiness of the varieties.
For bedding plants, the seedlings are best pricked out into large size plastic seed trays. Do not overcrowd them—about 24 seedlings to each tray (14×9in or 35×23cm) is usually adequate.
By leaving plenty of space, and giving more potting compost to each plant, much better and stronger specimens than you can buy in the shop will result. Do not forget too, that you can take advantage of all the offers of new varieties and novelties that the seeds sellers list each year.
If producing bedding plants on a large scale there is no need to keep the air of the greenhouse or the frames at a high temperature. It has been found that keeping the roots warm is far more important.
By so doing, the roots are encouraged to make sturdy development, but the top growth is not; this means a much faster establishment when the plants are bedded out, and the top growth is not forced only to become checked when exposed to the rigors of the weather.
The most convenient way to grow the bedding plants is to plunge the seedling trays in a layer of moist peat spread over sand-covered warming cables. About 10W/90 cm2 (1 It should be allowed when distributing the cable.
The greenhouse or frame air temperature need be only little more than frost free: 2-4 C (35-39°F) is certainly adequate. Often it is possible to enclose the warmed bench or area on which the seedlings are growing with transparent plastic film thus economizing in heating even further.
All bedding plants must be hardened off before planting out; this means gradually getting them accustomed to full exposure to the weather.
It should be started about two to three weeks before the beginning of summer—before the time when all risk of frost has passed in the area where you live—this may vary by a few weeks from north to south.
The hardening off is best done with frames, gradually increasing ventilation until the covers are left entirely off. At first, night cover must be given if there is risk of frost. Also at first frames may have to be slightly shaded if they are in a position of full sun.
Use white cool glass paint for this, gradually wiping it off by running a finger across the glass every few days. Remove the paint in lines, gradually increasing the width until the whole of the shading is removed.
2) Sub-Tropical Bedding And Garden Features
This is where a greenhouse can really have a dramatic effect on the appearance of a garden. Ideally the most useful type of greenhouse for the job is a fairly spacious one, but in most cases little more than frost-free conditions need be maintained in winter.
In some mild areas no artificial heat may be necessary. A great number of the sub-tropical looking plants do, in fact, not demand tropical winter temperatures. The plants can be used in various ways. They can be plunged in beds or borders so that the pots are concealed.
The pots can be slipped into slightly larger ornamental pots or standing on patios or terraces and the like or, in some cases, they can be permanently planted in ornamental containers which can be made mobile for easy moving in and out of the greenhouse.
Tubs can be fitted with wheels, or wheeled stands made for other types of pot.
Many garden centers are also now selling special pots fitted with castors. This is no doubt partly due to the latest fashion of patio gardening.
Some suitable plants to be found described in this website are the hardy, or almost hardy, palms (particularly Phoenix camariensis which is a beauty and easy to manage; Musa ensete, not so easy but worth a try.
Grevillea robusta and jacaranda mimosifolia, both from seed) cannas which are very popular and are used by many municipal gardeners (especially in seaside resorts), neriums (usually pretty easy), Ricinus communis varieties also very popular Erythnna crists-galli (easy but not so often seen).
Agave americana which is bound to attract attention, and sometimes Strelitzia regina where the flowers will not be exposed to too much weather punishment.
To this lot can be added the very beautiful Phormium tenax. This is hardy in some mild areas and will reach a considerable size.
It makes a good pot plant and will survive well often in an unheated greenhouse. Two very lovely forms are ‘Purpureum’ which has purplish foliage and Variegatum’ which has green leaves striped with cream.
The foliage is erect and sword-shaped. Cordyline indivisa and C. australis also have sword-shaped sharp foliage, and are again quite easy to overwinter. They can be simply and cheaply raised from seed and, in their young stage, can make useful greenhouse foliage plants for the staging.
Among the easiest tropical-looking plants to grow from seed, and fast growers, are the various forms of ornamental maize, Zea mays. The varieties with cream, green, and rose-pink striped foliage are especially attractive, and can be raised along with the summer bedding plants.
Another easy group is the amaranthus notable for richly colored foliage. ‘Red Fox’ and the new more ‘bushy tailed’ ‘Pigmy Torch’ have quite eye-catching erect bold catkin-like reddish tails, and are easy half-hardy annuals.
All plants to be used as temporary features for beds and borders, with their containers plunged, should be given clay pots.
