Coldframes and mini-greenhouses
Coldframes and mini-greenhouses are useful accessories to a greenhouse, and can also be a partial alternative to a greenhouse. Frames are merely boxes that lie flat on the ground with a glazed, sloping lid and mini-greenhouses are glazed boxes that stand vertically with openings on one side.
Mini-greenhouses are a space-saving way of covering plantsGlass is better than plastic
Gusts of wind are the main cause of damage
Why use coldframes and mini-greenhouses
There are sound reasons to use coldframes and mini-greenhouses, particularly where greenhouses are not an option, perhaps because of the cost or lack of space. Coldframes and mini-greenhouses can be easily moved to a spot suitable for growing a particular plant or crop (e.g. brighter for vegetables or shadier for cuttings).
Glasshouses and polythene tunnels still provide the best growing environment as they are walk-in and the conditions inside are easier to manage. This is because the larger volume of air acts as a buffer, reducing fluctuations in temperature and humidity - so are less severe than inside coldframes and mini-greenhouses.
When to use a coldframe or mini-greenhouse
In spring and early summer, coldframes and mini-greenhouses are a useful place to harden off tender young plants, such as half-hardy annuals and vegetables. These structures can be used to provide a halfway house between the greenhouse and the harsher conditions experienced in the garden.
In late spring and summer, use coldframes and mini-greenhouses to provide extra warmth for tender summer crops, such as aubergine, tomatoes and chillies. This encourages quicker ripening and a larger crop.
In autumn they are useful in protecting cuttings and young plants from cooler, unsettled weather and the first frosts (but only mild frosts).
In winter, coldframes and mini-greenhouses can be used to shield alpines from rain and so prevent them from rotting. They can also be used to overwinter young annuals and other plants. A layer of bubble plastic can be installed on the inside to provide some insulation against frost and provide additional draught proofing. However, the bubble plastic will reduce light and ventilation.
Throughout the year, these structures can be used to grow a range of edible crops, and other plants.
- Coldframes with clear sides and lids are the most popular, especially those made of aluminium and glass
- Where protection from cold is required, coldframes with wooden or brick sides are better, but less commonly seen
- Lids should be robust and easy to open. Ideally, they should allow varying degrees of ventilation, while protecting the contents from heavy rainfall
- Lightweight frames are cheap and easy to move, but vulnerable to wind damage and need to be pegged down
- Frames can be set up on soil, gravel or paved areas
- Mini-greenhouses usually have loose plastic covers or a skin of rigid plastic sheets; glazed versions are available too
- The covers on plastic mini-greenhouses are often difficult to replace, so despite low initial cost can prove poor investments in some cases
- Mini-greenhouses can protect taller plants than frames but often lack ways to adjust the ventilation, which leads to damaging fluctuations in temperature
- These structures rely on shelving to extend their limited floor space. However, the shelves cast shade and this is not ideal for many purposes, such as raising young plants
- Place mini-greenhouses against a wall to help overcome one common problem: the ratio of large surface area to internal volume leads to uneven growing conditions, particularly when the structure is exposed on all sides
- Mini-greenhouses can also be placed on paved areas, or on soil if necessary
As coldframes and mini-greenhouses can be costly to buy and make, many gardeners use ingenious makeshifts.
- Cut down plastic bottles can be used to cover seedlings or individual plants
- Plants and seedlings grown in larger areas or rows can be protected with panes of glass raised on bricks and old window lights (frames) resting on sides made of wood
All these are inexpensive and, if care is taken with ventilation, often prove good enough for most purposes.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.