How to Sow Annuals and Biennials
Sowing Outdoors
When the soil has warmed up to at least 7°C (45°F) in the spring, you can sow hardy annuals where you want them to flower. lf sown in successive batches until mid-summer, they can provide a long summer display. Some hardy annuals, such as poppies (Papaver), marigolds (Calendula) and candytuft (Iberis) can be sown outside in the autumn. They will germinate to produce small plants that overwinter satisfactorily outdoors and flower in late spring or early summer the following year.
Most hardy biennials can be sown outdoors from late spring until mid-summer, depending on the plant. For example, forget-me-nots (Myosotis) grow very quickly, so should not be sown until mid-summer, whilst Canterbury bells (Campanula medium) take longer to develop, so should be sown in late spring or early summer. Do bear in mind that early flowering before winter will diminish the spring display, so pinch out any flower buds that form during the first season.
In warm regions, half-hardy annuals and biennials can be sown in spring directly outside once the soil temperature reaches the optimum for germination, otherwise, sow indoors and transplant when the climate is more favourable.
Hardy annuals are able to withstand frost, whereas half hardy annuals cannot, and must be raised under glass and planted out after all danger of frost has passed.
Sowing Indoors
When you begin to plan where and when you are going to sow your annuals and biennials, you will need to take into account the temperature they need for germination and when they are required to flower.
Hardy annual seeds can be sown in seed trays or pots and either planted out in their final positions in late autumn or overwintered in a cold frame and planted out in spring. The latter technique is particularly useful for gardens with clay soils that are slow to warm up in spring.
Half-hardy annuals can be sown in containers during spring, and planted out when conditions are favourable; usually in temperatures of 13-21°C (55-70°F), depending on the particular genus. Half-hardy biennials should be sown in containers and planted out in mid-summer.
Some seeds require special conditions in order to germinate successfully:
- Seeds of Thunbergia alata and pelargoniums require a temperature of 21-24°C (70-75°F) to germinate.
- Begonia, Coleus, and busy lizzie (Impatiens) seeds prefer a constant temperature of 21°C (70°F) and plenty of light.
- Primulas prefer light, and need temperatures of no more than 20°C (68°F).
- Some seeds, such as Phacelia should be germinated in the dark.
- Seeds of Moluccella are difficult to germinate unless they have been stratified: the seed pan should be kept in a refrigerator for two weeks before bringing it into a temperature of 18-21°C (65-70°F) for a further two or three weeks.
See Also:
In Propagation, you'll find a guide to what you need to consider when choosing and buying seeds, along with step-by-step instructions on how to plant seeds indoors and out.