What to Grow in a Greenhouse

Before any crop list is useful, decide which of three greenhouses you actually have. The answer changes everything about what is realistic.

• Unheated. The greenhouse buys you a few degrees and, more importantly, shelter from wind and rain. It shifts your season by roughly three to five weeks at each end. It will still freeze inside on a hard night.
• Frost-free, held at 35 to 40F. Tender plants survive winter, hardy salads keep producing, and you can start seeds well before it is safe outdoors.
• Cool to warm, held at 50 to 60F and up. Continuous production year-round, including winter fruiting crops. Costs real money to run in a cold zone.

Most home greenhouses are unheated or frost-free, and that covers the great majority of what people actually want to do.

These are the crops that reward covered growing enough to justify the structure on their own. All of them want warmth, all of them want light, and all of them do noticeably better indoors in most of the country than they do outside.

Winter greenhouse growing is not about heat, it is about light. Below about ten hours of daylight, most plants effectively stop putting on new growth and simply hold. The trick is to sow in late summer and early autumn so plants reach usable size before the light goes, then harvest through the dark months from a standing crop.

Crops that work: winter lettuce and mizuna, mustards, rocket, spinach, chard, kale, corn salad, pak choi, winter-hardy spring onions, parsley, coriander and chervil. Radishes if you are impatient. Garlic and broad beans overwintered for an early outdoor spring. Forced chicory, rhubarb and sea kale in the dark under the bench, which is the best possible use of that space.

Water sparingly in winter, and water in the morning so foliage dries before nightfall. Cold wet foliage overnight is what causes botrytis, and botrytis is the main thing that ends a winter crop.

If you costed a greenhouse purely on what it saves you, propagation would carry it. A tray of forty tomato seedlings costs the price of a packet of seed and a bag of mix; the same plants bought in cost many times that, in three varieties instead of thirty.

From late winter onward the propagation bench runs continuously: tomatoes, peppers and aubergines first because they need the longest run to fruiting; then brassicas, lettuce, leeks and onions; then squash, cucumbers and beans nearer the last frost; then dahlias, half-hardy annuals and cuttings from tender perennials. A heat mat under the trays is the single most effective accessory here, because most of these want warm compost far more than they want warm air.

Two rules that prevent most propagation failures: sow into modules rather than a shared tray so roots are not torn apart at pricking out, and get seedlings into full light the moment they germinate. Leggy seedlings are a light problem, never a heat problem.

Greenhouse floor space is the most valuable growing area you own. Some crops simply do not earn it.

• Sweetcorn. Wind-pollinated, tall, and needs to be grown in a block. It does all of that better outdoors.
• Maincrop potatoes and pumpkins. Enormous space for a crop that stores perfectly well from an outdoor bed. Early potatoes in pots are a different matter and are worth it.
• Anything you can buy cheaply and store. Onions, maincrop carrots and cabbages take a lot of covered area for a modest return.
• Full-size fruit trees. They outgrow the structure and shade everything under them. Dwarf citrus or a fig in a container is the version that works.

The test is simple: does covered space make this crop meaningfully better, earlier, or possible at all? If not, it belongs outside.

Growing directly in the greenhouse soil gives plants the largest root volume and the most forgiving water regime - it is the best option for tomatoes and cucumbers if you can manage it. The catch is that a greenhouse bed cannot be rotated the way an outdoor plot can, and growing the same family in the same soil year after year builds up root diseases and depletes it. This is the classic greenhouse problem and it arrives quietly, usually in about the fourth year.

The workable answers: alternate the two long beds between crop families, replace the top several inches of bed soil every couple of years, grow a winter green manure or a completely different family over winter, or switch problem crops to large containers and fresh mix. Grafted tomato plants on disease-resistant rootstock also side-step much of it.

Whatever the method, feed heavily. A fruiting tomato in a greenhouse is a high-demand crop in a warm, high-light environment and it will run out of nutrition long before it runs out of season.