How to Save Zinnia, Cosmos & Marigold Seeds Before the Frost

Zinnias, cosmos and French marigolds will flower right up to the first frost — and then they are gone. Unlike dahlias or pelargoniums, these tender annuals cannot be lifted or overwintered; the only way to carry them into next year is as seed. The good news is that all three are among the easiest flowers to save seed from, and the window is now: in most of Britain the first proper frost arrives between mid and late October. This guide shows how to save zinnia, cosmos and marigold seeds — how to tell when they are ripe, where the seed actually is in each flower, and how to dry and store it so it comes up next spring.

• When September until the first frost · on a dry afternoon
• Ripe when Head brown, dry and papery · seeds come away at a touch
• Drying 1–2 weeks on paper, indoors · never above 35 °C
• Storage Paper envelopes in an airtight tin · cool, dry and dark · sow from April

Will the seed come true? Check before you collect

Before you collect anything, check what you are saving from. Seed from open-pollinated varieties produces plants much like the parent. Seed from F1 hybrids does not: the next generation splits into a mixed bag of the grandparents' traits, as we explain in heirloom vs F1 vs organic seeds. If the packet said F1, enjoy this year's flowers and buy fresh seed.

All three flowers are pollinated by insects, so different varieties of the same species growing near each other will cross. That matters more for some gardeners than others. A white cosmos such as 'Hummingbird White' grown on its own will give you white cosmos again; seed from a bed of mixed zinnias will give you a mix, often a pleasing one, but not necessarily the colours you started with. Different species — Cosmos bipinnatus and the orange Cosmos sulphureus, for example — do not cross in the garden. If you want a particular colour to stay true, save seed from plants growing apart from other varieties, and always take it from your strongest, best-flowering plants.

Zinnia seeds: look at the base of the petals

Zinnia seed is hidden in plain sight. Each "petal" around the edge of the flower is a separate ray floret with a seed at its base, and the small florets in the centre carry seed too. Leave the chosen flowers on the plant until the whole head is brown, dry and crisp — this takes several weeks after the flower opens, so mark a few heads now rather than waiting for the last flush. Pull the dry head apart over a sheet of paper: the fertile seeds are flat, arrowhead- or shield-shaped and noticeably plump in the middle, often with a darker stripe. Thin, papery, grey flakes are empty and can be discarded.

Cosmos seeds: the starburst

Cosmos is the quickest of the three. A week or two after the petals drop, the centre of the flower develops into a spiky starburst of long, slender seeds. They are ready when they turn from green to dark brown or black and pull away easily — at that point they also fall on their own, so check plants every few days. Pinch off the whole head and tip the seeds into an envelope. Goldfinches love cosmos seed, so leave a few heads for them; it is the same principle as in our guide to what to leave standing in autumn.

Marigold seeds: inside the papery case

French and African marigolds (Tagetes patula and T. erecta) hide their seed inside the flower's green outer case, the calyx. Once the petals have shrivelled and the calyx has turned brown and papery, snap off the head and pull the case open: inside is a neat bundle of thin, black seeds with pale tips, where the old petals were attached. The black part is the seed. Pot marigolds (Calendula) are a different plant with curled, crescent-shaped seeds; they are covered in our general guide to seed saving.

How to collect and dry the seed, step by step

1. Stop deadheading a few plants Choose two or three of your best plants and let some flowers fade and set seed. Tie a piece of wool round the stems so they are not cut by mistake.
2. Wait for brown, not green Seed is ripe when the head is dry and brown and the seeds separate easily. Green seed taken early rarely germinates.
3. Pick on a dry afternoon Collect after the morning dew has gone, never straight after rain. Put each variety in its own labelled paper bag or envelope.
4. Beat the frost with a paper bag If frost or a week of rain is forecast and the heads are nearly ripe, cut them with a length of stem and hang them upside down in a paper bag in a dry room. Seed that is already brown finishes ripening indoors.
5. Clean out the chaff Break the heads apart over paper, pick out the plump seeds and gently blow away the light, empty material. It does not need to be perfect.
6. Dry, then store Spread the seed on paper for one to two weeks in a dry, airy room, never above 35 °C. Then seal it in labelled paper envelopes inside an airtight tin or jar with a sachet of silica gel, somewhere cool and dark. Our guide on how to store seeds has more on containers.

Ripe or not yet? How to tell

Common mistakes

• Storing seed in plastic bags — plastic sweats and the seed goes mouldy. Use paper, then an airtight tin.
• Picking too early — green seed looks fine but rarely germinates. Wait for brown.
• Saving from F1 varieties — the seedlings will not look like the parent.
• Keeping seed in a damp shed — warmth and humidity shorten its life. A cool cupboard indoors is better.
• Forgetting labels — by spring every brown seed looks the same. Note the plant, colour and date.
• Sowing it straight away — these are frost-tender annuals; seed sown in autumn dies over winter.

Frequently asked questions

Will seed from mixed zinnias come true?
Not exactly. Zinnias cross freely between varieties, so seed from a mixed bed gives a mix of colours and flower forms. It is usually attractive, but if you want one particular variety, buy fresh seed or save from plants grown well away from other zinnias.

Can I still collect seed after a frost?
Yes, if the seed was already ripe — brown and dry — before the frost. Collect it as soon as the plants have thawed and dried out. Seed that was still green when frost struck is unlikely to germinate.

How long do zinnia, cosmos and marigold seeds last?
Zinnia seed stays reliable for three to four years; cosmos and marigold seed for around three, if kept cool and dry. After that, do a quick germination test first — our guide on how long seeds last shows how.

When do I sow the seed I have saved?
In spring. Zinnias and cosmos are frost-tender, so sow them indoors from late April or outdoors from late May — our guide to growing zinnias from seed walks you through it. Marigolds can be started a little earlier, from March, under cover.

Ten minutes with a few paper envelopes this month gives you a drawer full of next summer's flowers. If you are new to sowing flowers, start with our beginner's guide to growing flowers from seed. Browse the zinnia, Cosmos bipinnatus and French marigold collections, the wider annual flowers, or all our seeds, and keep our growing guides to hand for the season ahead.