The very idea of growing anything in a Northern winter, even in a greenhouse, seems unlikely to many, yet this is done right around the UK, even in Scotland.
A greenhouse is exploiting, quite literally, the greenhouse effect, to maximum effect, where any existing heat and light sources are brought in and trapped, so as to get as close as possible to optimum growing conditions for the plants.
The first consideration is whether to use an existing greenhouse, or one built for purpose.
An alternative to the traditional glass and frame structure is a polytunnel, in which more can be grown for the same cost as the former.
Obviously, a polytunnel is far less attractive, and needs more space, so tends to be built in an open space rather than a town garden.
A stand alone greenhouse will lose heat on all exposed surfaces, both by radiation and the heat stripping effect of cold wind and rain.
To absorb the maximum heat and light, when daylight hours are shorter and the sun’s radiation weaker, a balance needs to be struck between the proportion of glass surface and supporting frame work, as well as the thickness and transparency of the material used.
One solution is to build the house conservatory like, adjoining a wall, ideally south facing, especially if the wall is part of a heated building.
Also advantageous is constructing against the inside of a walled garden, also south facing, where protection from the wind is improved.
Obviously, an inner heating source is ideal, but the cost has to be considered.
Far more surprisingly, there are many who claim to manage winter crops without using a heater.
Another advantage can be to ‘double glaze’ either with a second layer of glass, or Perspex, or other material, but the advantage, again is set off by the filtering effect of light, especially in the short wavelength, or ultra violet.
All plants thrive on warmth, good growing medium, but also the right air quality inside, which in daylight hours is ideally carbon dioxide rich.
One of the more ingenious methods I heard of involves circulation of the atmosphere inside between the greenhouse and a premises like a restaurant, teahouse, or pub, in which both the cooking activity and human bodies in the premises release copious carbon dioxide and heat. This air can even be piped directly into the base of the plants, so as to ‘drown’ each one in a plume of this ideal mixture.
While polytunnels are more aerodynamic than an angled structure, even more so is a dome, a striking example of which is the Eden Project in Cornwall, in which both tropical and Mediterranean plants thrive. Most remarkable is the double layer of very thin plastic, which gives the surface its ‘bubble’ like appearance.
Many of the plants that winter grow in the Mediterranean region, need an artificial climate to approximate this. Plants like citrus and broad beans, and a host of herbs and vegetables can be grown this way.
To overcome the challenge of winter growing, the Victorians had a method that I have tried.
Melons love warmth as well as light, good compost and atmosphere.
To optimise this, the raised growing surface has a layer of manure, partly ‘matured’ for obvious reasons!
On top of this six inch deep layer, the growing compost forms the next, with a moisture proof layer between.
It is the decomposition of the manure that produces heat, which is why heaps of it are seen steaming in cold weather.
In addition, a supply of CO2 is released.
Obviously, the light inside needs to be maximised, so in a conservatory style greenhouse, it helps to paint the non-glass surface, i.e the wall behind, with either white or reflective material, such as foil. This can even be done on the floor surface, reflecting both heat and light within.
Some parts of the world, such as Iceland, though ferociously cold in winter, are blessed with naturally occurring geothermal water or steam, which are piped into the greenhouses there, with the result that there is a thriving banana growing industry. Tropical plants like these do need internal heating, unlike the Mediterranean ones.
It is not just plants that enjoy this climatic shift in the protection and warmth of a greenhouse, so do the vital bugs that both pollinate them, and others that may be used in biological control of pests, especially for organic growers who do not like using toxic chemicals.
Bees have been in decline in recent years, and some believe this is because the winters in northern climates are wetter, and bees suffer from this.
A smart idea is to build an ‘annexe’ at one end of a long greenhouse, or polytunnel, a dry sheltered place for placing bee hives. In the extended warm climate, they are supplied with a source of food, within the greenhouse, right on their doorstep.
This whole subject is constantly being updated by new innovations, partly because of advances in suitable materials, such as controlled ventilation.
I heard recently that a small proportion of neodymium, infused in glass, changes its properties, so that a larger proportion of ultra violet is transmitted through, but I believe this is very expensive.
Here is a brief list of just some of the plants that can be winter grown this way.
I am sure the list could be even longer.
Good luck with this interesting and rewarding challenge, all the more possible in the milder winters of today.