Half-hardy annuals - growing them from seed

Grow the border’s tarty barmaids from seed

The big show-stoppers in the garden are, more often than not, half-hardy annuals, like these lovely Cosmos. They need a long season to flower well, which means indoor sowing in a propagator sometimes as early as January (Antirrhinums), but usually February/March.

Sometimes they’re tender perennials, like dahlias, grown as annuals to save overwintering. You can usually spot them, as they’re sold as plug plants in catalogues and garden centres, costing a small fortune. What is critical for all is light intensity. If you don’t have a brightly-lit place indoors, wait to sow until March. Warmth and poor light will lead to leggy, weak seedlings. Read about award-winning Optigrow seeds.

Propagator’s a good buy for half-hardy annuals

A propagator with a constant temperature is a good buy too – here’s my guide on the best buys. Plug plants cost a lot, and a propagator will soon pay for itself. What all half-hardy annuals need is good seed compost. Buy the best you can afford. Some people like to cover seeds with Vermiculite or Perlite instead of compost – use what you know best. (I find the latter two don’t form a crust which is hard to wet, which is why I use them).

Ensure the soil is watered before you sow and doesn’t dry out. Sealing trays/pots in cheap freezer bags works well. There are a lot of old bedding favourites here – this is a quick guide to their germination temperatures, sowing depth, days to germination, and any special instructions.

Potted guide: half-hardy annuals

Ageratum: 15-18ºC, surface sow, 10-15 days, don’t exclude light.
Antirrhinum (Snapdragon): 20-25ºC, surface sow, 10-21 days, don’t exclude light.
Busy Lizzie (Impatiens): 21-24ºC, fine sprinkling compost, 14-21 days, don’t exclude light.
Cosmos: 18-25ºC, 3mm, 7-15 days, don’t exclude light.
Dahlia: 20-30ºC, 6mm, 7-21 days, don’t exclude light.
French/African marigold (Tagetes): 21-24ºC, a fine sprinkling of compost, 7-21 days, don’t exclude light.
Gazania: 18-25ºC, fine sprinkling compost, 14-30 days, don’t exclude light.
Heliotrope (Cherry Pie): 20-25ºC, 1.5mm, 14-21 days, lowering the temperature at night helps germination.
Lobelia: 18-24ºC, surface sow, 14-21 days, don’t exclude light.
Mesembryanthemum (Livingstone daisy): 15-20ºC, surface sow, 15-21 days, don’t exclude light.
Nemesia: 13-15ºC, surface sow, 7-21 days, don’t exclude light, keep soil temperature below 19C.
Nicotiana (tobacco plant): 21ºC, surface sow, 10-20 days, don’t exclude light.
Osteospermum: 15-18ºC, fine sprinkling compost, 10-15 days, don’t exclude light.
Petunia: 21-27ºC, surface sow, 10-21 days, don’t exclude light.
Phlox drummondii: 18-20ºC, surface sow, 10-21 days, don’t exclude light.
Rudbeckia: 20-25ºC: fine sprinkling compost, 7-21 days, don’t exclude light.
Salvia splendens: 21-24ºC, surface sow, 10-14 days, leave in a shady spot for 24-48 hrs; then cover seed lightly.
Verbena x hybrida: 24-27ºC, surface sow, 14-21 days, don’t exclude light.
Zinnia: 20-25ºC, 1.5mm, 10-24 days, sow individually in 7.5cm pots.

Annuals and biennials — Orange Dahlia Bishops’ Children, which I grew from seed

Single, not double dahlias

I’m not a fan of the big show dahlias, but I do love the simpler, single types, such as Bishops’ Children, with dark foliage and striking flowers – ideal for a sunny, hot-themed border. Bees and insects can’t get to the nectar in double flowers – another reason to grow singles. They are half-hardy perennials, but are usually grown as annuals

You can lift the tubers in autumn for next year. I did this – they do survive well in a frost-free place. They need a long growing season, hence the early sowing, but will flower until the first frosts. Sow 0.6cm deep in trays and cover with compost, Vermiculite or Perlite. They should germinate in 7-21 days – don’t exclude light.

Antirrhinums from seed

Cottage garden favourites Antirrhinums or snapdragons are a half-hardy perennial, but are best grown as half-hardy annuals, which means an early start. Sow seeds January-March, thinly, in trays of seed compost. Don’t cover the seeds, or exclude light, as this helps germination. Keep moist and maintain a temperature of 18-22°C (64-72°F). Germination should take 10-29 days, but may be erratic.

The seedlings are tiny. Don’t be put off if they seem to be making little progress. I always think they’re going to die young. Seeds can be sown July-September in a cold frame for planting out the following spring, but that’s too nerve-wracking for me. When they are large enough to prick out, transplant them 5cm apart in seed trays ( or I use small modules) and grow on. Gradually acclimatise the plants to outside conditions before planting out 12-25cm apart (depending on the size of the variety) in a sunny position.

Potted guide: Antirrhinums

Sow: January-March or July-Sept.
Flowering: June-October.
Position: full sun.
Hardiness: Although a half-hardy perennial, treat it as a half-hardy annual.
Recommended varieties: Available in dwarf, intermediate, tall and trailing. Tall: Royal Bride; Purple Twist, Chantilly Bronze (up to 90cm/36in). Intermediate: Tootsie F1, Black Prince; Day and Night, 45cm/18in. Dwarf: Bronze Dragon 30cm/12in.