The question of what to grow in a greenhouse in spring has a misleading answer on most of the web: a tidy list of crops pinned to month names. The trouble is that March in coastal Cornwall and March on a Pennine hillside are different countries, and a seed does not read the calendar. It reads the temperature of the compost it sits in. So this is a sowing calendar built around the thermometer first and the date second, the way Monty works a season: you go out, you feel the night air, you check the soil, and you sow when the conditions say go, not when a magazine says March.
What follows runs March to May, month by month, for a UK greenhouse. It covers what to sow, what to pot on and what to harden off, the soil and night-temperature thresholds that actually trigger each step, and the honest gap between a heated greenhouse and the cold one most of us own. It also covers what not to rush, which is where most spring losses happen.
Why temperature, not the calendar, tells you when to sow
Germination is driven by soil temperature, not air temperature, and that single fact reorganises the whole spring. Around 18–22°C suits roughly 80 per cent of UK garden crops. Heat-lovers want more: peppers, chillies, aubergines and cucumbers germinate reliably nearer 24–28°C. A cold March greenhouse rarely holds those numbers in the compost, whatever the air does at noon.
This is why a cheap soil thermometer earns its place on the staging. Push it into a seed tray, read it first thing, and you stop guessing. A bright afternoon can lift greenhouse air to 25°C while the compost an inch down still sits at 9°C overnight, and that night-time low decides what the seed does next.
Germination thresholds worth knowing
These are minimum and optimum soil temperatures, the figures that decide whether a seed races up or sulks and rots.
| Crop | Minimum soil °C | Optimum soil °C |
|---|---|---|
| Lettuce | 4 | 10–18 |
| Carrot | 7 | 10–20 |
| Beetroot | 10 | 15–22 |
| Tomato | 15 | 21–24 |
| French bean | 15 | 20–25 |
| Courgette | 18 | 21–24 |
| Basil | 18 | 20–22 |
| Cucumber | 20 | 24–28 |
| Sweet pepper | 22 | 24–27 |
| Chilli | 22 | 24–28 |
| Aubergine | 22 | 24–27 |
Read the bottom of that table and the spring plan writes itself. Lettuce will start in cold compost, so it goes early. A cucumber needs about 21°C just to wake up, so it waits, or it sits on a heat source. Sow it cold and it does nothing but soak and slump.
Heated or unheated: the honest reality
Most UK greenhouses are unheated, and the RHS is blunt about what that means: an unheated greenhouse may not be warm enough for tender seedlings until April. A heated one buys you early sowing and year-round use, but the RHS also notes that heating a greenhouse is rarely cost effective. So the practical route for a cold house is to sow hardy crops direct from early spring and either start tender seed on a heat source or simply buy ready-grown tomato and pepper plants in mid-spring.
The clever middle path costs little. A heated propagator or a heat-mat set to about 20–25°C sits inside a cold greenhouse and supplies the germination warmth to a few seed trays while the air around them stays cool. The RHS recommends exactly this for tender seed. You heat a square foot of compost, not 200 cubic feet of air, and that is the difference between a £20 problem and a £200 one.
What a heat-mat changes
- It lets you sow tomatoes, peppers and chillies in March in a cold house, because the tray, not the room, hits the 20–25°C they need to germinate.
- It does not warm the seedlings once they are up. They still need a frost-free spot and good light, so a cold greenhouse means moving trays to a windowsill on hard nights.
- It comes with honest limits. Without bottom heat, heat-lovers sown into cold compost tend to rot rather than race ahead.
One word of caution on the other extreme. A bright March or April day can cook a closed greenhouse while you are at work, killing seedlings faster than any frost. A solar auto-vent opener, the wax-piston kind, starts opening at around 12–15°C with no electricity and keeps the house from overheating when nobody is home. Our own guide to keeping a greenhouse cool works through venting and shading in full; the short version is that spring overheats houses as readily as summer does.
March: sow hardy crops, start tender seed under cover
March is the month for hardy, cold-tolerant crops that will germinate in a chilly greenhouse without extra heat. Inside an unheated house, nights average around 6–10°C and growth speeds up noticeably compared with the open garden. The RHS lists chard, coriander, lettuce, parsley and radish among crops you can sow direct into borders or containers without heat.
Sow direct in March (no extra heat)
- Salads: lettuce such as Little Gem or Tom Thumb, plus spinach and Swiss chard.
- Roots in containers: radishes, early carrots, and beetroot from late March.
- Alliums and legumes: spring onions, broad beans and peas.
- Brassicas: pak choi from late March.
- First early potatoes in deep containers for an early crop under glass.
Start under cover with bottom heat
If you have a propagator or heat-mat, March is when you sow tomatoes, peppers, chillies and aubergines. Give them the 20–25°C germination warmth, then grow them on somewhere bright and frost-free. Without bottom heat, hold these until the house itself warms up. A greenhouse buys you roughly a month's head start over outdoor sowing dates, and that month is most of the advantage of owning one.
April: the tender crops join in
April is when the fast tender crops can finally go into compost, with the slowest still kept under cover. The greenhouse air is warmer and more reliable now, though nights can still bite, so anything sown stays inside until the frosts have truly passed. This is also the month to move tomato and pepper seedlings into the greenhouse so they begin acclimatising to its light and swing in temperature.
Sow in April
- French beans from mid-April, kept under cover and not planted out until late May.
- Courgettes in pots from late April.
- Cucumbers from late April, on a heat source if the house is cold.
