Raising plants from seed is the most economical means of providing a long-lasting display for the cool greenhouse. Where a minimum night temperature of about 45°F (7°C) can be maintained the choice of flowering and decorative foliage plants is considerable. Many plants that we normally grow as annuals in the open may prove to be perennial when grown under glass, as they are in their native conditions.
For example, the fast-growing Mexican cup-and-saucer vine, Cobaea scandens, has large purple and green cup-and-saucer flowers over a long period. This evergreen plant which clings by means of tendrils can make as much as 20 ft. of growth in a season, so it is too rampant for a small greenhouse but most decorative for a sunny wall in a large conservatory.
The popular morning glory (ipomoea or pharbitis) requires much less space, twining up supports to a height of about two ft. and bearing a succession of large sky-blue funnel-shaped flowers, as will the Chilean glory flower, Eccremocarpus scaber, which bears clusters of bright orange flowers for many months. Even in the open, in milder parts of the country, the last named will sometimes seed itself when grown in a sheltered sunny corner, so it is easy enough to raise from seed. All these flowering climbers can be grown in a sunny greenhouse border or will flower happily when grown in pots or other containers.
Seeds of these plants, in common with many others, will germinate readily if sown under glass in March when the days are beginning to lengthen and to get warmer. An ideal temperature for germinating most seeds of this type is 65°F (18°C) and to maintain this as well as a moist atmosphere it will be found economical to have a propagating frame within the greenhouse. This need not be a costly affair, just four sides of a flat and a sheet of glass will suffice. Such a frame heated by an electrical soil-warming cable is convenient and economical in that the whole greenhouse does not have to be maintained at this temperature.
With an electrically heated frame the thermostat can be set at a slightly higher temperature to encourage germination and when this has taken place the temperature can gradually be lowered. Whatever means of heating a greenhouse or frame is used the aim should be to keep the temperature as constant as possible and to avoid wide fluctuations of day and night temperatures.
Most seeds germinate best in the dark and it is wise therefore to shade the frame or the pots and flats in which the seeds are sown, with brown paper. This will provide sufficient shade and at the same time reduce the amount of watering that may be necessary. The soil should be kept just moist in the early stages after sowing, but as germination takes place a little more watering will be required, and as soon as the seed has germinated the shading should be removed. However, should germination coincide with a spell of really bright sunny weather it is as well to shade the tiny seedlings from the midday sun.
These days seed-sowing and potting composts, based on loam, or of the soilless, peat-moss, and sand type, are readily obtainable at garden shops in various-sized polyethylene bags or cartons and are usually moist ready for use, which makes life easy in this respect.
While cyclamen are perennial tuberous-rooted plants they are often treated as biennials. They are easily raised from seed sown under glass in summer and will flower within about 15 months, that is by Christmas the following year when an individual plant will be quite an expensive item. Seed germinates readily at a temperature of 60°F (16°C), which means that the house or frame will need shading to keep the temperature down in August, and the atmosphere should be kept moist in hot weather by spraying water around the pathway in the greenhouse. When the seedlings have made two leaves prick them out in seed trays, or singly in small pots, and grow them on in gentle warmth. Put the little corms into larger pots when necessary, but do not bury the corm when potting, merely making it firm in the surface potting compost.
There are various excellent large-flowering varieties in shades of salmon, crimson, mauve, and white, also a new double-flowered `Kimono’ strain of Japanese origin, in a pleasing range of mixed colors. After flowering, the corms can be discarded, or treated as perennials. The dainty Cyclamen persicum, the parent of modern hybrids, has charming white or pink, sweetly fragrant flowers and is normally spring flowering.
Obviously one wants color in the cool greenhouse during the winter and early spring and cyclamen will help to provide this for a long period. Other decorative plants that are easily raised from seed include Primula malacoides with candelabrum-like flowers on strong stems in shades of pink, lilac, and mauve; the drum-head Himalayan Primrose, which is, in fact, a hardy perennial but easily raised from seed, will flower from February onwards under glass. The long-lasting heads of flowers are in shades of purple, mauve, or white. And don’t forget fragrant polyanthus and primroses in many colors. These flower for months in cool conditions.
Cinerarias sown in June should provide bold splashes of color from February onwards. At this time the little Limnanthes douglasii should be a mass of yellow and white fragrant flowers and the sky-blue nemophila which also makes a charming pot plant for a partially shaded place in the cool house, should be in full flower. Schizanthus, a butterfly flower, is available in an astonishing range of glorious colors — salmon, apricot, pink, yellow, mauve, and purple. Many of the flowers have attractive markings. They thrive in cool house conditions as do calceolarias, but the latter does not usually start to flower until June, from seed sown the previous May.
The Chilean salpiglossis is remarkably beautiful, with a richness of color and splendid veinings on the trumpet-shaped elegant flowers. The color range includes rose, crimson, purple, cream, and many blends. There are dwarf forms about 1 ft. in height and also large-flowered hybrids up to 3 ft. Seed sown under glass in August or September will make delightful flowering plants the following May.
There are numerous varieties of Coleus blumei which are grown for their decorative foliage. As they are natives of Java and Central Africa they like a warm, moist atmosphere which may make them a little difficult to accommodate in a cool greenhouse. Seed sown in warm conditions in February will provide large plants later in the year. While coleus are in fact perennial shrubby plants, given sufficient heat, they are more often treated as annuals. The leaves are in various shades of red-bronze, yellow, green, and maroon. The spikes of blue flowers are best pinched out to encourage bushy, leafy plants.
Seed of Celosia plumosa sown under glass in March will provide summer-flowering annual pot plants of striking appearance with bright red, feathery tassels in summer. There is also a golden form. These fast-growing plants require a rich soil and plenty of light.
Some annuals that do not transplant easily may be sown thinly in pots in which they are to flower. After germination, the seedlings should be thinned out to five or six plants in a pot, according to the size of the pot. It is as simple as that. Plants that can be treated in this way are the Californian poppy (eschscholzia), in shades of orange, copper, yellow, carmine, and ivory; linaria, viscaria and the charming long-lasting mignonette (reseda), which is uniquely fragrant. Many other annuals can be sown in the normal way and will transplant quite readily when large enough to handle. A few pots of such plants will come in very useful to fill gaps in the sequence of flowering.
Other plants that will give a good display in a cool greenhouse are antirrhinum, Brachycome, Campanula isophylla, of trailing habit and useful for growing over the edge of a shelf or in a hanging basket; Campanula pyramidalis, the chimney bellflower, with wide-open pale blue flowers borne on erect stems from 5-6 ft. in height, and clarkia sown in the fall will flower freely the following spring. There are annual and biennial stocks (matthiola) which may be sown at different times of the year to provide color and delicious fragrance for many weeks.
There is a wide range of hybrid petunias that are admirable as pot plants, in tubs, or in hanging baskets. They are outstandingly colorful and long-lasting large flowered singles and doubles, compact bedding varieties, pendulous hybrids suitable for window boxes, some with fringed and ruffled flowers, bicolors, and many self-colors.
These are but a few of the flowering plants easily raised from seed which is so welcome in the cool greenhouse throughout the year.
How to Plant Annual Flowers
How to use Annual and Biennial flowers