With a greenhouse, you are able to dramatically extend the range of plants that you are able to grow. With the huge recent increase in the cost of fuel, it is not really economic to heat a greenhouse, nor environmentally friendly. With suitable insulation and positioning the greenhouse to make best use of sunlight, a so-called cool greenhouse will enable you to raise and grow plants that would not normally grow at our latitudes.
When purchasing a greenhouse, price is obviously an issue, which depends on the size of the greenhouse and what it is constructed from. If there is any chance of vandalism, such as happens all too often on Allotments, plastic glazing will be more practical than glass. Get as large a greenhouse as you can afford as you will soon find things to plant in it. A basic size would be 8 ft X 10 ft, and make sure that it is high enough for you can stand up in it, at least down the middle.
A poly tunnel would be a good choice for cost and space, but the plastic covers do have a limited life span, though they can be replaced.
Making your own greenhouse is popular on Allotments. There is so much waste timber, old windows, old reinforced UV stabilised polythene,and off cuts of rigid plastic sheets thrown away, in skips etc, that you can construct your own for just a few pounds for nails, preservative etc. However, do remember to put in bracing members to resist the force of the wind.
Remember to obtain any permissions, such as from the Local Authority, or the Allotment Association, before you order or construct the greenhouse.
Positioning the Greenhouse correctly, is vitally important if you are going to get the best benefit. You should aim to position it so that it is in the sun for as long as possible during the day and not shade any adjoining plot.
Anchoring the greenhouse down to the ground is essential to resist Winter gales. I have seen so many greenhouses blown away and shattered during gales, all because there had not been adequate holding down arrangements. People forget that greenhouses are usually very light structures, easily overturned by the force of the wind during a gale.
Perhaps the best arrangement is to have an anchor at each corner, tying down the base of the greenhouse to the ground. Tie-downs made of metal angle irons are good as they will not rot. They should be driven into the ground with a sledgehammer as far as they will go, and very securely attached to the greenhouse base with stout nails or screws. If you look closely at the top photo of the homemade greenhouse, you will see angle irons at each corner.
Greenhouse staging or shelving. A common arrangement inside a greenhouse is to have one half of the greenhouse without any staging or shelving, so that plants such as tomatoes can be grown from ground level to the full height of the greenhouse.
The other side of the greenhouse can have intermediate staging or shelving so that plants such as peppers, aubergines and seedling plants can be placed in a warmer part of the greenhouse. Because heat rises, higher areas of the greenhouse will be warmer than the floor level. Staging can be made of protected timber to prevent rot, or metal or strong wire mesh.
On the staging it is convenient to have large plastic trays in which you can place a selection of pot plants to catch the excess water after watering the plants.
A further refinement would be to have the pots standing on a layer of wicking material, to ensure that all the pot plants get a supply of moisture from a supply tank of water.