What you do regarding the cultivation of individual plants will largely depend on type and variety, but there are general rules of maintenance which will apply in most cases. If the greenhouse is constructed of timber, make sure it is regularly treated or painted.
Always keep the glass clean outside and in, especially in winter when light is at a premium. Dirty glass can considerably reduce the amount of light reaching the plants, particularly if it is dirty on both sides. Thorough washing with detergent and water, to which has been added some weak bleach, is usually adequate, though greenhouse glass cleaners containing a disinfectant and algicide and cleaning agents to make the glass sparkle are available. Unlike bleach and many disinfectants, they can be used without taking the plants out of the greenhouse as they are harmless and can be hosed off if necessary rather than being removed with a cloth or sponge. Do not forget to get as much of the algicide as you can down the cracks where the panes of glass overlap each other, or the glazing bars.
Keep your eye on the thermometer, especially in very cold or very hot periods. Forgetting to use shading or to increase the ventilation in summer, or to close the vents and turn up the heating in winter, can be fatal to plants.
One way of making conditions more tolerable in sunny summer weather is to ‘damp down’ the floor and staging with water during the day when the heat builds up. Do not do this during the evening as it can encourage moulds, rotting and other diseases when the temperature drops.
If you do not have any automatic watering aids, keep a watchful eye on the watering. In winter, spring and autumn you will have to observe your plants carefully to see if and when water is needed, but in summer, especially if you are growing plants in pots and bags, you may have to water several times a day. It is best not to water late in the day when the greenhouse is cooling off.
At least once a year, try to give your greenhouse a thorough clean-out. Remove the plants and all debris, and scrub down the benches, staging, timbers, glazing bars, structural members, the floor, and anything else you can lay your hands on, with a disinfectant. At the same time you can wash the glass thoroughly and paint or treat anything that needs it. Late summer is the best time, especially if you have to move out tender plants, unless you have tomatoes or other crops which are not movable, when you should do the job in autumn or spring.
In order to reduce pests and diseases hiding in the nooks and crannies, as you do this you can treat the greenhouse with pest and disease smoke cones to fumigate it. It is especially important, however, to ensure that any chemicals used are safe in the confined atmosphere of the greenhouse.
Inspect the greenhouse regularly, but more often in winter, and remove all dead bits that have dropped off on to the pot tops, staging, floor, etc., before they can start to rot and start diseases. Check plants over as a matter of routine as well and take off all dead parts, dying leaves, spent flowers and the like, for the same reason. You will also notice any signs of pests and diseases, which can be dealt with accordingly.