A greenhouse extends the growing season to such an extent that all year round cultivation becomes a possibility and, if you aren't quite ready for that, sowing can be brought forward by weeks and yields increased substantially.
Key to the growing process is light and heat, both will be very adequately supplied by a greenhouse that has excellent light transmission and, when natural warmth is in short supply, the greenhouse can be insulated and heated.
Before a greenhouse is even in place, you'll probably have a good idea of what you want to grow, so it should be equipped with staging and any other accessories you require.
Layout and Heating
You may have beds along the sides where plants can either go directly into the soil or grow bags. There'll be an area for sowing and raising seedlings, and another space for potting on plants or a propagator, plus storage space for bags of compost and pots.
Once you have the greenhouse interior to your liking, you can get on with sowing seed and cultivating plants but not before deciding on whether or not to heat the greenhouse.
If heated, with the temperature around 18°C - 21ºC, you can sow many different types of vegetables much earlier in late winter and early spring.
In an unheated greenhouse, the growing season will be pushed back. But you can always raise tomato, cucumber, aubergine, capsicum and other seeds on a bright windowsill and then transfer them to the greenhouse when temperatures rise later in the spring.
When transferring seedlings to the greenhouse, one important consideration is the difference in day and night temperatures, if this is too great, your plants will suffer. Throughout the growing season try and keep any temperature variation to a minimum, definitely not more than 10ºC.
Choice of Vegetables
Many vegetables can be sown in early spring that will be subsequently planted out including peas, cabbage and sprouts and onions. When the days get a little longer, aubergines, courgettes, sweetcorn and other non-hardy plants can be started off and then moved outside after hardening off when the weather is warm enough.
Tomatoes are probably the most common greenhouse crop. Depending on the temperature they can be sown from late winter, transfer the seedlings to 3inch pots, and then plant in growbags or the border. When the first fruit has set, start to feed with a high potash fertiliser. Pinch out any side shoots, and then the top when there are four trusses.
If you like a curry, chillies, whether your taste is for the mild or the red-hot Naga Morich, can be sown from January to April but they need the temperature to be 25°C - 30°C so will need a heated propagator.
In late summer and autumn, further lettuce and other salad such as radishes can be sown and eaten over the winter, even carrots and new potatoes for your Christmas lunch. Broad beans can also be sown to be planted out in the following spring, and many herbs will grow throughout the winter. To retain heat in cold winter weather and bring the temperature up a few degrees, insulate the greenhouse with bubble wrap.
For most crops, the atmosphere inside your greenhouse should be warm and dry with good ventilation. Louvres near ground level are used to allow in cool air that will then heat up and rise, exiting through top ventilators, thus providing an effective throughput of air. On very hot days, opening the doors, as well as the windows, is a good idea.
However, some of the more exotic vegetables such as cucumbers and okra like some humidity and this is achieved by 'damping down' the area around the plants. If you are growing a variety of crops that require different conditions, a partition will be a good idea.
Water regularly, not allowing the plants to dry out. If you go on holiday, install an automatic irrigation system. You'll also need to feed plants frequently, a seaweed-based fertilizer provides the full range of nutrients.
In mid-summer, to prevent overheating, the glazing may need shading, a specialist paint or whitewash that can be easily wiped off or you may fit blinds. Again, if you go away for anytime, think about installing automatic vents.
Finally, keep your greenhouse clean and tidy. There is a myriad of pests and diseases waiting to attack your plants, from aphids to mildew, so remove any plant debris, wash both the interior and exterior thoroughly each year and clean all the pots and trays that you use.