Many plants which we call half-hardy annuals are natives of much warmer lands where frost is unknown or a rare occurrence. Some are perennial in their native conditions. It is necessary therefore in some colder areas of the country to germinate the seed with the aid of artificial heat in a greenhouse or frame. It is also advisable to sow the seeds of plants that need a longer season of growth or have very fine seeds in this way.
Sowing should be carried out in February or March in flats, pans, or pots using a compost that is purchased ready mixed, or made up as follows, 2 parts loam, 1 part peatmoss, and 1 part sand. All soil used for seed sowing must be put through a fine sieve. The containers should first have a layer of crocks (broken earthenware pots) or stones placed over the drainage holes, covering these with a small quantity of peatmoss, or partly decayed leaves. Now add the compost which should be firmed with a flat piece of board so that there is a space of about 1 in. between the surface and the top of the container.
Sow the seed sparsely, then sprinkle a fine covering of compost over the seed (very fine seed will not require covering).
After sowing water larger seeds from above with a fine spray, very small seeds may be watered by placing the pots in water and allowing it to rise through the soil by way of the drainage holes, on no account should the surface be allowed to flood.
Remove the glass daily, wiping off the condensed moisture and reverse the glass when you replace it. When the seed germinates remove the glass and paper and put the container in full light giving some protection from bright sunlight. On cold nights the seedlings should be protected with newspaper or may be moved away from the glass.
When the seedlings are large enough to handle, that is usually by the time they have developed two pairs of leaves, lift them carefully by levering up the soil with a dibber and prick them out into a flat, spacing them about 2 in. apart. Plant them firmly and water them into the compost. Keep them growing steadily under glass until the beginning of May. Remember that most annuals should have their growing points pinched out when they are 4-5 in. high to encourage bushy growth. If the weather is reasonably mild, stand the flats in the open for a week or so to harden the plants before they are planted out where they are to flower. If the flats have been in a frame it is a simple matter of just removing the frame light for this hardening-off process. Be sure that the flats are standing level, and on a firm base, otherwise some plants may lack water. Give the flats a good watering an hour or two before transplanting the seedlings so that they can be removed from the flats with plenty of moist soil attached to their roots. Water them again when they are planted out.
Among the half-hardy annuals which may be sown direct in the open where they are to flower are aster, cosmos, African and French marigolds, mignonette and zinnia. The last two resent transplanting and germinate better when the soil is made firm after sowing. May or early June is the time to sow half-hardy annuals in the open in a sunny position and in well-drained soil.
Whichever method is used, sow thinly and do not make the common mistake of covering the seed with too much soil. As a general rule cover the seed with its own depth of soil. Nature just scatters it on the surface and lets the rain wash it in, but Nature is very wasteful, and sows with abandon.
Where the seed is sown in a cold frame leave some ventilation or the frame may get too hot. Some shading on the glass will also reduce the need for frequent watering and the same applies to seed sown in pots in a cold greenhouse which should be covered with brown paper or newspaper until the seed germinates. These are small points which make all the difference between failure and success. Biennials are sown one year and flower and produce seed the next, which completes their life cycle. Some will perpetuate themselves by self-sown seedlings, for instance, foxglove, forget-me-not, mullein, hollyhock, among many others.
How to use Annual and Biennial flowers