Seeds are living things. Given the correct conditions, they will generally germinate quickly and give satisfaction. The correct conditions vary for each species, specific recommendations are given for temperature and other factors against each species. The general guidelines below for greenhouse-grown plants should be of additional help. With field sown plants we are all at the mercy of the weather.
A soil-less seed compost is best. Soil based composts generally give less good results. Maintaining the compost moist enough for the seeds and in the correct temperature range for the variety is critical.
Emerging seedlings are vulnerable to disease, particularly damping off which is an invasive fungus (Pythium spp.). A precautionary drench of the compost prior to sowing with a fungicide is helpful. Good fungicides can be bought from any garden centre.
Sow seed at the recommended time. Bear in mind that southern Europe and northern Europe sowing periods will need adjusting to local conditions. As a general guide, these are the usual sowing times:
HA (hardy annuals): February - May. If sown direct outside, sowing should not be made until the soil has started to warm up after winter. (Some hardy annuals can be sown outdoors in late August/early September for flowering early summer the following year.)
HHA (half hardy annuals): January - April.
HP, HB (hardy perennials, hardy biennials): April - July
TP (tender plants, including most bedding plants): December - March.
How to sow seed: use plastic plant-pots or seed trays, preferably new and hence sterile. For all but the largest seeds, fill the container with compost to within about 3cm from the top and lightly firm down. Then add about 1cm of compost that has been sieved through a 0.5cm mesh sieve and level the surface.
Next, water the compost by immersing the pot in water, being very careful not to over-water and spray the surface with the fungicide.
Sow the seed thinly and evenly. Firm the seed down and then cover as appropriate, preferably with vermiculite. Generally, leave fine seed uncovered, otherwise cover the majority of the seeds with a thin layer of vermiculite. Some species require light to germinate, some dark, so please check for any specific recommendations.
Spray the surface again with the fungicide and wrap the sowing container with clear plastic There should be no need to water the seeds again until after they germinate. The compost will stay moist; drying out is a common cause of germination failure.
Germinate the seeds at their appropriate temperature. Most species, with some exceptions, are happy to germinate in the range 18 20oC. A common cause of poor germination is temperatures that are too high, too low, or too variable. A certain way to overheat seeds is to leave them in direct sunshine! Temperature or moisture stress at this critical period will dramatically affect the success of germination.
Maintaining the seeds at the correct moisture levels and temperature is critical to success in germination.
Dormancy: some species, particularly ornamentals, have a dormancy mechanism that delays germination sometimes by months. There are methods to overcome this, noted below:
Hard-coated seeds: these should be soaked in water before sowing for 24 hours or the coat can be scratched or chipped (scarified) with a sharp knife, care being taken not to damage the embryo.
Vernalisation and stratification: some seeds require exposure to moisture and low temperature for an extended period. This can be achieved by sowing the seed and leaving the container outside in January or February for the appropriate period and then bringing inside (to about 15oC). Alternatively, put the seed in a refrigerator at a temperature of 4ºC for a similar period.
Dont be put off sowing a seed because you feel it may need special treatment one thing you can be sure of - it wont grow if left in the packet.
Pricking out: when large enough to handle, the young plants will need pricking out and, if not destined for planting out in the garden, eventually potting on.
When pricking out your seedlings often you will have more plants than you will need, it is a good idea to select individuals with an assortment of sizes. It is often the less vigorous seedlings that eventually produce the best flowers with the most interesting colours. This is particularly true when dealing with mixtures.
Tips: one of the commonest causes of failure of plants is incorrect watering. When first transplanted, water with extreme caution until the plant has recovered from the shock. Thereafter water thoroughly and then leave alone until the soil is again practically dry.
If you wish to keep seed, it should be kept cool and dry. It can be stored in a moisture-proof container in the main body of a domestic fridge. Although the seeds of many species can remain viable with storage at room temperature for a year or two, cold storage takes care of those with a relatively short life and extends considerably the life of others.
Download Germination Notes PDF file