A greenhouse is a garden under glass — or plastic — where you control the weather, at least a little. It keeps out frost, wind and heavy rain, it warms up in sun, and it lets you grow tender plants, start seeds early, and harvest crops out of season. It is the most useful structure a gardener can own, and also the most demanding: a greenhouse that is not ventilated overheats, a greenhouse that is not watered dries out, and a greenhouse that is not cleaned breeds disease. This guide explains how to choose a greenhouse, how to use it through the year, and how to avoid the mistakes that turn a greenhouse into a problem.
The first thing to understand is that a greenhouse does not make gardening easier. It makes gardening possible in conditions where it would otherwise be impossible — tender plants in a cold zone, early tomatoes in a short season, salads in winter — but it does so by asking you to manage temperature, water, light, air and pests yourself, every day. A gardener who enjoys that daily attention will love a greenhouse. A gardener who wants a garden that looks after itself will not.
Choosing a greenhouse
Size is the first decision, and the usual advice is to buy the largest greenhouse you can afford and have room for, because a greenhouse fills up faster than anyone expects. A small greenhouse — two metres by three, for example — is enough for seed starting, a few tomatoes and some overwintering plants. A larger one gives room for beds, staging, a path and a water supply, and it is easier to keep at a steady temperature, because a larger volume of air changes temperature more slowly. Site it where it gets the most sun, sheltered from strong wind, close to water and power, and on level ground. A greenhouse on a slope is hard to build and hard to use.
The material matters. Glass is clear, lasts well and looks good, but it is heavy, fragile and expensive. Polycarbonate is light, tough and insulating, but it can discolour and it lets in less light than glass over time. A metal frame lasts longer than a wooden one, but it conducts heat and cold; a wooden frame looks better and insulates better, but it needs maintenance. Whatever you choose, make sure it has vents — at least one roof vent, and ideally side vents too — because ventilation is the most important feature of a greenhouse, and a greenhouse without enough vents will overheat in summer and breed disease in winter. A door wide enough for a wheelbarrow is worth having, and a water supply inside the greenhouse saves endless carrying. If you are unsure about siting or size for your area, check your local extension service for advice.
Ventilation and temperature
Ventilation is the main job of greenhouse gardening. On a sunny day, even in winter, a closed greenhouse can heat up quickly — far hotter than the plants can tolerate — and the temperature can then drop sharply at night. Open the vents and the door on sunny days, even in winter, to let hot air out and fresh air in, and close them up before night. An automatic vent opener, which opens as the temperature rises and closes as it falls, is one of the best investments for a greenhouse, because it ventilates the greenhouse when you are not there. In summer, shade the greenhouse with shading paint or a blind on the hottest days, because even with vents open, a greenhouse in full sun can overheat. In winter, an unheated greenhouse can still freeze on a cold night, so tender plants inside it still need fleece or a second layer of protection. Our winter frost protection guide covers the cold side, and the winter heat protection guide covers the freeze-thaw and overheating problems that greenhouses share.
Heating a greenhouse is possible, but it costs money, and it should be done with care. A small electric heater with a thermostat is the usual choice for a home greenhouse, and it is safer and more controllable than a gas or paraffin heater, which produces moisture and fumes and needs ventilation. Do not heat a greenhouse with an open flame or a heater that is not designed for the job, and do not leave a heater running unattended without checking that it is safe and suitable. In most cases, a greenhouse is best used unheated, or heated only to keep it frost-free — a few degrees above freezing — rather than kept warm all winter. Heating a greenhouse to summer temperatures in winter is expensive, and it is rarely worth it for a home gardener.
What to grow in a greenhouse
A greenhouse earns its keep in three ways: starting seeds early, growing tender crops, and overwintering plants. Seeds started in a greenhouse in late winter grow into plants that are ready to go out weeks ahead of outdoor-sown seed. Our winter seed starting guide covers the seedling side, and the seed starting calculator works out the timing. Tender crops such as tomatoes, peppers, chillies, aubergines and cucumbers grow far better in a greenhouse than outdoors in a cool zone, because they need warmth and a long season. Our growing tomatoes guide covers the most popular greenhouse crop. Salads and herbs can be grown in a greenhouse through winter in mild zones, and tender plants such as pelargoniums, citrus in pots and fuchsias can be overwintered in it. In summer, a greenhouse can also grow melons, grapes and other warmth-loving crops that would not ripen outdoors.
