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Greenhouses

 

Greenhouses

Greenhouses are now available in many shapes and sizes, and prices can range enormously, anywhere from £250 to £10,000.

The main question you need to ask yourself is what you want to use it for, and whether you want it to be purely functional or have ornamental value.

Greenhouses are mainly used to propagate plants, start young plants off earlier and grow warmer climate plants like tomatoes and other ornamentals.

You need to consider several factors:
• Position, size, style and shape
• Frame and glazing materials
• Ventilation
• Guttering
• Staging and shelving
• Heating, if required
• Shading, if required
• Access to water, electricity and ease of loading/unloading
• Need for planning permission

Where to site your greenhouse
Positioning of the GreenhouseThe plants in your greenhouse need as much natural light as possible and so the greenhouse should be in an open position, away from any trees or other shade giving elements. As well as creating shade, trees can also drop branches, and insects on trees can drop honeydew and make the greenhouse sticky.
If you are going to have part of the base as soil to plant into, then the soil needs to be free draining.
Avoid positioning a greenhouse in a frost pocket. These are often found in low areas. Equally, avoid windy areas.
If you mainly want to use the greenhouse to propagate plants and start plants off early, then you want to maximum the light levels in the spring, and so the ridge should run east to west.

What size do you choose?
Think about how you intend to use the greenhouse and how many plants you will have in there. You also may want some space for a chair.
In general, go one size larger than you think you need, if budget allows.
Choose a greenhouse that has a suitable height, both of the doorway and inside. If you are tall, you may not be able to stand upright in some of the smaller ones.

Style and shape
Most greenhouses are the apex type with a single ridge, however you can also get octagonal shapes, lean-to shapes and additions like porches. In fact, many companies offer bespoke greenhouses although the price can be high.
Lean-to greenhouses have the added benefit of being slightly cheaper as they are using a house wall as one side. This is also beneficial as the wall absorbs the heat during the day and releases it as the outside air cools, thus buffering the greenhouse against temperature extremes. It is also easier to install electricity and water from the house. However, you may only have one choice of wall and this may not be the best for obtaining maximum light.

Greenhouse frame
Most frames are either aluminium or timber.
For a timber frame it is best to use Western Red Cedar as it has a longer life than softwood, although this will still need treating every 2-3 years. Timber is a good insulator, heating and cooling the greenhouse more slowly.
Aluminium frames are generally less expensive and are generally maintenance free but are not aesthetically pleasing unless you go for a powder coated option which will add to the cost. Make sure all the fixings are also aluminium and not steel as they will rust.

Glazing
GlazingThere are several types of glazing. The most common is glass.
Horticultural glass with a thickness of 3mm is the cheapest and offers good light transmission but if damaged, it breaks into shards which could be dangerous if there are children around.
Toughened (safety) glass is 4mm thick and is more expensive but is less likely to break, and if it does, it breaks into relatively safe pieces.
Polyethylene (plastic) is cheap but wears quickly and will need to be replaced every two years.
Polycarbonate is more expensive than glass but more durable, however, it will scratch easily and thus the light transmission will be reduced.

Greenhouses can be fully glazed or part glazed, i.e. the bottom half is constructed from brick or timber. Part glazed greenhouses can look nice but you will not be able to grow crops at ground level as they will not get enough light.

Ventilation
Good air circulation is important for healthy plant growth. Most greenhouses come with vents or windows in the roof. However, it is wise to also have vents (louvres) in the sides of the greenhouse which will create better circulation of air. The ventilation should be 1/6 of the floor area.
Vents can be manually controlled or automatically controlled when the air reaches a pre-set temperature.
Be wary of using the door as ventilation as unwanted animals can get in.

Guttering
Check to see if the greenhouse comes with guttering as collecting water from the roof into a water butt will provide a good source of water close to where it is needed.

Staging and shelves
ShelvingStaging is usually made to be at waist height. If you are using some of the greenhouse to be planted at ground level, use the sunnier side for this, and put the staging on the side away from the sun.
Slatted staging offers better air circulation than solid staging, although may be more difficult to clean. When installing solid staging keep a 150mm between the sides and back to aid air circulation.
You may also want extra space by adding narrow shelving at ridge height.

Heating
If you only want to bring on plants a little earlier and store slightly tender plants over the Winter then an unheated greenhouse is fine. But if you want to grow plants that will not survive a frost, then you need to think about using a heating system.

  Advantages Disadvantages
 Electric Fan Heater
  • Can have hot and cold settings
  • Also circulates air which is good for healthy plant growth
  • Can be thermostatically controlled so it only operates when needed
  • Can be expensive to run
  • Electricity is required
 Paraffin Heater
  • Cheap to buy and run
  • Emits carbon dioxide which is good for plant growth
  • Don't need to install electricity supply
  • Manually controlled so may waste heat
  • Fuel may be difficult to source
  • Emits water vapour which encourages mould
 Bottled Gas
  • Can be thermostatically controlled
  • Need to replace cylinders
  • Needs ventilation as it emits water vapour and fumes


Some protection can be given by insulating your greenhouse. The cheapest method is by using clear plastic bubble wrap. You can also buy specifically designed thermal screens and base cladding.

If you only want to heat a small number of pots you could use a heated bench or propagator.

Shading
If you keep plants in your greenhouse over late spring and summer, it can often get too hot and shading will be required. The cheapest option is to use shading paint although this is messy to apply. Alternatively you can install shading blinds. These are more versatile as they can easily be rolled up and down as required.

Looking after the plants in your greenhouse
Looking after the plants in your greenhouseRemember to water your plants regularly especially on hot days as they can dry out very quickly.
There should be a humid atmosphere to prevent too much water loss from the plants. On hot days, spray the path with water or fill trays with water and let it evaporate slowly.
Ideally, clean the greenhouse glass in the spring and autumn to ensure that the maximum amount of light is getting in.
The greenhouse is a good place for pests and diseases to spread so clean the inside with a garden disinfectant at least once a year, remove any dead leaves and discarded plants and check the plants regularly, dealing with any issues as soon as they arise.
Irrigation systems e.g. drip systems, mist units and soil warming cables can be used if you want more control over the environmental conditions.

Cold frames and ‘hardening off'
If you are starting plants off early in a greenhouse, remember that you need to harden them off before planting them outside. This allows the plant to acclimatise to different environmental conditions and they will grow more strongly as a result.

A cold frame is an ideal place to harden off plants as it is sheltered and relatively warm. To harden off, move the plants to the cold frame and open the lid for a few hours in the middle of the day. Do this for 3-4 days, then another 3-4 days where you leave it open all day. After 7-10 days leave the lid open all day and night. After 14 days, the plants are ready for the outside world.
If during this time, a frost is forecast, keep the lid closed, and remember not to plant out until the danger of frost is passed in your area. (This is about the middle/end of May in Bedfordshire, but you need to watch the weather foreasts.)

If your budget does not allow for a cold frame, or you have limited space, you can put the plants outside and cover them with horticultural fleece using the same principles as above in terms of removing it as and when required.

 
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