Home Propagation Half-hardy annuals: how to grow them from seed

Half-hardy annuals: how to grow them from seed

Grow the border’s tarty barmaids from seed

The big show-stoppers in the annual garden are, more often than not, half-hardy types, like these lovely Cosmos – more tips on growing them here.

Sometimes they’re tender perennials, like dahlias – more on growing them here – 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.

French marigold Strawberry Blonde
French marigold Strawberry Blonde

Propagator’s a good buy

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 it 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’s 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-logoPotted 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, fine sprinkling of compost, 7-21 days, don’t exclude light.
  • Gazania: 18-25ºC, fine sprinkling compost, 14-30 days, don’t exclude light.

    Mesembryanthemum (Livingstone daisy) Sparkles Mixed
    Mesembryanthemum (Livingstone daisy) Sparkles Mixed
  • 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.

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 Bishop of Llandaff, with its dark foliage and striking red 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.


Antirrhinums from seed

Cottage garden favourites Antirrhinums or snapdragons, are a half-hardy perennial but are best grown as an annual – 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 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.

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