Do not try to grow everything in the greenhouse. Hardy vegetables such as potatoes, carrots and brassicas grow better outdoors, where they have room and the weather suits them, and a greenhouse full of hardy crops is a waste of its warmth. Use the greenhouse for the plants that need it, and the garden for the plants that do not.
Watering and feeding in a greenhouse
Plants in a greenhouse need watering more often than plants outdoors, because the greenhouse is warm, the soil dries out fast, and there is no rain to help. Water in the morning, so the plants have the day to use it and the leaves dry before night, and water the soil, not the leaves, because wet leaves in still, warm air encourage disease. Do not water late in the day, and do not leave the greenhouse damp and closed at night. The watering calculator gives a starting point, and a drip system or a seep hose is worth considering for greenhouse beds, because it waters the soil evenly and saves time. Feed greenhouse crops regularly, because they grow fast and their food runs out quickly: a balanced liquid feed every week or two in the growing season suits tomatoes and other hungry crops, and our fertilizing vegetables guide covers the detail. Stop feeding in autumn and winter, when plants are not growing.
Pests, disease and cleaning
A greenhouse is a warm, sheltered place, and pests love it as much as plants do. Aphids, whitefly, red spider mite and fungus gnats all thrive in a greenhouse, and they can build up fast because there are no predators and no weather to check them. Check plants regularly, remove affected leaves, encourage predators such as ladybirds if you can, and use biological controls where they are available. Our aphids control and whiteflies control guides cover two of the most common greenhouse pests, and the spider mites on plants guide covers the mite that thrives in hot, dry greenhouse air.
Disease is the other greenhouse problem, and it is usually caused by still, damp air. Ventilate every day, space plants so air can circulate, remove dead leaves promptly, and do not overwater. Clean the greenhouse thoroughly at least once a year, in late autumn or winter when it is emptiest: remove plants, wash the glass inside and out so winter light gets in, scrub staging and pots, and clear out debris. A clean greenhouse lets in more light, holds less disease, and is a better place to grow in spring. Our winter garden cleanup guide covers the end-of-season tidy, and the damping off seedlings guide covers the disease that kills seedlings in a damp greenhouse.
Pros and cons of a greenhouse
The advantages are a longer season, tender crops in a cool zone, early seed starting, winter salads in mild zones, and a place to overwinter tender plants. The disadvantages are cost, the daily attention to ventilation, watering and pests, the risk of overheating in summer and freezing in winter, and the fact that a greenhouse needs cleaning and maintenance every year. A greenhouse is an investment of money and time, and it should be used every month of the year — a greenhouse that sits empty for half the year is a waste.
Common mistakes
The most common mistake is not ventilating enough, so the greenhouse overheats in summer and breeds disease in winter. The second is overwatering and watering late in the day, so leaves stay wet overnight. The third is overcrowding, so air cannot circulate and disease spreads. The fourth is heating a greenhouse to summer temperatures in winter, which is expensive and rarely worth it. The fifth is forgetting that an unheated greenhouse can still freeze, so tender plants inside still need protection. The sixth is not cleaning the greenhouse, so disease carries over from year to year. And the seventh is growing hardy crops in the greenhouse that would do better outdoors, wasting its warmth. If you are unsure about greenhouse growing in your area, check your local extension service for advice.
Frequently asked questions
Do I need to heat my greenhouse in winter?
Usually not, or only to keep it frost-free. An unheated greenhouse still protects plants from frost, wind and rain, but it can freeze on a cold night, so tender plants inside still need fleece. Heating to summer temperatures in winter is expensive and rarely worth it for a home gardener.
Why is my greenhouse too hot in summer?
Because a closed greenhouse in full sun heats up fast. Open vents and doors, use shading paint or a blind, and consider an automatic vent opener. Water in the morning, and do not leave the greenhouse closed on a sunny day.
What grows best in a greenhouse?
Tomatoes, peppers, chillies, aubergines and cucumbers in summer; salads and herbs in winter in mild zones; and seeds started early in late winter. Hardy vegetables such as potatoes and carrots grow better outdoors.
How often should I clean my greenhouse?
Thoroughly at least once a year, in late autumn or winter when it is emptiest: wash the glass inside and out, scrub staging and pots, and clear debris. Clean pots and trays before spring sowing, so disease does not carry over.
How do I stop pests in a greenhouse?
Check plants regularly, remove affected leaves, ventilate so air circulates, do not overcrowd, and encourage predators. Biological controls are available for several common greenhouse pests. Do not ignore a small outbreak — in a warm greenhouse, it becomes a large one quickly.