Choosing a greenhouse

For gardeners, buying a greenhouse opens up all sort of exciting new opportunities: propagating your own plants; growing vegetables or even exotic, tropical plants. However, budget and space usually means compromises, so it’s worth deciding what’s most important and doing some research in order to make the right choice.

So, before you rush out and buy your lovely new greenhouse, there are a few questions to answer first:

Why do you want a greenhouse?

It seems like a silly question, but it’s worth thinking about! Look on Ebay or Gumtree and you’ll find plenty of people trying to sell greenhouses they no longer want or need.

A greenhouse is a commitment. Plants growing in your greenhouse are dependent on you for water, feeding and ensuring they don’t overheat in summer or get too cold in winter. You can’t leave them to their own devices as you might do for many garden plants. Greenhouse plants can be more susceptible to pests and diseases (which also love those cosy, warm conditions). The greenhouse itself needs a certain amount of maintenance. Don’t buy a greenhouse because you think it will save you money – it probably won’t! Buy a greenhouse because you enjoy growing plants – whether it’s vegetables, fruits or ornamentals – and because this will bring you pleasure.

Where will you put your greenhouse?

The next question to think about is location. For most of us, this will probably be a compromise as the best location horticulturally-speaking (ie best for the plants) might not be the best aesthetically. As part of the Hands-On Gardening Course we look at mapping the shade and sun in your garden throughout the year, and this can help to find the most practical location for a greenhouse.

South-facing location: Traditional advice is to site your greenhouse where it can get sunlight for most of the day, with the ridge of the roof being on an east-west orientation, out of the way of shade from buildings or trees. However, for many of us, this is impossible: our gardens aren’t big enough, and it would mean putting the greenhouse slap-bang-in-the-middle of the lawn – not really ideal!

And although this orientation provides maximum sunlight, it can also mean the greenhouse overheats in summer, damaging precious plants. Yes, you can put up shading, open and close vents, but this can be time consuming and expensive. For many hobby gardeners it might be easier to place the greenhouse where it will receive some shade some of the time.

My simplified graphic (ahem!) below, I hope, helps to illustrate the sunlight pattern for a South-facing location. Of course, you may have shade from fences, trees, walls and neighbouring houses to think about as well!

South-facing greenhouse as the sun moves from East to West

West-facing location: The second choice for orientation is to face the length of the greenhouse westwards, so that it receives afternoon and evening sun. This will allow the greenhouse to warm up in the afternoon and stay warm into the evening.

Greenhouse west-facing orientation

East-facing location: Third choice is to face the greenhouse eastwards. Your plants will still get half a day’s sun, but bright sunlight can scorch some sensitive plants in the morning and the greenhouse will be colder at night.

East-facing greenhouse will catch the morning sun

The only orientation that really won’t work well is north-facing, or where the greenhouse will be shaded by trees for much of the day.

Remember shadows are longer, and the sun takes a different path, in spring and autumn. So, for example, if you’re planning to grow vegetables from seed in the spring, you need to be mindful of the long shadows cast by trees and buildings at this time of year.

Finally, try to site your greenhouse in a more sheltered part of your garden. As well as causing lower temperatures, strong winds can damage the structure or the glazing.  This might mean another compromise, as the more sheltered parts of your garden may also be near the house, trees etc which also cast shade.

How big do you want your greenhouse?

It’s generally recommended to get a greenhouse at least 6ft x 8ft. Without getting too technical, what you need is something to buffer the temperature gradients in your greenhouse. The larger the volume of air in the greenhouse, the more the temperature will be buffered (ie avoiding extremes of temperature). In addition, no gardener has ever been known to wish for a smaller greenhouse!

In my opinion, the very small, cupboard-style greenhouses are not worth having. They’re so small that temperatures can get far too high or too low for plants to survive.

Frame material?

There are two choices for greenhouse frames: wood or aluminium.

Wood: Wooden-framed greenhouses are the more expensive option. They are more attractive, and are better at maintaining a constant temperature. (For the same reason that a larger air volume will buffer temperatures, so will a wooden frame: the wood will absorb heat and release it slowly.) A wooden-framed greenhouse will need a bit more maintenance than an aluminium-framed one.

Aluminium: Aluminium greenhouses are a cheaper option. The thinner struts mean they let more light in, and they need less maintenance. You can get them powder coated in different colours – usually green, which can look very attractive.

Finally, a style which has tall eaves and a high pitched roof will allow in more light.

Glazing material?

The three choices generally available are horticultural glass, toughened glass and polycarbonate sheeting.

Toughened glass is (as you’d expect from the name), stronger than standard horticultural glass. If it does get broken, the pieces won’t be so sharp. It’s more expensive, but would be a good choice if you have children or play a lot of ball games in your garden! Polycarbonate sheeting is cheaper and won’t get broken by footballs. However, it deteriorates quickly with age and lets in less light, meaning your seedlings and plants will suffer. The panes also tend to get popped out by the wind. (I speak here from bitter personal experience!)

The base:

Your greenhouse needs to have a solid foundation, to keep it steady and stop it warping. After all, you don’t want it ending up in the neighbour’s garden after the first storm…

Greenhouse manufacturers recommend using a greenhouse base  – a rigid metal frame which can be anchored to the foundation. The foundation itself can be compacted soil, paving slabs or concrete. You can also make a ‘perimeter base’, ie a square or rectangular base only under the greenhouse frame.

A solid, impermeable floor can become wet and slippery so whatever type of base you decide on, make sure it has drainage. A layer of gravel is useful as you can then wet the floor in summer to cool the greenhouse down. A plain earth floor is fine (again, useful if you want to wet it to cool it down), but it’s probably not a good idea to plant vegetables directly into the soil, as this can cause a build up of diseases.

If you don’t feel confident in being able to make the foundation yourself, it’s worth getting a good landscaper to do this for you.

Whatever your final decision, I’m sure your greenhouse brings you many years of joy and pleasure!

Image credits:
Greenhouse” (CC BY-ND 2.0) by Arty Guerillas
Tomatoes” (CC BY 2.0) by Ajith_chatie
Seedlings” (CC BY-SA 2.0) by peganum
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