Expand your horticultural horizons with a greenhouse

Gardening - growing plants, flowers, fruit or vegetables - is a hobby that fascinates thousands of us.

When you first start gardening, the techniques involved may seem daunting, and it's quite enough of a challenge to be able to tell plants from weeds, annuals from perennials.

But after a while, when it all starts to make sense, you may begin to look for ways of expanding your cultivation techniques. One good way of doing that is to try growing plants under glass.

It might be that you'd like to raise your own bedding plants, overwinter half-hardy perennials, grow tomatoes and peppers or start a specialist collection of alpines or insectivorous plants; whatever the challenge, glasshouse cultivation has the answer.

The simplest way of growing plants under glass is to use a cold frame, a modest construction of low walls topped with a wooden frame and glazed with glass or polythene. A cold frame will keep out all but the hardest frosts and will stop plants in pots from becoming waterlogged during periods of prolonged rainfall.

Frames are especially useful for hardening off plants on their way from a warm greenhouse or windowsill to the outside world.

If you are thinking of buying or building a cold frame, look for tops that open in stages, to allow varying amounts of ventilation, and ease of access for adding or removing plants. Cover the base with a weed-proof but porous membrane, topped off with gravel for good drainage and a professional appearance.

The next step up from a cold frame is a lean-to greenhouse, which literally leans against a wall of your house, deriving support, protection and a certain amount of heat from the house wall. Its proportions are only limited by the size of the back wall and the size of your wallet.

The smallest lean-to is a simple construction consisting of a set of shelves and a polythene covering that can be bought at the garden centre or DIY store for just a few pounds. My own veranda in Cockermouth is a larger example, as is the more ostentatious orangery at Brockhole.

The main disadvantage of a lean-to is that the amount of light it receives is limited because it has no glass on the house side. To be most effective, a lean-to should face either south or west; unless of course your new hobby is growing tender ferns, in which case east or north facing would be fine.

The free-standing greenhouse is something we are all familiar with. It is an extremely versatile concept, structures may range in size (and price) from the smallest 6ft by 8ft glass and aluminium house to a brick-based, timber-framed construction many times the size.

It's generally the case that the bigger the greenhouse the nicer the atmosphere inside and the easier it is to grow your chosen plants within it.

A small greenhouse heats up remarkably quickly when the sun comes out, and cools down equally rapidly at night. The plants may experience huge swings in temperature over very short time spans - ventilation can be a nightmare.

In a large greenhouse the temperature and humidity are much more stable and plants are more likely to thrive.

As a rough guide, most plants prefer to grow within a temperature range of 5 and 25C. Install a maximum/minimum thermometer in your greenhouse to make sure plants are not getting too hot or too cold.

All greenhouses need some form of ventilation, preferably automatic if you are out all day (oddly enough automatic window openers, operated thermostatically, are not expensive and can be used in the smallest of greenhouses).

Unless your garden is very shady, your greenhouse will also need shading in late spring and throughout the summer to prevent overheating and scorching of plants within. The simplest and cheapest method of shading is to use white shade-paint, available from garden centres for a few pounds, painted on in spring and wiped off in autumn. Netting or lath blinds do a better job, they are most effective when mounted on the outside of the greenhouse, trapping a layer of cooler air between the blind and the glass.

Undoubtedly the most decorative way of growing plants under glass is in a garden room or conservatory.

These have come a long way in terms of design over the last few years, though if you are buying one with the intention of growing plants you should make sure there is adequate provision for ventilation and shading. Generous window ledges are also important, together with a waterproof, tiled floor that can withstand watering and the odd compost spillage.

The addition of heating to your greenhouse or conservatory will expand your horizons even further, though it will also add to the expense.

There are many different types of heater that can be used but you must make sure, especially with electric heaters, that they are approved for use in a greenhouse. Always go for a heater with an automatic thermostat - you won't always be there to turn the heat on and off when required.

Remember too that when grown under glass our plants are dependent on us for all their watering requirements. This means that the conscientious owner of any greenhouse or conservatory must ask a friend or neighbour to water their plants whenever they are away. But be warned greenhouse gardening can be addictive, and you might not want to leave your plants to go on holiday ever again.