A greenhouse is the most expensive square metre in the garden. Glass or twin-wall panels, a foundation, often a water connection: it costs money and space. And then it stands empty from October to April, because the tomatoes are cleared and the new seedlings are still waiting on the windowsill. That is the most common mistake, and it costs you an entire harvest.
Yet the glasshouse is the only bed that belongs to you all year long. It freezes later, thaws earlier, keeps out rain and slugs. Grow in it wisely and you harvest something in all 52 weeks. This article shows you month by month what belongs inside, so not one square metre lies fallow.
Auf einen Blick
- A greenhouse should never stand empty. Plan in three layers: propagation, main crop, winter cover.
- The main crop (tomato, cucumber, pepper) runs from roughly CW 19 to CW 41. The rest of the year belongs to propagation and winter vegetables.
- From CW 8 onward you raise inside what moves outdoors in May.
- Corn salad, winter purslane, Asian greens and spinach carry you from CW 42 to CW 12 through the dark season.
- Rotate the crops under glass too: tomatoes in the same spot lead to soil fatigue.
The core idea: three occupancies in sequence
Do not think of the greenhouse as one bed, but as three uses that hand over to one another across the year. These three layers mesh like gears, and the handover between them is exactly where the craft lies.
- Propagation (CW 6 to 20)Seedlings for the open ground and the glasshouse. A cold frame or a table under fleece. This is where the head start is made that later becomes the harvest.
- Main crop (CW 19 to 41)The heat-loving fruiting vegetables: tomato, cucumber, pepper, aubergine, melon. They fill the house through summer.
- Winter cover (CW 40 to 14)Salads, spinach, herbs and root vegetables that shrug off cold and hold on in the unheated house.
Once you carry these three layers in your head, the rest almost plans itself. Propagation ends when the main crop moves in. The main crop ends when the October nights turn cold, making room for the winter salads you already raised alongside it in September.
January to March: harvest winter greens, start propagation (CW 1 to 13)
January feels empty, but it is not. If you planned ahead last autumn, you are now harvesting corn salad 'Vit', winter purslane and the last Asian greens. They barely grow, but they stand, and on a mild February day they pick up again. Always harvest only the outer leaves, then the plants keep bearing into March.
From CW 8, so late February, the house takes on its second role: propagation. Aubergines and peppers need warmth and want to germinate early; set their trays on a heat mat. Lettuce, kohlrabi and onions, by contrast, germinate cool and tough.
Frost in an unheated greenhouse is no drama, but the seed trays are sensitive. A second layer of fleece over the seedlings, or a small cold frame inside the house (a house within the house), holds 3 to 5 degrees more at night. On clear nights up to CW 12 it almost always pays off.
In practice: late February, trays with peppers, aubergine and chilli. Early March, tomatoes follow (CW 10 to 12, six to eight weeks before planting out). Mid-March, lettuce, brassicas, leek and herbs. Plan cleverly and you sow the salads straight into a free corner of the bed and clear the space in time for the tomato.
April to June: the great changeover (CW 14 to 26)
Now it gets crowded, and that is a good thing. In April, seed trays, leftover winter salad and the first direct sowings all stand in the house at once. It is the month of clearing out: corn salad and purslane are finished, the bed is loosened and fed with ripe compost. From around CW 17 the first outdoor seedlings move outside. The bed now belongs to the main crop. Tomatoes go in as soon as your nights are frost-free, under glass often from mid-April (CW 16), in the unheated house more safely from CW 19. Cucumbers are more cold-sensitive and wait until the soil temperature holds steadily above 15 degrees, usually from CW 20.
Solanum lycopersicum
- Standort
- Greenhouse, full sun, airy
- Saison
- Harvest CW 28 to 42
- Wasserbedarf
- Even, in the morning, at the base rather than over the leaves
- Frosthärte
- Frost-sensitive, plant from CW 19
Between the young tomatoes there is room for a quick catch crop: radishes, cut-and-come-again lettuce or spinach are ready in four to six weeks, long before the tomato needs the space.
Cucumbers are the second pillar of the summer planting. Train them vertically up a string, then they take almost no floor space and the air circulates better. That prevents mildew, the most common cucumber problem under glass. A grafted cucumber 'Picolino' or a hardy outdoor variety bears from July into October, as long as you harvest regularly and thin out the side shoots.
July to September: high summer and full occupancy (CW 27 to 39)
From high summer into early autumn the house works at full load. Tomatoes, cucumbers, peppers and aubergines are bearing now, and your main job shifts from planting to tending: pinching out, tying up, watering, ventilating. Above 25 degrees the doors and vents must stand wide open, otherwise tomatoes set no fruit and the humidity lets fungi thrive.
The most important summer move takes ten seconds: ventilate in the morning, before it gets hot. A greenhouse heats up faster than you think. How to ventilate and shade properly through a hot summer is covered in detail in the article Greenhouse in Summer: Heat and Ventilation.
And already now, in the middle of the harvest, you begin the next round. Sow nothing in September and you have an empty house in winter. From CW 32 sow corn salad, from CW 34 winter spinach, from CW 36 Asian greens and winter purslane. Raise these seedlings in trays beside the tomatoes and plant them out as soon as the first fruiting crop is cleared.