These pots should have a layer of sharp sand at the bottom to discourage entry of earthworms (and some soil pests). Clay pots are porous and will allow moisture to reach the roots from surrounding soil—even so, watering must not be forgotten.
All perennial plants must, of course, be taken up and returned to a greenhouse well before the first frosts.
For pots to stand above ground, it’s best to avoid plastic, unless they are to be used as liner pots. Many plastics soon become brittle when exposed to sunlight; when you attempt to pick them up, they may fall to pieces or split!
Before returning sub-tropicals to their winter quarters, make sure they are not harboring any garden pests, and cut away cleanly any damaged or dying vegetation.
3) General Propagation
Apart from bedding plants many choice perennials can be grown from seed and kept in a greenhouse until well established in their pots. The seedlings can be given a good start and protected from both weather and pests that often cause young subjects significant damage.
The vegetable garden can be made more productive by raising many vegetable seedlings and pot growing in the early stages in the greenhouse, provided the plants are not coddled. This way you also get much earlier cropping, e.g. sweet corn, sweet peppers and outdoor tomatoes.
Many of the favorite garden shrubs, trees, and other perennials can be propagated from cuttings. This work is also often best done in the greenhouse. Those gardeners especially interested in the more choice shrubs will find a mist propagation unit useful.
4) Overwintering And Early Starting
A great many of the more tender garden plants need a greenhouse to survive the winter. Pelargoniums, fuchsias, garden chrysanthemums, and dahlias are typical important plants benefiting from such accommodation. A greenhouse is essential for propagating the chrysanthemums which send up plenty of shoots from the cut-back plants to use as cuttings in early spring.
As well as saving the old plants over winter, a greenhouse will allow cuttings of most types of pelargonium to be taken in late summer to autumn. These, overwintered in frost-free conditions will yield masses of new plants for bedding out the following summer.
Dahlia tubers can be started into growth in late winter and the shoots used as cuttings to increase stock. A trick for getting unusually early blooms—not generally known—is to pot up, in large pots, in early spring.
Grow on the plants in the greenhouse to an advanced stage and until the beginning of summer when it is safe to plant out in permanent positions.
Often the plants will start to bloom almost immediately and continue all the rest of the year until cut down by frost. By using this method you can enjoy these lovely flowers for the maximum period and they will enhance the garden enormously.
Terrace and patio pots can be kept colorful over a long period by growing the plants in ‘liner pots’—plastic pots just large enough to fit inside the ornamental pot.
The ‘liner pots’ can be planted up or sown with seed and kept in the greenhouse until ready to put out. Quick changes according to season can be made, resulting in a wide variety of color.
5) Growing Your Own Houseplants
Many of the more exotic houseplants need congenial warmth, and often moderate humidity, to grow well and to maintain their attractive appearance. They do not make reliable long-term subjects for the average greenhouse with low winter temperatures.
However, with the aid of a large warmed frame in the greenhouse as described on in this post*. You can use an ordinary cool greenhouse to grow many houseplants from seed, or to propagate those you have, or from cuttings donated by friends.
To germinate seed of choice kinds a temperature of about 24°C (75°F) must be attainable if needed. Some, such as Saintpaulia, may be happy enough with several degrees lower. Similarly, to root cuttings quickly and well, a brisk ‘bottom heat’ is desirable.
The frame should be large enough to accommodate seedlings or rooted cuttings until they are well established, and until the greenhouse temperature will be high enough for a transfer to the staging, where the plants can often be grown on further during the summer months, prior to being moved to the home.
Some houseplants to raise from seed, freely available, are Ficus elastica and F. benjamina (do not cover the seed with compost when sowing); Monster a deliciosa; philodendrons; Stephanotis floribunda; Anthurium scherzerianum; Crossandra infundibuhformis; Gardenia jasminoides and Ruellia macrantha. You may also like to try seed of many tropical palms, bromeliads, cacti and other succulents, and warm climate fern spores.
By reproducing ‘jungle-like’ conditions in your warmed frame—sufficient heat and humidity—you can get most tropicals to root from stem and often leaf cuttings, very easily. Most of the begonias and members of the gesneria family will root readily from their leaves.
Those that form clumps can be propagated by simple division—marantas and peperomias, for examples. Houseplants are never particularly cheap to buy, so it’s worth exploiting this possibility. The plants can be useful as gifts too.