- Basil from late April, though under glass the RHS will start it from as early as January, with or without heat.
- Sweetcorn in modules for transplanting in June.
The honest April mistake is reading one warm week as summer. A run of mild days tempts you to sow everything at once, then a clear night drops the temperature and the cucumbers sit and rot. Sow in stages, keep a soil thermometer in a tray, and let the compost tell you whether the week is real or a tease.
May: the month of transplanting, not new seed
May is mostly about moving plants into their final greenhouse positions rather than starting new seed. The tomatoes, cucumbers, peppers and chillies you raised earlier now go into borders, beds or large pots once the night minimums hold. The seed sowing is largely done; the job is establishing strong young plants in the spots where they will fruit all summer.
Plant out by the night temperature, not the date
Each crop has a night-time floor it wants before it settles in. These are the numbers that matter more than any calendar square.
- Tomatoes: plant out once greenhouse nights reliably clear about 10°C. They want nights of 15–18°C to thrive and take cold-check damage below roughly 12°C.
- Peppers and chillies: from mid-May, once nights hold above about 12°C.
- Cucumbers: when nights stay above about 15°C, usually late May. Cucumbers are the fussiest crop of the spring. Do not rush them.
Frost is the other gate. There is no single UK last-frost date; regional averages from Met Office 1991–2020 normals show frost finishing earliest in coastal Cornwall and Devon, while exposed Pennine, Welsh upland and Scottish sites add at least a fortnight. As a rough guide the last frost passes around mid-May in southern England, late May in the north and early June in Scotland. In any given year the actual last frost can land two to three weeks either side of the average, so an exposed plot rewards patience over optimism. Once your tomatoes are settled, our guide to growing greenhouse tomatoes takes them from planting to harvest.
Harden off before anything leaves the glass
Hardening off means acclimatising greenhouse-raised plants to outdoor conditions over about two to three weeks before they go outside for good. Plants raised in still, warm air are soft; thrown straight into wind and cold they suffer a severe check to growth that can cost weeks. The RHS sets out a staged routine, and it is worth following closely.
- Stand plants outside by day in a sheltered, south-facing spot or under an open cold-frame lid, and bring them in at night through week one.
- Add a single layer of fleece for cold nights early in week two.
- Remove the fleece by day after about ten days and leave plants out overnight if the weather is mild.
- Have them fully uncovered and outside by the end of week three.
This matters even for crops staying in the greenhouse if you bought them as plugs from a warm nursery, because the swing between a heated polytunnel and a cold home greenhouse is itself a shock. Move them in gradually rather than all at once.
Succession sowing: keep the salads coming
Succession sowing means making small repeat sowings on a regular interval so you crop steadily instead of facing a glut then a gap. The RHS suggests fortnightly sowings as the default, sowing the next batch when the previous one is well developed at around four true leaves rather than counting strict days. Lettuce goes in roughly every three weeks in early spring, rising to weekly as summer warms.
It pays off most on quick, perishable crops. Salads, spinach, carrots, peas, French beans and bolt-prone leaves like coriander or rocket all reward succession. Most salad leaves are ready to harvest within about four to six weeks of sowing, and cut-and-come-again types sown early under glass give pickings from mid-spring on. There is no point successionally sowing tomatoes, peppers or aubergines: you raise those once and crop the same plants for months. For the cold-season companion to all this, our piece on growing winter salads under glass carries the leaves through the dark half of the year.
What not to rush, and why patience wins
The most common spring failure is sowing heat-lovers too early into compost that is too cold for them. An unheated greenhouse is not reliably warm enough for tender seedlings until around April, French beans should stay under cover until late May, and cucumbers are the last things to go out, only once nights hold above 15°C. Sown into cold compost, heat-lovers check or rot rather than racing ahead.
Two habits save a spring. First, watch the nights, not the headlines: a single warm afternoon proves nothing, and it is the overnight low the seedling answers to. Second, lead with a temperature trigger rather than a date for every crop, which is why a soil thermometer in a tray of compost is the most useful tool on the bench. The calendar gives you the rough window. The thermometer tells you whether to act.
This calendar works whatever greenhouse you own. A frost-free corner, a heat-mat set to 20–25°C and the patience to read the nights matter far more than the badge on the frame. If you are starting from scratch and want a proper walk-in, the smallest we make is the NORDIC 8x6 ft timber greenhouse from £1,049, glazed in twin-wall polycarbonate that diffuses light and holds heat well, which helps a cold house hold its overnight low. Delivery is typically one to two weeks, free to the UK mainland, and a SmartVent auto-vent window is an optional £99.99 add-on for hands-off daytime cooling in spring. The advice above stands on its own; the greenhouse is just the box you do it in.
The growing season does not end where it begins. You sow in spring and you ripen in autumn, and when the light fades and the last trusses refuse to colour, our guide to ripening end-of-season tomatoes closes the loop you open in March.
The short version
Put a soil thermometer in a seed tray and let it run the spring. In March, sow hardy salads, roots and legumes direct, then start tomatoes, peppers and chillies on a heat-mat at 20–25°C if the house runs cold. In April, add French beans from mid-month, courgettes and cucumbers from late April. In May, transplant rather than sow, planting out tomatoes once nights clear 10°C, peppers above 12°C and cucumbers above 15°C. Harden off over two to three weeks, sow salads every fortnight for a steady supply, and hold the heat-lovers until the compost, not the calendar, says they are ready.
