How to Manage a Garden Greenhouse
Managing a Greenhouse
Maintaining the right environment in the greenhouse involves a range of factors: watering, ventilation, shading, heating, and good hygiene.
Getting this right is important. If plants have too little water, then they become stressed and are more prone to pests and diseases. If they have too much, then they are liable to attack by a variety of fungal diseases.
When to Water in the Greenhouse
Unfortunately, there is no easy formula for deciding when to water. Different plants have different needs, which also depend upon the time of year, the weather, and the growing medium. Plants in pots will need watering much more than those in the greenhouse border.
In general, plants need most water when they are growing quickly, especially on sunny, windy days, and least water when they are growing slowly, especially on cold, dull days. The following suggestions will help you to judge when to water.
- Scratch the surface of the , or of the compost in pots, to test whether it feels dry.
- Observe the colour of the soil or compost. Dry soil or compost is lighter in colour than that which is moist.
- Lift up pots. Dry pots feel lighter than moist ones.
- Watch for signs of wilting. Leaves will often look dull and slightly droopy before the plant wilts.
- Try to avoid watering when it is cold and damp, or late in the evening when a cold night is expected, as this will encourage fungal diseases and slugs.
How to Water
Water plants in a greenhouse border as you would in the garden. A mulch of organic material, such as leaf mould, lawn mowings, or compost, will help to retain moisture and protect the soil surface from the impact of heavy watering. Some materials can also help to feed the plants.
Water plants in pots until the compost is soaked, but stop before the water streams out of the bottom of the pot. If they have become so dry that water is not absorbed, then stand them in a deep tray of water and let it soak upwards. This is also a good way to water trays of seeds and seedlings. In summer, put plants on trays of moist sand or special capillary matting material to help conserve moisture and raise humidity.
On dull cold days, water carefully using a watering can without a rose. Try to avoid splashing water on plant leaves or around the greenhouse, because dampness in these conditions can cause fungal diseases. On very hot days in summer, however, it is often beneficial to spray the plants and greenhouse floor with water. This cools the greenhouse and creates humidity, which helps reduce the need for watering. It can also help to control attacks of red spider mite.
Avoid spraying delicate plants and seedlings in bright sunshine, as water droplets on plant leaves focus the suns rays and cause scorching.
Greenhouse Watering Equipment
- A good watering-can, ie. one with a long spout and a removable fine rose that does not dribble. Fill the can and leave it in the greenhouse so that the water has a chance to warm up before you water tender plants.
- Water-butts for collecting rain help to conserve water resources. Provided that the butts have tight-fitting lids to keep out debris and insects, the water should be clean enough to use for established plants, but it will inevitably contain some bacteria and fungi. It is better to water seeds, seedlings, and cuttings with fresh tap-water to minimize the risk of fungal disease.
- A hose connected to the mains supply will be needed, as water-butts and watering-cans are unlikely to supply sufficient water in summer.
- You can also buy a variety of more sophisticated watering systems, which make the job easier and less time-consuming. Devices such as sensors, timers, and reservoirs mean that, in theory, you can leave the greenhouse to water itself. However, it never hurts to check it. Few automatic devices are as good as an experienced greenhouse gardener!
Greenhouse Watering Systems
- A sprinkler is good for watering trays of seedlings and lots of small pots. However, it has limited use in a small mixed greenhouse because it delivers the same amount of water over a fairly wide area, which may contain plants with differing needs. Also, it wets the plant foliage constantly, which can cause fungal diseases.
- A drip system is good for individual pots, trays, and even hanging baskets.
- A seep hose for plants in borders is more economical on water and does not create humidity.
Good ventilation is essential for keeping the temperature in the greenhouse down. Most plants that will grow in anprefer temperatures below about 28°C, although short periods of higher temperatures will not harm them. Fungal diseases are encouraged by stale air, and ventilation will help prevent their spread Whenever the weather permits, open the greenhouse vents a little for at least part of the day, even in winter when cooling is not necessary.
Greenhouse Ventilation Equipment
Louvre vents, which most greenhouse manufacturers will supply as an extra, can replace any of the lower panes of glass and allow good air circulation. Manyon the market have too few vents and rely upon the door to provide ventilation.
- An automatic opener on a roof vent is a worthwhile investment for the summer, especially if you are away from home during the day. These respond quickly to temperature changes, and are easy to buy and fit. Make sure that the type you buy will open the vent wide and can be adjusted to start opening over a range of temperatures. Most will not open at low enough temperatures to allow ventilation in winter; instead, you have to put back the manual catch or use the other vents.
In summer, despite good ventilation, too much sun can overheat a greenhouse, and this is when shading is necessary. Give priority to shading the south and west sides of the greenhouse.
- The outside of the greenhouse can be coated with a white shading paint. Traditionally, this was made from a stiff mixture of garden lime and water. Modern alternatives use chemical compounds, such as titanium dioxide, which stick well to the glass in rain yet wipe off easily with a dry rag. Shading paints are cheap, easy to apply, and effective. However, once they are on they stay on all season, so wait until light levels are generally high (around late spring) before applying them and wipe them off in early autumn.
- An alternative is to fix shading material on to the outside of the greenhouse for a similar time (shading on the inside, although cutting out the sunlight and preventing scorching, does little to reduce the temperature). Exterior blinds, which you can roll up on dull days to let in more light, are ideal but expensive.
In winter and early spring, plants need all the light that they can get, but you can provide temporary shade for seedlings and vulnerable plants by covering them with white paper, an old net curtain, or horticultural fleece.
Read more on Greenhouse heating and insulation