Growing undercover

A polytunnel in an allotment.

The vast majority of popular fruit and vegetable crops can be grown outside in the UK; however our growing season is relatively short and can be quite variable weather-wise to say the least.

As you get into your hobby you will soon appreciate the benefits a protected growing space, in the form of a greenhouse or polytunnel, could provide. However, as with a vegetable plot or garden, even those with limited space can gain many of the benefits by installing a mini greenhouse or cold frame on a sunny patio.

So what are the benefits?

The benefits of protected growing space include:

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  • The protection provided allows you to sow earlier and harvest later, so extending the season by up to amonth either end.
  • Offers morecontrol over growing conditions, helping to even out variable weather patterns, keeping crops warmer and protecting them from wind and excessive rain.
  • Provides acomfortable allweather area for growing and tending to your crops.
  • Agreenhouse or tunnel raises the temperatureallowing tender crops such as tomatoes, peppers, sweet potatoes and aubergines to be grown with moreconfidence.
  • An enclosed structurecan be heated to allow awider range of crops to be cultivated and for sowing to start earlier in the spring.
A colourful allotment with a greenhouse.

Greenhouse or polytunnel?

Having decided to install a protected growing area, the first choice you will have to make is whether to invest in a greenhouse or polytunnel and the choice is likely to be determined by one or more of the factors outlined below.

Greenhouse Advantages:

  • Attractive –this may be an important factor in a garden situation.
  • Low maintenance –(however, wooden structures do require painting every 1-2 years).
  • Better insulation –the ‘greenhouse effect’ produced by glass tends to keep the structure warmer than polythene although this is less true with specialist modern plastics.
  • Easier to maintain/control growing conditions – it is generally easier to control temperature and humidity in a well ventilated greenhouse than in a polytunnel.

Disadvantages:

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  • More expensive –the cost of a greenhouse is usually at least twice that per square metre of covered area than a polytunnel.
  • Glass can be a hazard where young children or the elderly are concerned.
  • More complicated to construct and ideally needs a firm base on flat ground.

Polytunnel Advantages:

  • Cheaper to buy.
  • Easy to build, even on gently sloping sites.
  • Provides adequate protection for most crops.

Disadvantages:

  • Not as aesthetically pleasing.
  • Polythene generally does not hold heat as well as glass.
  • Lack of ventilation may cause problems with diseases such as mildew and botrytis.
  • Depending on the plastic cladding selected, the cover may require replacing everyfive years although some last significantly longer.
Fully grown plants inside a polytunnel.

What to look for when purchasing

A greenhouse:

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  • Good, firm base. (Is it included in the price?)
  • Efficient and easy to use glazing system.
  • Adequate ventilation to suit the size of building.
  • Toughened safety glass/polycarbonate glazing options?
  • Free running, well-designed door.
  • Preferably no doorstep/ledge to create a trip hazard.
  • Guttering (is it included in the price?).
  • If choosing a wooden structure, western red cedar is more expensive but generally longer lasting than softwood. It smells good too!

A polytunnel:

  • Strong frame, preferably with strengthening bars (these can also serve as crop/hanging basket supports).
  • Doors/ventilation both ends.
  • Good range of cladding plastics on offer (it is worth paying more for a cladding with along guaranteed life, UV protected and with anti-fog coating).
  • Ease of re-cladding. The polythene is usually tensioned/secured by burying in a trench along the length of the structure, but can be fixed to wooden battens and bolted to the frame. This makes tensioning/replacement easier, but does add to the initial cost.
The side of a greenhouse with the windows open.

Install staging

In both cases you will need to consider whether you wish to install staging (benching) and if so whether you want it down both sides or just one side of the structure.

Staging has the advantage of lifting crops from the cooler floor to the warmer areas of the building, also saving your back when working on potted plants and when propagating seedlings. However you may wish to leave one or both sides clear for planting into the soil or growing taller crops in growing-bags.

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Where to site your building

In a small garden options may be limited, but try to choose a sunny, sheltered, level spot, avoiding the shade of trees, buildings and tall hedges. If you intend to run awater/electricity supply to the building, it makes sense to site it as close to the source of supply as possible, so reducing installation costs.

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