April is the greenhouse gardener’s busiest month, and for good reason, the days are lengthening, temperatures are rising, and the window for getting key crops started is firmly open. Get the timing right now and you’ll be harvesting well into autumn, leave it too late and you’ll spend the season playing catch-up. Here’s how I’m thinking about my sowings this month, grouped by what each type of seed actually needs.
In the heated propagator first
Some seeds need a little persuasion to germinate, warmth they simply won’t get from the ambient greenhouse temperature in April, even on a sunny day. These go into my propagator, set to around 20–24°C, until they’ve germinated and are big enough to prick out.
Tomatoes are the obvious one. If you haven’t sown already, do it now, any later and you’ll be stretching the season at the other end. Sow thinly in small pots or modules, cover lightly with compost, and expect germination in a week or so. Once they’re through, move them to the greenhouse staging in full light, it will be warm enough by then. Peppers and chillies are the same story, though they take a little longer to germinate and appreciate slightly higher heat — 24°C is better than 20 for these. Aubergines too, if you grow them.
Basil is one I always start in the propagator as well. It’s a Mediterranean plant and simply sulks in cool conditions. A small pot now will give you a decent-sized plant by early June.
Straight into pots in the greenhouse
These are the more accommodating crops, seeds that will germinate happily at the cooler end of the greenhouse temperature range (10–15°C is fine) without any special treatment.
Cucumbers are the big one for April. Sow one seed per small pot, on its edge, in warm compost. They germinate quickly and grow fast, so don’t ever be tempted to sow too early — a plant sown now will catch up with one sown in March before you know it. Summer squash and courgettes follow the same principle: sow on edge, one per pot, and they’ll be ready to harden off and plant out by late May.
Climbing French beans are worth starting undercover in April too. Sow in root trainers or deep modules — they put down a long taproot — and they’ll be ready to go outside once the last frost has passed. It gives them a good head start over direct sowing.
Herbs can go in now as well: coriander, dill, and parsley all germinate well in the greenhouse without bottom heat. Sow thinly in pots and thin rather than prick out.
A third category: cut flowers
It’s worth reserving a corner of the staging for a few cut flowers this month. Cosmos, zinnias, and sweet peas (if you haven’t already started them) can all be sown in the greenhouse now. They germinate quickly and will reward you with months of cutting. Dahlias can also be started from seed this month if you want a huge range of variety beyond what tubers allow, they need the propagator and a bit of patience, but it’s a deeply satisfying experiment.
A word on watering and labelling
Whatever you’re sowing, water gently from below where you can, standing pots in a tray of water until the surface looks moist, then removing them. And label everything the moment you sow it. Not next week, now. April sowings have a habit of all looking identical three weeks later.
This is the month that sets the tone for the whole growing year. Sow well, give your seedlings enough light, and the greenhouse will be overflowing with promise before May is out.