This is the number one enemy of young seedlings grown under glass where the air circulation is poor and is caused by various fungi which live in the soil. Affected seedlings rot off at soil level, suddenly collapsing and withering away.
Still healthy seedlings away from the edge of the affected patch can often be saved by quickly transferring them to fresh compost but, once this problem appears, it is almost impossible to cure and so it is important to prevent it occurring in the first place. The keys to doing this are, as already mentioned, clean containers, fresh compost and the use of tap water, not rainwater. Compost can also be watered with the fungicide, Cheshunt Compound, and this is recommended in cases where seedlings are known to be particularly susceptible. If the above rules are observed, however, and seeds are sown thinly and not overwatered, fungicide should not be necessary in most cases. It is certainly not a substitute for good hygiene.
Abnormal growth or development
We sometimes receive reports of plants growing taller or less tall or maturing more or less quickly than stated on our packets. As living things, however, the performance of plants is very much subject to the conditions under which they find themselves. The notes on seedpackets are for guidance only as no seedsman can guarantee that a particular variety will perform in exactly the same way in all summers, on all soils and in all parts of the country.
On some soils a crust or cap forms on the surface after heavy rain or watering. This can make it very difficult for smaller seeds to germinate. If such a cap forms soon after sowing and before seeds have had a chance to start germinating it can be carefully broken up by very light cultivation. In vulnerable areas, however, it is best to avoid the problem by using general purpose compost rather than soil to cover seeds.
Weak, leggy seedlings
Seedlings which receive insufficient light will often become drawn up, thin and weak and this problem most often occurs when temperatures are also high. Most likely to be affected are those from early spring sowings, when light levels are naturally very low, and ones grown on windowsills, especially windowsills over radiators, where light is only received from one side. In so far as this is possible, the remedy is to move them to a position where they receive more light and to reduce the temperature if this is too high. When pricking out or potting on such seedlings the problem can often be corrected by planting them deeper than previously, as long as their lowest leaves are still above the surface of the compost.