Greenhouses in school grounds

Information sheet

Temperature significantly affects the growth rate of plants. Greenhouses are very useful because they provide an increased temperature around plants and protect them from the weather.

  • School term: All year round
  • Level of experience: No experience needed
  • Subject(s):

 

A greenhouse or polytunnel will:

  • Extend the growing season allowing you to sow plants earlier and harvest plants later.
  • Allow crops such as tomatoes, cucumbers, melons and aubergines to crop more successfully.
  • Provide a protected environment to overwinter tender plants like pelargoniums, fuchsias and citrus trees.
  • Improve the chances of propagation success when taking cuttings.

From a practical point of view a greenhouse can provide an outdoor classroom. This can be used to carry out gardening tasks, such as pricking out and potting on during wet weather. Science experiments e.g. investigating global warming and the greenhouse effect can also be staged.                                                 

Where to position it?

  • A sunny, south-facing position that is sheltered from strong winds is best to maximise the potential of your greenhouse.
  • The base of the greenhouse will need to be placed on a level surface. A flat playground is ideal, however you will have to grow things in grow-bags and pots rather than in the soil. Ensure that the greenhouse is anchored into the ground. If the site is grassed or bare soil, level footings will have to be constructed which will add to the cost of installation. Any existing vegetation will have to be thoroughly cleared to prevent it invading your greenhouse in future.
  • Access to a water supply and electricity will also be important if you want to have an automatic irrigation system (very useful for school holidays) or want to provide heating to your greenhouse.
  • Is the site near the school garden or building? Can wheelchair users access the site? Is the site a ‘stone’s throw away’ for potential vandals?  

Glazing

You need to be satisfied that the greenhouse you purchase fulfils the requirements of your risk assessment. Broken glass can be a potential hazard. There are however a number of materials available that minimise this risk: 

  • Tempered glass: much stronger than normal horticultural glass and shatters into lots of small, blunt pieces rather than shards when it breaks. Benefits include high light transmission and longevity although it is more expensive than other glazing materials.
  • Rigid plastic: includes polycarbonate and acrylic which do not last as long as glass but are incredibly difficult to break, polycarbonate which is stone proof. They do not transmit as much light as glass and can be blown out of their frames during windy conditions because they are lightweight.
  • Plastic film: includes polyethylene (polythene). This is the cheapest cladding material but only lasts 3 – 5 years before it is degraded by ultraviolet light and will need replacing. You should budget for this. 

Frame material

  • Aluminium greenhouses are often labelled ‘maintenance-free’ but still benefit from an annual clean to remove algae and dirt and to prevent the spread of plant diseases. Because the structural supports are slim, they allow more sunlight into the greenhouse than a bulkier wooden frame. They can have sharp edges. Look for a model with rounded corners and well-made benching that is unlikely to cause abrasions and cuts.

Health and safety considerations

  • Look for a ‘kick-board’ that spans the width of the bottom of the door. This prevents accidental breakage and provides strength.
  • Try to avoid large threshold steps which present a trip hazard across the doorway. A small step allows easy wheelbarrow access too.
  • A glasshouse can get very hot in summer, which can cause dehydration and heat exhaustion. It is important to avoid working inside it for prolonged periods in hot weather. White-wash shading can be applied to the glass or green shading fabric can be attached to the inside of the glass. 
  • Damping down (with tap water only), which involves throwing water onto the floor of the greenhouse, also helps to reduce temperatures. Ground surfaces can be slippery, especially when wet and children should be advised of this.
  • There should be no way that children can get locked inside while working – remove any padlocks etc. and place out of sight.
  • Because they can get very dry, greenhouses can get dusty which can present a respiratory risk. Keep them clean and tidy and wet surfaces down before working if necessary. Avoid storing compost, perlite and vermiculite in greenhouses which can cause bacterial pneumonia.
  • The greenhouse should not be used as a shed. 
  • Check the area before every session for broken equipment and glass.
  • Never store water tanks in a greenhouse for health and safety reasons.

Alternatives to greenhouses

Polythene tunnels (Polytunnels)

A large polytunnel can be a real bonus for your school garden. They consist of polythene film pulled tightly over a curved metal structure to provide a cost effective protected environment. They are however inefficient to heat, and should not be used for overwintering frost-tender plants. They are good for growing summer crops like tomatoes, cucumbers, melons and squashes and for raising salad crops earlier in the year. They are best installed by a professional because it can be particularly difficult to get the skin tight across the framework. If you can afford it, a poly tunnel with straight sides improves working conditions by providing more headroom and allowing more ventilation. Straight-sided poly tunnels often have a mesh that allows air in. They are adjusted by winding a sheet of polythene up and down over the mesh. 

Cold frames

These are smaller box-like structures made of wood, metal, brick or glass. They have a sloping, hinged lid glazed with glass or plastic.They are cheap to make and are useful for hardening off seedlings in the spring. They are also useful for growing on hardwood cuttings during the winter months. Consider carefully where you site cold frames. A south facing postion and away from areas of high foot traffic is best. 

Cloches

These are structures that are placed directly over the plants growing in-situ. Plastic drinks bottles with the bottoms cut off are also useful. It is important to add a few extra ventilation holes to allow some air to circulate around the growing plant. They also provide some degree of protection from garden pests such as slugs and snails. Other examples include bell cloches and sheet-cloches that are placed directly over the plants as they grow.