Introduction to Owning a Greenhouse
A greenhouse is valuable in an organic garden not only for extending the range of plants you are able to grow, but also for giving a good start to vegetables and that will be transplanted outdoors later. As in the garden, plants in the greenhouse will thrive if they have a balanced environment and the right growing conditions for tender plants. Site the greenhouse in a sunny sheltered spot, orientated so that the ridge runs from east to west if possible, as this will allow in maximum light.
A small aluminium greenhouse 2.4m x 3m (8ft x 10ft) allows you to raise your own plants for the garden, grow half-hardy summer crops and provides winter protection.
Creating a balance
Growing a varied range of plants from different families and with different habits will help to establish a balanced environment where pests and diseases are less likely to get out of hand. Many greenhouse pests have natural predators that will find their way in from the garden. You can encourage them by, for example, planting a few flowers in the greenhouse border that will attract hoverflies and other beneficial insects.
The right conditions
To get the best light, site a greenhouse away from buildings and trees. Good light is essential, especially for raising plants early in the year and for autumn, winter and spring crops. Some shading may be necessary in summer to reduce temperatures and prevent scorching. Exterior blinds are ideal for this, but shading paints are cheaper and easy to apply.
Correct watering of plants in pots is critical; over-watering is as damaging as under-watering. The amount of water required depends on the weather, how vigorously the plants are growing and the type of potting mixture you have used.
In cold, cloudy weather, plants may not need much water — check the look and feel of the mixture in the pot first. Water carefully if necessary: avoid splashing water onto plant leaves and round the greenhouse as this will encourage fungal diseases and increase humidity. In sunny summer weather, more watering will be needed. Conserve moisture and increase humidity in sunny, hot weather by standing pots on trays of moist sand or on capillary matting. Grow cropping plants in the greenhouse border whenever possible as watering is less critical here.
Ventilation in the greenhouse is essential, not only to keep temperatures down but also to increase the flow of air and hence aid in the prevention of fungal diseases.
Open the vents for a few hours whenever the weather permits, even if there is no danger of overheating.
You can help a greenhouse maintain heat by making sure that there are no gaps in the glass or structure. Put up a windbreak forin exposed positions. In winter, line the greenhouse with bubble polythene or other insulating material.
A little extra heat can be useful, especially during extreme winter conditions. However, factors such as light and day length also play a part in plant growth, so beware of upsetting the balance. Paraffin and gas heaters increase the humidity and make ventilation all the more important. For raising plants in spring, an electric propagator which provides heat beneath the seed trays and pots is invaluable.
The plants’ basic diet should come from the organic matter in the greenhouseor potting mixture. Treat a greenhouse border as you would an intensively cultivated plot outdoors. Fork in compost, well-rotted manure or other organic matter as appropriate to the crop. Mulch plants where possible: this conserves moisture, provides extra nutrients and protects the soil surface from the damage done by continual watering.
When plants are fruiting, you may need to top up this supply with an organic liquid feed. If you are not growing winter crops, sow a. Even green manures such as phacelia and fenugreek that are slightly tender outside will often overwinter in an .
For plants in pots, use a good organic potting mixture. When extra feeding is required, top-dress with organic fertilizers or a 1.5cm (3/4in) layer of worm compost or use an organic liquid feed.
Pest and disease control
Good hygiene is very important. Remove dead leaves and debris from the greenhouse regularly. Throw away plants that are badly affected by pests or diseases. At least once a year, have a thorough clean-up of the structure — glass, pots, seed trays and water butts — using hot soapy water.
Be particularly careful not to bring problems into the greenhouse with new plants. This is almost the only way several common greenhouse pests and diseases can arrive.
Plan a rotation of crops for the greenhouse border if possible. Where you wish to keep growing the same crop, change the soil every two to three years, replacing it with good topsoil from the garden.
Common pest and disease problems you are likely to encounter in a greenhouse include, grey mould, , sooty mould, aphids, vine weevil, red spider mite and whitefly. One of the most important ways of controlling the outbreak of many pests is by biological control.