The best June borders are sown in September. It sounds backwards, but hardy annuals — cornflowers, nigella, larkspur, ammi and their kind — are built for exactly this: they germinate fast in the warm soil of late summer, sit out winter as tough little rosettes, then race away in spring on a root system a March sowing can only dream of. The reward is flowers four to six weeks earlier, on taller, bushier plants that shrug off dry spells. Autumn sowing is the closest thing seed growing has to a head start, and the window is open from mid-August to the end of September.
- Sowing window mid-August to late September
- First flowers late May–June, 4–6 weeks early
- Hardiness young plants take −5 to −10 °C
- Golden rule small, hard plants overwinter best
What exactly is a hardy annual?
A hardy annual completes its whole life in one growing year and — the important part — tolerates frost while young. That single trait is what makes autumn sowing possible: the seedling simply pauses through winter and resumes in spring. It is not the same thing as a biennial. Foxgloves, wallflowers and sweet william must cross a winter before they will flower at all; a hardy annual will flower either way, autumn sowing just moves the show forward. We cover that group separately in our guide to sowing biennials.
The distinction that really matters is with the half-hardies. Cosmos, zinnias, sunflowers and morning glory are killed outright by the first proper frost — never autumn-sow those. If the packet says "hardy annual" or "HA", you are in business; if in doubt, our flower growing guide explains the categories.
Why an autumn sowing beats a spring one
Three things stack in your favour. First, germination: late-summer soil is 15–20 °C, so seeds that dawdle for three weeks in cold March ground are up in days. Second, roots: through autumn and every mild spell of winter, the top of the plant sits still while the root system quietly keeps working. By March an autumn-sown cornflower owns twenty times the root of a just-germinated one, which is why it ends up taller, bushier and far more drought-proof. Third, timing: overwintered plants flower from late May, bridging the "hungry gap" before spring-sown flowers begin in July — the very weeks a cutting garden is usually bare.
There is an honest trade-off: you will lose the odd plant to a brutal winter, waterlogging or slugs. Sow a little thicker than you would in spring, and keep half the packet back as insurance — seed is the cheapest part of gardening.
The best hardy annuals to sow now
- Cornflower — the most reliable overwinterer of the lot. Autumn-sown plants make branching, metre-tall clumps that flower for months.
- Nigella (love-in-a-mist) — sow direct where it is to flower; it resents transplanting. The seed pods are as good as the flowers.
- Larkspur — germinates best in cooling soil, which makes September its natural month. Autumn plants throw dramatically longer spikes.
- Ammi — the florist's favourite white umbel. Spring-sown ammi reaches a metre; autumn-sown easily passes 1.5 m.
- Poppies — Shirley and field types. Surface-sow direct and let autumn rain settle them in.
- Calendula — the workhorse. September-sown plants often open a first flower in a mild February.
- Scabiosa — the annual types overwinter well in modules and flower from early June.
- Annual gypsophila — quick, airy filler for early bouquets; sow direct in drifts.
- Clarkia — an old-fashioned charmer that dislikes root disturbance: direct sowing only.
Sweet peas belong on this list too — many growers rate an October sowing in deep pots above any other — and wallflowers squeeze in as the last of the biennials if you hurry. You will find the full range in our annual flowers collection, plus everything tagged for the season in the August and September sowing collections.
How to sow hardy annuals in autumn, step by step
- Pick a free-draining, sunny spot Winter wet kills more autumn sowings than cold does. Rake to a rough crumb; no compost, no feed — see preparing your soil.
- Sow direct, in marked drills Water the drill first, sow thinly at half a centimetre, and label — straight rows make weeding obvious. Our direct sowing guide has the full technique.
- Or use modules where slugs rule Two seeds per cell, kept in a cold frame or sheltered corner — not indoors. Plant out in October at rosette stage, or in March.
- Thin early and hard One plant per hand-width by mid-October. Crowded seedlings grow soft and rot in winter damp.
- Then leave them alone No feeding, no cosseting. A single layer of fleece in a hard snap below −8 °C is all the protection they should ever need.
Autumn-sown or spring-sown?
The honest answer for most gardens is both — the two sowings hand over to each other for the longest possible season:
Autumn-sown (Aug–Sept)
Flowers late May–June on big, deep-rooted plants.
- 4–6 weeks earlier into flower
- Taller, bushier, more drought-proof
- Small winter risk: wet, slugs, hard frost
Spring-sown (Mar–Apr)
Flowers from July on smaller, quicker plants.
- No winter losses to worry about
- Ideal for cold, heavy or exposed gardens
- Perfect follow-on for later colour
Run the two sowings of the same varieties back to back — a classic piece of succession sowing — and you will cut flowers from May until the frosts.
Common autumn sowing mistakes
- Sowing too early — August-sown plants in a warm autumn can get big and soft, and big soft plants flop and rot. If your garden is mild, mid-September is better than mid-August.
- Sowing too late — seedlings need four to six weeks of growth before the first hard frost. In cold districts, close the window at the end of September.
- Feeding them — nitrogen in autumn forces lush growth that winter kills. Save the feed for March.
- Ignoring drainage — a rosette can freeze solid and shrug it off, but it cannot sit in water for a fortnight. Heavy clay? Use the module route.
- Forgetting the slugs — overwintering rosettes are the only fresh salad in a winter garden. Patrol on mild damp evenings; our pest guide covers the rest.
- Autumn-sowing half-hardies — cosmos and zinnias sown now are dead by Christmas. Store those packets properly for spring instead.
Frequently asked questions
What is the difference between a hardy annual and a biennial?
A hardy annual flowers within one year from seed whenever you sow it — autumn sowing just brings flowering forward. A biennial (foxglove, wallflower, sweet william) must have a winter before it will flower at all. Both are sown in the same late-summer window, which is why they share the season.
How cold can autumn-sown hardy annuals take?
Established, well-drained young plants of cornflower, nigella, larkspur and calendula routinely come through −5 to −10 °C. Wet feet, not frost, is the usual killer. Below about −8 °C, throw a layer of fleece over them.
Should I sow direct or in modules?
Tap-rooted kinds — nigella, larkspur, poppies, clarkia — hate disturbance, so sow those direct. Cornflower, scabiosa, calendula and ammi are happy either way. Choose modules if slugs are relentless or the bed is still occupied by summer plants.
When will autumn-sown hardy annuals flower?
From late May in most gardens, five or six weeks ahead of a March sowing of the same variety — and the plants will be noticeably bigger, with more stems to cut.
An autumn sowing is a small act of faith — a few marked drills in September that look like nothing all winter, then make you look like a genius in May. Browse our cut flower seeds and the full flower seed range, dip into the rest of our growing guides, or check the overall sowing calendar — and if you sow one thing this month, make it a row of cornflowers. June will thank you.
