Seeds to Sow in October Greenhouse: 12 Crops to Start Now

If you’re holding a packet of seeds in mid-October and wondering whether the sowing season has already passed you by, I’ve got good news: there are still plenty of seeds to sow in October greenhouse conditions. I’ve been sowing into my unheated greenhouse every October for years, and some of my best harvests — crisp winter salads, fat spring garlic, sweet peas that flower weeks early — started life in a cold frame or greenhouse tray in the second half of October. The trick isn’t fighting the season. It’s picking crops that actually want to germinate now and grow through the cold months ahead.

October sowing is a different game from spring sowing. Days are shortening, light levels are dropping, and soil temperatures are falling. But inside a greenhouse — even an unheated one — you’ve got a buffer of several degrees and protection from the worst of the wind and rain. That buffer is exactly what lets a carefully chosen list of seeds get going now.

Seeds to Sow in October Greenhouse: Why It’s Still Worth Sowing

Most gardeners pack their seed trays away by September, and I understand why. Outdoors, October soil is cold and wet, and most seeds would just sit there and rot. But a greenhouse changes the equation in three ways.

First, temperature. An unheated greenhouse typically stays 4–8°C warmer than the night air outside, and a simple layer of bubble wrap insulation stretches that further. That margin is the difference between a seed that rots and a seed that germinates.

Second, control. Under glass, you decide how wet the compost is. October’s biggest seed-killer outdoors is waterlogging, and in the greenhouse that problem simply doesn’t exist.

Third, timing. Seeds sown now establish roots through autumn, sit quietly through the darkest weeks, then surge the moment light returns in late winter. A broad bean sown in October will be a sturdy plant by March, while the spring-sown one is still a seedling. You’re not really growing in October — you’re banking a head start.

Quick Salad Leaves That Germinate Fast

When I want a result I can actually eat before Christmas, I sow winter salad leaves. These are the sprinters of October sowing.

Winter lettuce is my number one pick. Look for varieties bred for cold-season growing — Winter Density, Valdor, and Arctic King are the names I reach for. Sow thinly in trays or directly into greenhouse borders, and you’ll be cutting baby leaves in about six to eight weeks. In a cold greenhouse they’ll slow down in December and January, but they rarely die.

Rocket germinates enthusiastically even as temperatures fall. I sow it in shallow trays and cut it young; the peppery flavour is actually better in cool weather, when it grows more slowly and the leaves stay tender.

Mizuna and mustard greens are the unsung heroes here. They germinate in days, shrug off light frosts, and keep producing cut-and-come-again leaves through winter. If you’ve never grown them, October in the greenhouse is the perfect introduction.

Corn salad (lamb’s lettuce) is practically designed for this job. It’s one of the hardiest salad crops there is, and October-sown plants will sit through winter and give you fresh leaves in the hungry gap of February and March.

Spinach works too, but be choosy about variety. Short-day types bred for autumn sowing — like Medania or Winter Giant — are far more reliable now than the summer varieties that bolt at the first stress.

Sow all of these shallowly (salad seeds need light to germinate well), keep the compost just moist, and thin ruthlessly. Crowded October seedlings in dim light stretch, go leggy, and damp off.

Hardy Herbs That Still Start Well Now

Coriander is the herb I sow every October without fail. It hates heat and bolts in summer, but in the cool greenhouse it grows slowly into lush, slow-to-bolt plants. Sow it in pots rather than trays so you can bring a pot into the kitchen when you need it.

Parsley is slow — famously slow — but that’s actually an argument for sowing it now. Get it going in October and you’ll have established plants ready to take off in spring, months ahead of a spring sowing. Curly types are the hardiest choice for overwintering.

Chervil is another cool-weather lover that does beautifully from an October sowing. It germinates best in cool conditions, which is exactly what you’ve got.

One honest warning: don’t bother with basil now. It needs real warmth to germinate and sulks in cold, dim conditions. Save it for March.

Overwintering Vegetables for Spring Payoffs

This is where October sowing really earns its keep. These crops won’t feed you this year — they’ll feed you next spring, weeks before anything sown in spring.

Broad beans are the classic. Sow winter-hardy varieties like Aquadulce Claudia about 5cm deep in pots or directly in greenhouse borders. They’ll grow a few inches, stop for winter, and then race away in February. I’ve had October-sown broad beans flowering a full month before spring-sown ones.

Garlic isn’t technically a seed, but October is its moment and it belongs on this list. Plant individual cloves pointy-end-up in the greenhouse border or in deep pots. They need the cold of winter to form proper bulbs — that’s called vernalisation, and it’s the reason autumn planting beats spring planting for garlic every time.

