For the greenhouse plants list, I have chosen a range of plants which the keen amateur might well consider as possible subjects for the greenhouse. The choice has been made to suit the warm or the cool house, and I have indicated that temperatures should help make a selection easier.
Plants to Grow in the Greenhous
Abutilon (Indian Mallow) A semi-shrubby, half-hardyperennial with bell-like flowers hanging from the axils of the leaves on short stalks. Some species are grown for their foliage value. Species and varieties: A megapotamicum, red and yellow; A. striatum Thomsonii, green and yellow foliage; A. Savitzii, silver, yellow and green foliage. Most others will come under a group called hybrids. Propagation by cuttings and from seed. Soil: an ordinary potting compost, with rather more loam added. The temperature in winter 45°. Acacia (Mimosa) Shrubby, hard-wood plants, flowering in spring. Flowers variously shaped but all rather beautiful. A few are ideal for pot culture in a cool house. The main requirements are water and food during spring and summer. Species: A. armata, A. Baileyana, A. decurrens dealbata (Silver Wattle), A. Drummondii, A. longifolia, A. lophantha (Albizzia) and A. Riceana. A very loamy, fertile soil, firm potting, and ample syringing in spring is required. Propagation: By half-ripe cuttings taken in summer or from seed.
Achimenes
Belonging to the gesneria family, this is a useful plant for the warm greenhouse or the cool house in summer. Its variously colored tubular flowers in many shades and its soft foliage make an interesting subject in pots or pans. Species: A. longiflora (violet-blue), A. grandiflora (reddish-purple). There are many varieties of these. Compost should contain at least one-third peat. Warm, moist conditions are necessary when growth is developing. Tubers are dried off in autumn and started again in spring—propagation by seed or offsets.
Agapanthus
This is a long-stemmed, blue-flowering plant, with strap-like leaves and many lily-like flowers radiating from the top of the stem. It has thick, fleshy roots and demands ample supplies of water during summer; but must be kept almost dry in winter. It is nearly hardy and is thus a good subject for cold houses, but the winter temperature should not go below freezing-point. Soil should be rich loam with only slight additions of peat or leaf-mold. Propagation is by division.
Ageratum
This well-known bedding plant grows and blooms exceptionally well in a 3-inch pot, and much more use might be made of it. Sow in autumn and very early spring, and grow in a temperature of not less than 40° in winter. Any ordinary potting compost will suit it.
Alonsoa (Mask Flower)
This is another of the neglected annuals which does so well in 5- or 6-inch pots. Sown in September, the plants bloom in May, the most useful species being A. Warscewiczii, with deep salmon-red flowers.
Anthericum (St Bernard Lily)
One species, A. Liliago, is worth considering as a pot plant for a cool house. Its flowers are not very striking, but their numbers make cultivation of this plant a great pleasure. It will stand temperatures down to 40° in winter. Likes a rich loam as a rooting medium. Propagation by division or from seeds.
Antirrhinum (Snapdragon)
Growing these from seed sown in September and giving the young plants a genial temperature of about 45° in winter should ensure good specimens in full bloom in April. Soil should be on the loamy side and rich. Choose from the group known as A. ma jus.
Aristolochia (Dutchman’s Pipe)
The species A. duration (sometimes called A. Sipho) makes an admirable climber for a cool greenhouse. It has yellow, pipe-shaped flowers, from which it gets its name. The soil must be loamy and rich. Much water in spring and summer, with less in autumn and very little in winter, should keep the plant safe from seed or cuttings.
Arum Lily
The botanical name is zantedeschia, but I include this plant as it is the name it is so well known. Best grown in a house kept at not less than 50° in winter. The roots are rested from April to August, being placed in a trench outdoors for that time. In late July drench with water, repot in August and grow in a cool, lighthouse. Soil, a rich loamy mixture.
Azalea
From a large number of species available, those most commonly used for pots are A. indica, requiring a warm house if it is to bloom in midwinter (though quite safe in a cold house, but blooming later), A. Mollis and the many hybrids of the Japanese kinds. Some of the best of the latter should be chosen and grown on year after year. The main points are syringing daily after blooming, standing outside when frosts are over, and repotting every two years. Much water in the growing season, which is from March to August. Soil: 2 parts of loam, two parts of peat, and 1 part of sand. Propagation by cuttings or grafting.