CW 32: sow corn salad
In rows straight into the cleared radish bed, or in trays. It germinates slowly but has enough time before winter.CW 34: sow winter spinach and Welsh onion
Spinach of the variety spinach 'Matador' or 'Verdil' takes the cold and delivers fresh leaves from October.CW 36: Asian greens and purslane
Pak choi, mizuna, winter purslane. Fast-germinating, tough, ideal for the gaps between cleared summer crops.CW 38 to 40: move in
As soon as cucumber and tomato weaken, set the seedlings into the bed. The house passes seamlessly into its winter planting.
October to December: follow-on crop and winter cover (CW 40 to 52)
In October the main crop ends. Once the nights drop below 10 degrees at the latest, tomatoes barely ripen further and grow prone to blight. Clear them in good time; green fruit ripens on the windowsill. Now it pays off that you pre-sowed in September: the winter salads move in.
The big soil change belongs in October. Lift the tomato root balls, loosen the bed, work in compost. If you can, move the tomato to a different spot in the house.
Valerianella locusta
- Standort
- Greenhouse, bright, cool
- Saison
- Sow CW 32 to 38, harvest CW 42 to 12
- Wasserbedarf
- Little, avoid waterlogging
- Frosthärte
- Very hardy, down to minus 15 degrees
From November to January nothing grows fast anymore, but everything stands. Corn salad, winter purslane, spinach and Asian greens outlast the dark weeks and gain a little on every mild day. Harvest sparingly and always the outer leaves, then the plants keep bearing into spring. Over the holidays you cut fresh corn salad while the ground outside is frozen: the reward for the planting that began back in August.
An empty greenhouse in winter is not a rest, it is a missed harvest.
Gardener's rule of thumb
Crop rotation and soil care under glass
The greenhouse tempts you into laziness: tomatoes grow so well there that people plant tomatoes in the same spot year after year. That is exactly what backfires. Under glass, mineral salts build up because no rain washes them out, and soil-borne diseases like corky root or nematodes accumulate.
- Move the spotDivide the bed into two or three zones and rotate tomato, cucumber and pepper through them each year. Never plant nightshades (tomato, pepper, aubergine) twice in a row on the same spot.
- Slot in a green manureWhere no salad stands in winter, sow phacelia or corn salad as ground cover. It keeps the soil alive and suppresses weeds.
- Water and ventilate the soilOnce a year, soak the soil thoroughly to flush out salts. On frost-free days, ventilate regularly so no pockets of mould form.
- Compost instead of mineral feedRipe compost in spring feeds slowly and improves the structure. That keeps the most expensive square metre in the garden fertile for years.
Run this way, the greenhouse is not a summer house for tomatoes but a year-round bed. The moment when you cut fresh salad indoors in January while snow lies outside is exactly the one that makes the most expensive square metre in the garden worth it.
Häufige Fragen
What can I grow in an unheated greenhouse in winter?
Hardy leaf vegetables are the answer: corn salad, winter purslane, winter spinach, Asian greens like mizuna and pak choi, plus Welsh onion and parsley. Sow them from CW 32 to 38 (August to mid-September) so the plants are sturdy enough before winter. They barely grow from November on, but they hold and deliver fresh leaves into spring. Important: harvest sparingly, always only the outer leaves, then the plants regrow on every mild day.
When can I plant tomatoes in the greenhouse?
In an unheated greenhouse, safely from CW 19, so mid-May, when the nights are reliably frost-free. In a well-placed, sheltered house it often works from CW 16 (mid-April), but then with fleece over the plants for cold nights. What matters is not the date but the night temperature: keep it above 8 degrees and the tomato is happy. If frost is forecast up to CW 20, keep fleece ready.
How do I stop my greenhouse standing empty in winter?
The key lies in August and September, not in winter. Anyone who thinks about winter planting only in October is too late, because the seedlings then no longer get enough light to establish. Sow corn salad from CW 32, spinach from CW 34, Asian greens from CW 36, ideally in trays beside the still-bearing tomatoes. As soon as the fruiting crop is cleared in October, the pre-raised winter salads move in seamlessly.
Do I need to rotate crops in the greenhouse?
Yes, even more urgently than in the open ground. Under glass, no rain washes the salts out of the soil, and soil-borne diseases build up faster. Never plant nightshades (tomato, pepper, aubergine) two years running in the same spot. Divide the bed into zones and rotate. If space does not allow it, swap out the top soil layer every two to three years or slot in a green manure of phacelia.
Can I raise seedlings for the open garden in the greenhouse?
That is exactly what it is ideal for in spring. From CW 8 to 20 you raise here what moves outdoors in May: lettuce, kohlrabi, onions, squash, courgette and the heat-loving peppers and aubergines. The greenhouse offers more light and steadier temperatures than any windowsill. On cold nights a second layer of fleece or a cold frame inside the house protects the tender seedlings. That way you make full use of the time before the main crop.
Spotted a mistake?
