Home Seeds and cuttings Half-hardy annuals – growing them from seed

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 – more tips on growing them here.

Sometimes they’re tender perennials, like dahlias – you can usually spot them, as they’re the ones being sold as plug plants in catalogues and garden centres, costing a small fortune.

They all need a long season to flower well, which means indoor sowing in a propagator sometimes as early as January (Antirrhinums), but more usually February/March.

What is critical here is light intensity. If you don’t have a brightly-lit place indoors, wait until March. Warmth and poor light will lead to leggy, weak seedlings which will easily fall victim to disease. Read about award-winning Optigrow seeds here.

  • half-hardy annuals
  • half-hardy annuals
  • half-hardy annuals

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. Bear in mind how much plug plants cost, and a propagator will soon pay for itself.

What they all 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).

Make sure the soil is watered before you sow and doesn’t dry out. Sealing trays/pots in cheap freezer bags work 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.

half-hardy annuals
French marigold Strawberry Blonde

Potted guide: half-hardy annuals

Potted guide
  • 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, 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 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.
half-hardy annuals
Mesembryanthemum (Livingstone daisy) Sparkles Mixed

Single not double dahlias

I’m not really a fan of the big show dahlias, but I do love the simpler, single types, such as the Bishop series and 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. You can buy tubers, but if you like a riot of colour and growing from seed, try Bishops Children (I bet the Church wasn’t happy about that name).

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. My packet cost £2.49 (40+ seeds) – the cheapest Bishop of Llandaff SINGLE tuber I could find was £3.25, ranging up to £9.99 for a potted plant.

  • half-hardy annuals
  • half-hardy annuals
  • half-hardy annuals
  • half-hardy annuals
  • half-hardy annuals

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.

  • Antirrhinum The Bride
  • Antirrhinum Chantilly Bronze F1 Hybrid. Picture; Chiltern Seeds
  • Antirrhinum Tootsie
  • Pure white Antirrhinum The Bride
  • Antirrhinum Purple Twist
  • Antirrhinum Angel Tower. Picture; Plant World Seeds

 Potted guide: Antirrhinums

Potted guide
  • Sow: January-March or July-Sept.
  • Flowering: June-October.
  • Position: full sun.
  • Hardiness: Although a half-hardy perennial, treat 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.

Create a border full of summer-blooming plants – read more here. Read more on my pick of the best half-hardy annuals for beginners here.

Half-hardy annuals – growing them from seed updated August 2022