Tips for Seed Sowing
With plastic pots it is perhaps doubtful whether the layer of roughage is justified (except for very fine slow-germinating seeds) especially if a modern soilless mix is used. For boxes or containers it is doubtful whether any crocks are needed, as the admittance of air and moisture movement are more influenced by evenness of consolidation than anything else, provided the compost has a good texture.
Always use warm compost, firming it well with the fingers to the corners and then levelling off with a pot-firmer or the base of another pot or suitably sized box about 1.25cm (1in)below the pot or box level. Compartmentalized trays should be filled in the same general manner, although firming is more readily achieved by ‘dumping’. The surface of the compost can be finished off with some light coloured fine sand, especially for very tiny seeds like lobelia or primula, as it not only enables the distribution of seeds to be observed but ensures intimate contact of seed on compost. Once again with special soilless seed composts this procedure may not be thought essential. A level surface on top of the receptacle is, however, necessary for fine seeds in all circumstances.
Prior to sowing, all compost-filled receptacles should be thoroughly watered with a fine rose and allowed to drain, during which interval labels should be prepared, showing plant species, variety and date of sowing. Plastic labels have almost entirely ousted wooden tallies, although there is a lot to be said for the cheapness of the latter on a short-term basis.
Open the seed packet by tearing off a corner, shake the seed into a piece of paper and use the finger and thumb to distribute it evenly on the prepared compost — this can be difficult if the seed is very dust-like. Alternatively a folded piece of paper can be used as a spill and the seeds encouraged to come out evenly by gentle shaking. At what density to sow seeds depends a lot on circumstances; it is always better to err on the thin side than to sow so thickly that the little seedlings are overcrowded, a state of affairs that leads to the encouragement of damping-off disease.
Fine seeds are not covered, merely being pressed into the top of the compost; most other seeds are covered with a little sifted compost, either scattered on with the fingers or worked through a fine 0.31 — 0.62cm (1/8-1/4in) riddle or sieve, a soup strainer being a useful item of equipment at amateur level. The depth of covering is related directly to the size of the seed. The covering can be gently firmed if thought necessary. When seeds are sown individually they are simply inserted into the receptacle by hand just below the surface. Pelleted and pilled seeds are best sown on the surface.
Whether to follow the traditional practice and cover seeds first with a sheet of glass and then paper, or to leave the seeds on an open bench, perhaps with mist irrigation, or alternatively to place them in a germinating cabinet, is definitely a matter of personal choice and the facilities available. Germinating temperatures vary greatly, but as a general rule 18-24°C (65-75°F) is desirable day and night. In 90% of cases the amateur gardener will use the glass and paper method to ensure humid conditions and avoid drying out, examining the seedlings regularly to observe signs of germination and wiping moisture from the glass, although use of dome covers or polythene bags is increasing and is a valuable technique. Where mist propagation is practised in boxes or receptacles such surveillance is not so necessary, but in a germinating cabinet (which is dark) constant vigilance is required. It is important, as soon as a major percentage of the seedlings germinate, to bring them into the light, otherwise etiolation occurs, which results in a badly weakened plant very prone to disease attack. At the same time care must be taken not to subject the little seedlings to excess sunlight, which can burn them up badly, and this means using temporary shading, blinds or green coloured polythene.
Whether watering is required before germination occurs depends much on species, coupled with temperature level and humidity of atmosphere. Drying out must not occur, and the practice of using capillary benches for germinating purposes has much in its favour in this respect, as applying water overhead can expose the seeds and leave them proud of the compost. Alternatively the receptacle can be immersed in a tray of water or the seeds covered to reduce moisture loss during the critical germination period, or polythene bags used.
One reason why germinating cabinets are favoured in commercial circles is to avoid the watering problem, as the humidity in them is so high that watering is seldom, if ever, needed before germination occurs. Where mist units are used for germinating purposes it matters little whether the seedling is exposed, as it is kept continually moist by the watering, and damage by excessive exposure to the sun is unlikely. The same is true of polythene bags.
Once seedlings are developing freely on the greenhouse bench in a position of good light they should be watered freely according to demands, avoiding over-watering and the provision of too humid an atmosphere which will encourage damping-off.
Under most circumstances and with seed of a non-temperamental nature, air, moisture and correct temperature will bring about germination in a specific number of days. Delay in germination may be caused by faulty seed or careless storing, or by the wrong germinating conditions for the species, especially in respect of temperature and light. Compost with poor structure, inhibiting the entry of oxygen, will literally stop the seed breathing, as indeed will saturation of the compost. Drying out could also be disastrous.
Some seeds have naturally hard testas or coats (eg sweet peas) and some pre-soaking treatment can speed up germination — by chipping, rubbing with sandpaper, soaking briefly in very hot water, or in warm water for a longer period, or alternatively for a specified period in strong sulphuric acid (this requires precise reference to a botanical list). Sometimes a period of cold followed by warmth will bring about germination; hardy primulas for example can be sown and left outside over the winter, then brought in to germinate in the spring with case. Such complications are, however, more the exception than the rule.