Babiana
These are small-growing bulbous or cormous plants, ideal for pot culture. Pot in September and grow as suggested for freesias. Propagate by offsets. Bartonia A perfect annual for pots, blooming in April if sown in the previous September and grown all the winter in a cool house. Grow one plant in a 6-inch pot for best results. Ordinary compost will suit, but firm potting, ample light, and air are essential.
Begonia
A very large family, offering many species suited to varying temperatures. For the type of house most generally used by amateurs I suggest the following selection: B. semperflorens (fibrous-rooted), B. fuchsioides (fibrous), B. metadata (fibrous), B. weltomensis (semi-tuberous), and, of course, the many tuberous hybrids, frilled, single and double. Tuberous roots are dried off in winter and the tubers stored in sand. The fibrous sorts are kept almost dry in winter, but not quite. Both types are started into normal growth in a warm house in spring; rich soil and firm potting are essential. They are propagated by cuttings and division. Winter-flowering species should be left to those who are experts and have warm houses.
Beloperone (The Shrimp Plant)
This plant needs a warm house to grow it well, and it is not so much its flowers as the salmon bracts, which almost enclose them that are attractive. Any good soil will suit it, and stock can be increased by taking cuttings in summer. Boronia Fragrant shrubby plants which can be adapted to pot-culture, all from Australia. A fifty-fifty mixture of loam and peat will suit, with some coarse sand added. Cool-house treatment is ideal. Species: B. elatior (rose-red), B. heterophylla (red), B. megastigma (pink). Propagate by taking cuttings in spring.
Bougainviilea
An attractive climbing plant, but best in a house maintained at 45-48° in winter. Always grows better if planted out in the house, as it hates root restriction. It is best when trained close to the roof-glass. A compost mainly of loam, but thoroughly drained, will suit. It is propagated by cuttings rooted in heat. The varieties of B. glabra are all excellent.
Browallia
Easily grown blue and white flowered subjects best sown in spring. The two most useful are B. speciosa major and B. viscosa ‘Sapphire’. Any good compost will suit them.
Brunfelsia
Shrubby plants with beautiful funnel-shaped blue flowers, requiring a minimum temperature of 50° in winter and a certain amount of humidity in the atmosphere. Soil must be rich, made up of 3 parts of loam, 1 of coarse peat, some dried cow or horse manure, and enough coarse sand to keep the soil well-drained. The species B. calycina and its varieties should be chosen. Propagation by cuttings struck in a warm pit.
Calceolaria (Slipper Flower)
The type which appeals to most gardeners is known as the herbaceous calceolaria, which has enormous pouches in May and June. To grow these well, sow seed in July, prick out and grow cold until large enough for pots. Then put into the 3-inch size and ultimately into the 6- or 7-inch size. Use a well-broken loam at all times, with only a quarter of leaf-mold or peat and enough sand for drainage. Keep cold and clean. Shrubby species can also be grown from seed or cuttings. C. nana is excellent for small houses.
Calendula (Pot Marigold)
An annual which not only makes a good pot plant but flowers in late winter and early spring from an August sowing. Subsequent sowings will ensure a sequence of blooming. It is essential that the soil used is nearly all loam. Grow cold.
Camellia
This lovely shrub is a fine greenhouse subject, more especially for the cold house. Potting soil must be well-drained and made up of 3 parts of fibrous loam, 1 part of leaf-mould (or peat) and some coarse grit. Pot firmly. Plants can stand outside most of the year but should be inside from November to March for the blooming period. Propagation is by grafting.
Campanula (Bell-flower)
This latter, treated as a biennial, will make plants 6 feet high in 8-inch pots, and between this and the miniatures are hundreds of species and varieties to choose from.
