Greenhouse Growing
Greenhouse growing is a daily commitment and you need to be prepared to check the conditions inside at least twice a day. A good routine is to check in the morning before work and then again at the end of the day. It’s important to keep a constant temperature, good ventilation and a reasonable level of humidity. All is not lost however if disaster does strike - there will still be time to in late Spring, as and seeds will germinate and grow quickly ready for a good show in the summer in the warmer weather.
The Greenhouse Effect
As the sun's rays enter a greenhouse, their energy heats up the air, as well as the soil or floor inside. At night, warmth radiates back from the soil or floor and is trapped by the glass. By contrast, a polythene tunnel is much less effective at transmitting the sun's rays or retaining re-radiated warmth. The downside of glass is that it can intensify the sun's rays, which is why it is important to provide protective shading.
A Breath of Fresh Air
Good ventilation is one of the essential parts of good greenhouse management. Without it, temperatures can rise to dangerous levels (easily 50"C/120'F). This will stress your plants, which can check their growth or easily kill them. Overheating can also cause compost to dry out or, if you have watered too much, it can lead to a build-up of mildew and bacteria. Make sure that the vents are open every morning during late spring and early summer on warm days, and then close them again at sundown. Shade netting on the roof has saved me from certain disaster. Think about which vents to open to encourage circulation, rather than just creating a draft, which will cause havoc. Don't open vents on the windy side of the greenhouse.
The Greenhouse Effect
As the sun's rays enter a greenhouse, their energy heats up the air, as well as the soil or floor inside. At night, warmth radiates back from the soil or floor and is trapped by the glass. By contrast, a polythene tunnel is much less effective at transmitting the sun's rays or retaining re-radiated warmth. The downside of glass is that it can intensify the sun's rays, which is why it is important to provide protective shading.
A Breath of Fresh Air
Good ventilation is one of the essential parts of good greenhouse management. Without it, temperatures can rise to dangerous levels (easily 50"C/120'F). This will stress your plants, which can check their growth or easily kill them. Overheating can also cause compost to dry out or, if you have watered too much, it can lead to a build-up of mildew and bacteria. Make sure that the vents are open every morning during late spring and early summer on warm days, and then close them again at sundown. Shade netting on the roof has saved me from certain disaster. Think about which vents to open to encourage circulation, rather than just creating a draft, which will cause havoc. Don't open vents on the windy side of the greenhouse.
Watering
You wouldn't feed or water a child irregularly, so why do it to a plant? Greenhouse plants are in a much more vulnerable position than outdoor plants as they rely on you to control their environment. Without regular watering, plants begin to behave in odd ways: leaves can wilt, fruit may split and pests are attracted to a weakened plant. Conversely, too much or irregular watering can lead to blossom end rot, wilting and mildew. Consistency is the key here.
- Water plants in the morning before the sun is at its harshest - this also stops the greenhouse becoming too damp overnight.
- Keep pots with similar water requirements in trays - it's much easier to water a whole bunch in one go.
- Use a surface mulch of fine gravel on larger pots.
- To see if a plant needs water, lift the pot. It's weight will quickly tell you just how dry it is.
- Water thoroughly rather than in small amounts, to encourage roots to grow down into the soil.
- Ask a friend to water if you are going away. Otherwise, place your plants on capillary matting with a feeder bucket of water.
Greenhouse Diseases
Damping off in seedlings is caused by the parasitic organisms Pithium and Phytophthora. Don't overwater and use vermiculite when sowing to help prevent it from developing.
Mildew spreads by spores - open side vents, top windows and the greenhouse door, when weather permits, as good air circulation stops spores from building up. Other ways to improve air circulation are to pick off lower leaves on tomato plants and avoid overcrowding plants on staging and below benches.
Viruses can often be introduced by plants brought into the greenhouse, or by sap-sucking pests. lnspect all new plants thoroughly, and remove any plants affected by viruses. Clean hands and tools with soapy water after handling infected plants. Use fresh compost and clean containers.
Mildew spreads by spores - open side vents, top windows and the greenhouse door, when weather permits, as good air circulation stops spores from building up. Other ways to improve air circulation are to pick off lower leaves on tomato plants and avoid overcrowding plants on staging and below benches.
Viruses can often be introduced by plants brought into the greenhouse, or by sap-sucking pests. lnspect all new plants thoroughly, and remove any plants affected by viruses. Clean hands and tools with soapy water after handling infected plants. Use fresh compost and clean containers.
Greenhouse Pests
Red Spider Mite
Check the underside of leaves for these tiny mites and their eggs. They suck sap from the leaves, leaving a fine pale mottling. Particularly common when the air is dry, so regularly damp down the greenhouse. Spray with an appropriate chemical or introduce predatory Phytoselulus mites.
Whitefly
An infestation is hard to control so be vigilant. Check the underside of leaves for tiny pinpricksized spots or white insects. Use an insecticide spray or try introducing parasitic Encarsia wasps. Let carnivorous plants do some of the work: try butterworts (pinguicula) or sundews (Drosera) which attract whitefly to their sticky leaves.
Greenfly
Small numbers may be picked off by hand, otherwise use an organic spray (most contain fatty acids and work on contact with the insect). Conventional pesticides are also effective, as is the parasitic wasp Aphldius or midge Aphidoletes.
Mealybug
These small insects also suck sap from leaves and weaken them, leaving sticky 'honeydew' that encourages the growth of sooty mold. You may notice a powdery white residue on hidden parts of the plant or small brownish eggs. Use conventional pesticides, organic fatty acid sprays or the Cryptolaemus ladybird to control them.
Scale Insects
Look out for small round bumps or lumps on the underside of leaves - these are the outer shell of this sap sucking insect. Treatment is more effective when the insects are smaller. Use pesticides or organic sprays, or try a predatory Metaphycus wasp.
Check the underside of leaves for these tiny mites and their eggs. They suck sap from the leaves, leaving a fine pale mottling. Particularly common when the air is dry, so regularly damp down the greenhouse. Spray with an appropriate chemical or introduce predatory Phytoselulus mites.
Whitefly
An infestation is hard to control so be vigilant. Check the underside of leaves for tiny pinpricksized spots or white insects. Use an insecticide spray or try introducing parasitic Encarsia wasps. Let carnivorous plants do some of the work: try butterworts (pinguicula) or sundews (Drosera) which attract whitefly to their sticky leaves.
Greenfly
Small numbers may be picked off by hand, otherwise use an organic spray (most contain fatty acids and work on contact with the insect). Conventional pesticides are also effective, as is the parasitic wasp Aphldius or midge Aphidoletes.
Mealybug
These small insects also suck sap from leaves and weaken them, leaving sticky 'honeydew' that encourages the growth of sooty mold. You may notice a powdery white residue on hidden parts of the plant or small brownish eggs. Use conventional pesticides, organic fatty acid sprays or the Cryptolaemus ladybird to control them.
Scale Insects
Look out for small round bumps or lumps on the underside of leaves - these are the outer shell of this sap sucking insect. Treatment is more effective when the insects are smaller. Use pesticides or organic sprays, or try a predatory Metaphycus wasp.