Running an unheated greenhouse through the year

January and February are the quiet months. The greenhouse protects overwintering salads — corn salad, land cress, winter purslane, hardy lettuces like Arctic King and Winter Density. You are not growing them, just keeping them alive and pickable. Ventilation matters even now: crack a roof vent on any day above 5°C to prevent fungal rots. Water sparingly, only when the compost is dry to the touch.

March is when the real work begins. You sow tender annuals — tomatoes, basil, French beans, courgettes — in modules or small pots. Minimum night-time temperatures should be above 10°C before you sow tomatoes and peppers; before that, they germinate slowly and suffer. A heated propagator on a greenhouse bench solves this and costs pennies to run. By late March, you are pricking out seedlings and potting on anything that has outgrown its module.

April to June is organized chaos. Tomatoes go into their final positions — either large pots or a greenhouse border if you have one. Plant them deep, up to the first true leaves, and they will root along the buried stem. Cucumbers, peppers, aubergines, and melons follow. Ventilation becomes critical: above 27°C, plants stop photosynthesising efficiently and just stress. Open doors and vents wide on warm days. Consider shading paint or mesh if your greenhouse regularly exceeds 30°C in summer.

July and August are harvest and maintenance. You are picking tomatoes, cucumbers, and peppers daily. Water twice a day in hot spells. Feed weekly with a high-potash tomato feed once fruiting begins. Remove lower leaves from tomatoes to improve airflow and reduce blight risk. Watch for red spider mite — they thrive in hot, dry conditions and can strip a plant in days.

September and October are the second spring. As tomatoes finish, clear the space and sow a fast winter salad crop — rocket, mizuna, mustards, baby chard. These will give you leaves from November through to March. You can also sow broad beans in pots for an April harvest, earlier than anything sown outside. Ventilation is still important, but you begin closing vents earlier in the afternoon to retain warmth overnight.

November and December return to stillness. The greenhouse is a holding zone for tender perennials that will not survive frost — pelargoniums, salvias, dahlias in pots. It is also insurance for late-sown winter salads that need just a few degrees of extra warmth to keep growing. You check it weekly rather than daily, water sparingly, and let it rest.