Greenhouse Heating Levels
In the early spring when it can be quite cold during the night it is important to water plants sparingly, and for preference to do any watering that is necessary early in the day as it will be harmful to plants to lie wet and cold overnight. The temperature of the cold house can be lifted a few degrees by lining the entire inside of the house with thin gauge clear polythene, but condensation dripping from the polythene can be harmful to any plants that may be in flower. Alternatively, if it is a small house you could put the thing to bed on very cold nights by covering the entire greenhouse in a polythene wrap.
Daft you may think, but this we did with a low, 60ft (18m) house in the winter of ‘63/64 and raised the temperature inside the house by 10°F (6°C), which was enough to save the plants. Climbing onto the ridge to do the job is not recommended, but it should be little trouble to envelope an average size house in a polythene shroud, and save the plants. This is a tip that may also help with the warm greenhouse when the heating plays up and plants are in peril. From thethe next most natural step is to the cool house which maintains a minimum temperature of not less than 40°F (4°C) — this, however, does not mean that the house is permanently at 400, it will be somewhat higher most of the time and at this level when the temperature is low outside.
When you maintain this sort of house you can really begin to feel that you are in the business of growing more interesting plants than your neighbour can manage on her window sill. It is a kind of refuge on colder days, and a house where endless hours will be spent attending to all manner of interesting plants — yet you will never reach the point where you can stand back and feel that there is nothing more to be done.
You can now grow all sorts of interesting and colourful plants, particularly in the flowering line — zonal pelargoniums, primulas, betterthan in a coldhouse, cinerarias, perpetual flowering carnations, and you could have a go with spring sown cyclamen that will flower in about eight or nine months and be ready for early December. With the cool house you have to be especially careful with watering during the winter and early spring when temperatures drop and, because of heat loss there is a reluctance to air the house.
Dank conditions that are a perfect breeding ground for fungus troubles will be the result, and these conditions can create havoc with all manner of plants. Dank conditions also result where plants are crowded too close together preventing free circulation of air. Plants that are packed in very tightly may seem to give more value to your efforts as there are so many more plants, but this is a lot of baloney. It is far better to grow one good plant than it is to grow half a dozen in the same space if few or none of them will be any good. Each plant should have its own space on the staging, and not be crushed up against its neighbours so that they all deteriorate — if the plant is getting too large for its allotted space then it is time to get rid of it, give it to the man down the road with a new greenhouse to fill(!), or sell it to the local flower shop. Alternatively, you could make a wire hanger to fix around the rim of the pot and hang it above your head where there is usually more than enough room to spare; remember the point made earlier about having a greenhouse with reasonable height!
The warm greenhouse is where it all happens, where you can grow almost anything in addition to the cool house subjects if you have a reasonable amount of skill to help the plants along the sometimes rough road to maturity. The minimum temperature here is in the region of 55°F (12°C) and, depending on the heating system you have installed it may be getting on for double the cost to maintain this temperature compared to the cool greenhouse. How do you overcome this — perhaps you could stop smoking, or persuade yourself that it is no more costly than your neighbour’s tobacco bill. Or you could follow my example of earlier years with my first greenhouse and sell some of the plants when they are mature. A bit mercenary, you may feel, to try to make money from what is ostensibly your hobby. Not a bit of it, the best thing for any greenhouse crop regardless of where it may have been grown is to get rid of the plants when they are at their best, either by selling them or donating them to the many worthy causes that exist in every locality.