Overwintering peas work in mild areas and in the shelter of a greenhouse almost everywhere. Choose round-seeded, hardy varieties rather than the wrinkly sweet types, which are less cold-tolerant. A variety like Douce Provence or Meteor has never let me down.

Spring onions from hardy varieties sown now will overwinter as small plants and bulk up fast in spring. I sow them in clusters in modules — it’s far easier than thinning a row in February.

Spring cabbage sown in October and overwintered as young plants gives you the earliest cabbages of the year. It’s a long game, but if you have the space, it’s worth it.

Sweet Peas for Flowers Weeks Earlier

Here’s one for the flower growers: sweet peas sown in October produce stronger plants that flower earlier than spring-sown ones. I sow them in deep pots or root trainers — they make long roots fast — and keep them in the cold greenhouse all winter. They need no heat at all, just protection from the worst weather and from mice, which adore sweet pea seeds. A few mousetraps near the pots have saved many a sowing of mine.

Hardy annuals like cornflowers and calendula can also be sown now for earlier flowers, but sweet peas are the star of the October flower sowing.

How I Actually Sow in October

My method is simple and it works. I fill trays or pots with a good seed compost — not multipurpose compost, which is often too rich and too coarse for seeds. I water the compost before sowing, not after, so the seeds aren’t washed around. I sow thinly, cover with a fine layer of vermiculite or sieved compost, and label everything with the variety and date. Then the trays go onto the greenhouse staging, and I cover them with a sheet of newspaper or a propagator lid until germination, just to hold in a little warmth and moisture.

The biggest mistake I see is sowing too deep. In October, when every bit of energy counts, a seed buried too deep exhausts itself before it reaches the light. As a rule of thumb: sow a seed about twice as deep as it is wide, and leave the tiniest seeds — lettuce, for example — right on the surface.

Getting Germination When Light Is Fading

October’s real challenge isn’t cold, it’s light. Here’s what actually helps:

Sow early in the month, not late. Every week matters now. Seeds sown on October 1st get meaningfully more light than seeds sown on October 31st. If you’re reading this mid-month, sow this weekend, not next.

Give seedlings maximum light. Put trays on the sunniest staging, keep the glass clean, and don’t let taller plants shade the youngsters. I move my seed trays to follow the sunniest spot as the season progresses.

Don’t overwater. Cold, wet compost is where damping-off lives. Water in the morning so surfaces dry by evening, and let the top of the compost dry slightly between waterings. Seedlings in October need far less water than you think.

Ventilate on mild days. It feels wrong to open vents when you’re trying to keep warmth in, but stagnant damp air causes more October seedling deaths than cold does. A little airflow on a bright afternoon is worth it.

Use a soil thermometer, not guesswork. Most of these crops germinate happily at 10–15°C, which an October greenhouse delivers easily. But if your greenhouse is running below 7°C consistently, germination will be painfully slow — that’s your signal to add a heated propagator or a simple soil-warming cable for the seed trays.

Thin early and hard. Leggy, crowded seedlings in low light are an invitation to every fungal problem going. I’d rather have six strong plants than sixty weak ones.

If you want the full method for getting seeds going without any heating at all, I’ve written a detailed guide to starting seeds in an unheated greenhouse that covers the setup tray by tray.

What NOT to Sow in October

Honesty matters here, so let me save you some disappointment. Don’t sow tomatoes, peppers, chillies, cucumbers, or courgettes now — they need warmth and long days, and October seedlings will just sit, sulk, and die. Don’t sow summer basil, as I mentioned. And don’t sow anything just because the packet is open and you feel guilty — a failed October sowing wastes compost, space, and enthusiasm. Sow what suits the season, and save the rest for spring.

My October Sowing Checklist

If I had to boil my whole October routine down to one tray, it would be this: winter lettuce, rocket, mizuna, coriander, broad beans, garlic, sweet peas, and a pinch of parsley. That single tray, sown in an afternoon, gives me salads through winter, beans and garlic in spring, and sweet peas flowering before anyone else’s. Not bad for a month most gardeners write off.

For the general technique behind all of this — compost, depth, watering — the RHS’s guide to sowing seeds is the authoritative reference I check myself against every season.

Conclusion

October isn’t the end of the sowing year — it’s the start of the quiet one. The seeds you sow in your greenhouse this month won’t shout about themselves the way spring sowings do. They’ll germinate slowly, grow steadily, and then, when everyone else is just filling their first trays in March, yours will already be plants. That head start is the whole point. So grab those packets, fill a tray, and get sowing — your future self, harvesting salads in February, will thank you.

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