Canna (Indian Shot)
A fine tropical plant of easy cultivation, giving spikes carrying a noble mass of gorgeously colored blooms. Though tropical, it grows. Splendidly in a cool house. The fleshy roots, which are-rested in dry soil in winter, are potted into 7-inch pots. In March, watered and given daily spraying. Soon they will unfold their ornate leaves and with ample watering, and feeding will quickly be in bloom. The spikes grow 3 or 4 feet high and give a long period of brilliance before being dried off in autumn. Propagation is by division or from seed.
Carnations
This subject is too large to be dealt with within a few lines. Only a website can really deal with it, and there are many excellent ones. For winter and spring, the perpetual-flowering type should be grown and a house given up to them; these being followed by the borders, the-‘Cottage’ type and all the many hybrids between carnation and pink which, like the Allwoodii group, make. Fine pot plants. Soil for all types must be nearly all loam, with lime added when necessary. Propagation is by layering, cuttings, and seed.
Celosia
An ideal pot plant which, being a summer plant, is quite happy in a cold house. Seed should be sown in April and the seedlings potted on as soon as they are ready, first into 3-inch pots and then, long before they become starved, into 5- or 6-inch pots. Avoid starvation and dryness. Use a rich loam with one-third of peat added, plus fertilizer and sand. Give some humidity all the time, and once the red or gold plumes develop, give more air and almost full light.
Chorizema
This is an Australian shrub that makes a grand pot plant and will go on for years with normal top-dressing or repotting. It needs rich, loamy soil with some peat added, plus perfect drainage. Being nearly hardy, it only requires a cool house and normal attention to develop into a flower-smothered shrub. Little water is needed in winter, but normal watering should begin in February. Propagation by cuttings taken in summer. The favorite species is C. cordatum, with attractive flowers of orange and purple.
Chrysanthemum
Here again, is another plant that I will not attempt to deal with. There is so much to be said about it that only a work entirely devoted to it can be of service, and the reader is recommended to refer to Amateur Gardening Handbook No. 7, Greenhouse Chrysanthemums, by E. Morley Jones, in this series. However, as a greenhouse plant, it is the outstanding flower of autumn, and as such, it demands the premier position. The emphasis will be upon the Decoratives, Large-flowered, Singles, and perhaps the Cascade and Charm types.
Cineraria
This must be one of the most popular of all greenhouse plants, responding ideally to cold conditions so long as the plants are not actually frozen. Sow seed at intervals from April to August, grow on in frames, and use a very loamy soil. Beware of placing the collar too low in the soil, and water from September onwards with the utmost caution. All types are best raised from seed. There are tall or stellate forms, dwarf, large-flowered, small-flowered and semi-dwarf types, and a wide color range.
Clarkia
One of the favorite annuals for pots. Sow in October and pot into 3-inch pots for the winter, keeping them on a shelf in a frost-free greenhouse. An ordinary soil mixture will suit. Any of the newer varieties of C. elegans will do well in pots. Stake early to prevent excessive swinging and breaking.
Cobaea (Cups and Saucers)
An admirable climbing plant for the roof of a cool or cold greenhouse. It is best treated as an annual, being sown in a warm house in February and potted first into 3-inch pots and ultimately into 8-inch. Use an ordinary compost, but feed with liquid manure when the large pots are full of roots. The plant climbs by its tendrils, and so long as there are wires or wood trellis to hang on to, no other support is necessary. The best species is C. scandens, with large bell-shaped flowers of mauvy-blue, purple, or white. Propagation is by seed.
Summary
Avoid growing hot-house and cool-house plants together, or those who depend on high humidity with those who require a dry atmosphere.
Purchase young plants from a reliable source, and remember that many a well-stocked greenhouse today had the foundations of such a collection laid by the raising of many plants from seed.
As time goes on, one will wish to adventure into the lesser-known families of plants and, helped by the knowledge gained when passing the novice and elementary stages, there are indeed no limits for the keen grower, so long as he has the suitable heating and structure— coupled with his enthusiasm.
It is then that he may require more detailed help in the cultivation of certain genera, but as there are many works on greenhouse plants, these should provide all the instruction necessary and at much greater length than is possible in a work of this size.