Sowing seeds in pots or trays
When sowing seeds in pots or trays it is important to use the right type of compost. A proprietary seed compost can be purchased or you can mix your own. For a soil-free seed compost use 3 parts peat/coir, one part fine bark and one part perlite. For soil based seed compost use half sterilised loam (ie good garden soil), quarter peat/coir and quarter sharp sand. In both cases you can add chalk or limestone to reduce the acidity of the mix, or exclude these for a more acidic mix (acidic loam can also be used for this purpose). Seeds from bulbous plants prefer a more open mix, so add three parts of grit to every two parts of compost used.
Select a container which is appropriate to the type of seed you are sowing. Bulbous plants should be sown in pots or deep trays to accommodate the developing bulb. If the plant has roots which grow deeply (including tap roots) then use a tall pot, specialist root trainer pots or cardboard tubes to grow them in. If the plant does not like to be transplanted, then degradable pots made of peat, paper or cardboard are best as you can plant them without removing the seedling.
Large seeds can be sown in rockwool modules to create compact ‘plug plants’. These are easy to transplant and handle compared to seedlings growing in compost, but they are a much more expensive way of growing seed.
Fill the container with sieved seed compost to the depth at which the seed should be planted – as a general rule you cover seeds with their own depth of soil. Tap the pot or tray on your work surface a couple of times to settle the compost then press down on it (ideally with a piece of wood cut to the right size) to firm it, without pressing so hard that you compact it.
Sow the seeds evenly across the surface of the compost (or singly if you are putting large seeds into pots) and cover to their own depth with sieved compost. Fine seeds can be mixed with silver sand before being sown and placed on a piece of folded card which is then tapped to release seed over the surface of the compost to ensure a more even coverage. If the seeds are very fine you can leave them uncovered or cover them with a thin layer of vermiculite, which allows air and light to reach the seeds while keeping them moist.
If you are sowing tree seeds which are likely to take a while to germinate then cover the compost with a 5mm layer of 7-12mm gravel to prevent mosses or liverworts growing on the surface. Slow growing shrub seeds like the same treatment, but with coarse sand or fine grit being spread over it, unless the seeds are very small.
The compost surface and resulting seedlings can be sprayed with a copper based fungicide to reduce the risk of damping off and similar fungal infections.
Water the compost with a fine rose (or by sitting the pot/tray in a tray of water for 30 minutes if you have sown fine seeds) and don’t forget to label the container. If the surface covering is still dry (eg if you have used vermiculite over fine seeds and watered from below) then you can spray it with water from a fine spray to moisten it.
Different plants will have different temperature requirements for germination; the more tender the seed the warmer it needs to be to germinate. All seeds prefer to be kept in a sheltered location and in a well lit spot, which is out of direct sunlight, until the plants are established (you can loosely cover them with netting or muslin to keep them shaded). Ensure that you keep the compost moist.
If you are placing the pots outside (eg in a cold frame) to germinate, or for cold stratification prior to germination, ensure that they are protected from rodents, particularly if you’re sowing things like peas and acorns which make tasty treats for local mice or squirrels.