Annual flowers go from seed to bloom in a single season. You can raise almost all of them yourself, and a packet of petunia among our flower seeds holds 50 seeds. Snapdragon, petunia and annual phlox are started indoors from February, while cornflower, poppy and calendula (pot marigold) are sown straight outdoors once the soil can be worked. Count back from planting-out time, which in most of the UK and northern Europe falls between late May and mid-June.
How to decide whether to sow indoors or outdoors
It comes down to how long the species needs to reach its first flower and how well it copes with being moved. Petunia needs 10-14 weeks from sowing to flowering, so a June sowing outdoors would not flower until August or September. Species that germinate in 5-15 days and grow fast are sown where they are to grow instead.
Start indoors: snapdragon, petunia, annual phlox and China aster.
Sow direct outdoors: sunflower, cornflower, calendula and nasturtium. Annual poppy belongs here too, because the roots are damaged when the plant is moved.
Either works: zinnia, cosmos and sweet pea. Started indoors, they flower earlier. In milder areas, zinnia can be sown direct once the frost is over.
We often do both. With nasturtium, we start a few plants indoors for early flowers and sow the rest direct.
Sowing dates for twelve annual flowers
The table assumes the last frost falls at the end of May. In milder areas, such as much of southern England, the frost is often over earlier. In a colder spot or further north, move the dates 2-3 weeks later.
| Flower | Sowing | Time indoors | Plant out | Flowers from |
|---|---|---|---|---|
| Snapdragon | Indoors February-March | About 12 weeks | After the last frost | July |
| Petunia | Indoors late February to mid-March | 12-14 weeks | Late May to mid-June | June |
| Annual phlox | Indoors in March | 12-14 weeks | After the last frost | July |
| China aster | Indoors March-April | 6-10 weeks | After the last frost | July |
| Sweet pea | Indoors February-April, or outdoors in soil at 12°C | 5-7 weeks | When nights stay at 5-8°C | June-July |
| Zinnia | Indoors in April | 4-8 weeks | After the last frost | July |
| Cosmos | Indoors in April or outdoors in May | 4-8 weeks | When days reach at least 15°C | Around midsummer |
| Sunflower | Outdoors mid-May to early June | Sown direct | Sown in place | July-August |
| Cornflower | Outdoors March-April, another round in May | Sown direct | Sown in place | July |
| Poppy, annual | Outdoors March-May | Sown direct | Sown in place | June-July |
| Calendula | Outdoors April-June, in batches | Sown direct | Sown in place | June-July |
| Nasturtium | Outdoors May-June in soil at 15°C | Sown direct, or 2-3 weeks indoors | After the last frost | June-July |
Start indoors from February to April
February: snapdragon, sweet pea and, towards the end of the month, petunia. Snapdragon and petunia need light to germinate, so scatter the seeds on moist seed compost and put a clear lid on top instead of covering them with compost. Sow sweet peas 1 cm deep in tall pots.
March: annual phlox, China aster and petunia by the middle of the month at the latest. Annual phlox germinates in darkness at about 18°C, so cover the tray with cardboard until the leaves show after 2-3 weeks. Cover China aster with 0.5 cm of compost.
April: zinnia and cosmos, which grow fast. Zinnia germinates best at 20-22°C.
After germination the seedlings want plenty of light and cooler conditions. In February and March the light from a window is rarely enough, so the early sowings need a grow light. Trays, compost and lamps are covered in the article on starting seeds indoors, and petunia has its own timeline in the article on how to grow petunias.
We fill the pots with potting compost at the bottom and seed compost on top, so the roots reach the nutrients when the plant needs them. We sow zinnia straight into pots at least 7 cm deep, because the roots are sensitive and the plant then does not have to be pricked out. We pinch out snapdragons at 8-10 cm and zinnias at about 10 cm so that they branch.
Sow direct outdoors from March to June
Direct sowing saves space indoors, and you skip hardening off. Soil temperature decides when you can start.
As soon as the soil can be worked: cornflower and annual poppy, often in March or April. The seedlings tolerate a little frost. Sow cornflower 0.5 cm deep, while poppy is only lightly raked in because the seeds need light to germinate.
April and May: calendula, 1 cm deep and thinned to 10 cm. In May also cosmos, 1 cm deep and thinned to 25 cm.
When the soil is 12°C: sweet pea 10 cm apart, thinned to 20 cm.
Mid-May to early June: sunflower, 1 cm deep in a sunny, sheltered spot.
When the soil is at least 15°C: nasturtium, 2 cm deep after soaking the seeds for 24 hours.
We sow cornflower once in March or April and again in May for later flowers, and calendula in batches until June. Poppy and calendula are welcome to self-seed in the vegetable patch. More species are covered in the article on direct sowing and among our seeds for direct sowing.
Harden off and plant out
A plant that goes straight from the windowsill into full sun is damaged by sun and wind, and a single frosty night can kill a zinnia or petunia. Harden off over 1-2 weeks. Start with a few hours in shade and shelter, and increase the time and the sun each day. Cover with horticultural fleece on cold nights.
Tolerates light frost: sweet pea can be hardened off as early as April and planted out when nights stay at 5-8°C.
No frost tolerance: zinnia, petunia, cosmos and nasturtium are planted out once the risk of night frost has passed, in late May or early June.
Spacing: 20 cm for petunia and annual phlox, 20-25 cm for dwarf snapdragons and 40 cm for tall ones, 25 cm for zinnia and cosmos.
Sun, partial shade, pot or vase
Full sun: zinnia, sunflower, petunia, poppy and sweet pea. Petunia needs at least five hours of direct sun a day.
Partial shade: snapdragon, annual phlox, China aster, calendula, cosmos and nasturtium flower most in sun but cope with partial shade.
Pots and window boxes: choose low varieties, such as snapdragon at 20-50 cm, cosmos at 30-40 cm and cornflower at about 40 cm, or petunia, annual phlox and trailing nasturtium. Suitable varieties are among our seeds for container growing.
Cutting: zinnia, snapdragon, cosmos, sweet pea and annual phlox form new buds the more you pick. Tall snapdragons and cosmos grow to over a metre. China aster lasts a long time in a vase and can be dried. See more seeds for cut flowers.
More flowers all summer
Deadhead: a flower left to set seed takes nutrients from new buds, and sweet pea stops flowering once it sets pods. Deadhead twice a week.
Feed by species: petunia in a pot needs liquid feed every week and zinnia in a pot every third week. Cosmos and nasturtium manage on the fertiliser added at planting.
Common mistakes and what to do instead
Sowing everything in February: a zinnia sown then stays indoors for over three months and becomes tall and weak. Count back from planting-out time.
Covering seeds that need light: snapdragon, petunia and poppy germinate poorly under compost. Annual phlox is the opposite and wants darkness for the first 2-3 weeks.
Leaving seedlings in the warmth: at 22-25°C without extra light they stretch and become weak. Move them somewhere cooler once they have come up.
Transplanting poppies: the roots cope badly with it. Sow poppy where it is to flower and thin to 15-30 cm.
Feeding cosmos and nasturtium: in soil that is too rich you get lots of leaves and few flowers. Choose a lean spot.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-11-04