Sowing tiny seeds and watching the whoie process develop, no matter how long it takes, is a never-ending pleasure to those who become deeply involved in gardening. At the same time there is nothing more maddening than careful sowing, a long wait, and then no results. Many of us will never attempt to acquire the right mental attitude to raise rare shrubs or difficult alpines from seed. But there is room for all tastes in the greenhouse and a wide variety of reliable seeds will give rapid and predictable results with reasonable care and reasonable luck. Some plants regarded as difficult are in fact easy, if one can sow fresher seed than is often available in a packet. The length of time seed remains viable is extremely varied and greatly affected by the way it is kept. Although one is advised not to discard pans of rare seeds for two or three years, this is unpractical and depressing in the conditions of the average small greenhouse.
The equipment used for seed sowing under glass ranges from the fast disappearing clay seed pan and wooden box through plastic trays, pots and pans to peat pots and compressed soil blocks as well as the plastic cast-offs of modern living, Every amateur evolves a personal method, but all successful arrangements ensure a temperature sufficient to allow germination and moisture that is constant without being excessive. It is sometimes said that the ideal temperature for germination of seeds is 10°F above the optimum growing temperature of the plant concerned. This is only a very rough guide and one cannot give each packet of seed a different temperature. As 21 °C (70°F] is satisfactory for the germination of a great many seeds of greenhouse and bedding plants, most seed raisers are designed to raise the temperature to this level. The simpler types lift the temperature but do not have thermostatic control, so that one must guard against the sun shining on them. The most elaborate kinds with both soil and air heating on separate thermostats are very costly. However, soil warming cables can be put into benches or frames and there are many ways of arranging a warm corner in a cooler greenhouse. The highest temperature is only required until the seeds germinate. All seeds need to be sown much more thinly than comes naturally, with the smallest ones merely pressed lightly into the surface and the larger ones buried to their own depth in the compost.
The traditional way of covering seed boxes is a sheet of glass and a sheet of brown paper or newspaper. The glass should be turned each day to remove condensation and let in fresh air. Many seed-raisers have stiff plastic covers and only need shading from sunlight. No matter how the seeds are covered, daily inspection is advisable since they must have good light as soon as they germinate. A few plants, including Primula obconica and P. sinensis, germinate best in light and some only in the dark.
A realistic assessment of how many seedlings can be grown to maturity may mean that only quite small receptacles are needed. I use J in. (12mm) square pots, which fit closely together in a seed tray. This goes into a simple seed raiser in my kitchen. The moment any seedlings appear, they are moved to stronger light and cooler conditions. They must never dry out or be allowed to stop growing; therefore early pricking out is usually advised. This is always a matter of judgement if the weather is bad and the temperature rather low, but crowding does weaken growth.
Most plants are pricked out into larger, deeper trays about 2 inches (5cm) apart in a slightly richer compost. These are easier to keep evenly moist by hand watering than individual small pots. However, if one does not want enough plants to fill the boxes, individual plastic pots may be better, particularly on capillary benches. Quantities of seedlings urgently needing pricking out at the same moment are one of the nightmares of gardening.
Here are some suggestions for plants to grow from seed for greenhouse decoration with sowing dates and the approximate time taken to flower. Annuals are marked A, biennials B, perennials P, MWT is the minimum winter temperature advised for over-wintering.
Abutllon xhybridum. P. Spring, about 16 weeks. MWT 7°C (45 °F).
Alonsoa. P, grown as A. March to May, About 16 weeks.
Asparagus meyerii. P. Spring, Foliage plant. MWT 7°C (45°F).
Begonia (fibrous rooted). P. February or March. Nearly 6 months.
Brachycome (Swan river daisy). A. March, About 12 weeks.
Browallia. P. March or June. About 6 months. MWT 10 °C (50sF) for winter flowers.
Calceolaria, A and P. June for flowering following spring. MWT 7°C (45°F).
Coieus, P as A. Early spring in heat. Foliage plant. Discard autumn.
Cuphea. A and P. March about 16 weeks. MWT 7°C (45°F).
Gerbero, P. March or when new seed available. About 15 months for tall kinds but new compact hybrids flower same year. MWT 10°C (50°F) for reliable winter flowering.
Gilia rubra. B. Early spring for late summer and autumn or July to over-winter and flower next year. MWT 7°C (45 °F).
GreviJIeo robusia. P. March. Foliage plant. MWT 7°C (45°F).
Heliotropium (Cherry pie) P. March. About 15 weeks.
Hypoesies snnguinolentu. P. Foliage and house plant. MWT 16°C (60° F).
Impotiens, A. and P, grown as A. Spring in warmth. About 11 weeks. MWT 10°C (50°F) to over-winter.
Limonium suworowii. A. Early spring for summer flowering.
Lobelia ienuior. A. Spring for summer and summer for winter in cool greenhouse.
Nemesia. A. March to June. About 13 weeks.
Nierembergia caerulea. P. March and April for late summer. Trostfree to over-winter.
Pelargonium (Geranium) P. The hybrids sown in heat from January flower in 4 to 5 months according to temperature. MWT 7°C (45 °F}.
Petunia. P, grown as A. April. About 14 weeks.
Plumbago capensis. P. climber. Spring. 18 months. MWT 4°C (40 °F).
Primula acauJis (modern hybrids) and P. auricula. March to May. For winter and spring in unheated and frost-free greenhouse.
Primula xkewensis and P. malacoides. May to June for winter. MWT 4°C (40°F).
Primula obconica and P. sinensis. May to June for winter. MWT 4°C (40°F).
Punica granatum 'Nana' (dwarf pomegranatej. Spring for autumn. Frost-free.
Rehmannia. P. May for following year. Frost-free.
Opposite above: Primulas provide colour in winter; this is a form of Primula obconica (p. 41),
Opposite below: Cinerarias can be grown from seed to flower in the winter (p.41).
Top left: Pelargoniums are popular and easy to grow from seed or cuttings (pp. 41, 48).
Top right: Coleus are grown for their decorative foliage [see page 51). Above left: Campanula isophyiJo, a pretty! it tie trailing plant [see page 40).
Above right: The well known Busy Lizzie (Impotiens) easily grown from seeds or cuttings (see page 41).
Schizanthus hybrids are available in a wide range of colours.
Schizanthus hybrids are available in a wide range of colours.
Was this article helpful?