When I was a child, my parents had a greenhouse. They used it for sowing and germinating seeds, and also grew lots of plants in it, including tomatoes, cucumbers and various exotics, but all in pots and grow bags. So I grew up thinking that greenhouse gardening was all about pots.
Then I discovered the joys of planting direct into soil under glass. This has huge advantages over growing in pots, not least that your plants don’t dry out quite so quickly if you forget to water them for a day or so. They also have much greater reserves of nutrients to draw on, because the roots can go deeper.
But there are still difficulties. The old systems of crop rotation were designed to ensure that each field had a chance to recover from each crop. Crop rotation also minimises the risks of diseases, as most won’t last the four years or so of the rotation. But you can’t rotate crops in a greenhouse, or not much. Assuming that you want to grow more or less the same things each year – tomatoes, cucumbers, peppers – the space soon runs out. What’s more, pests and diseases do tend to thrive in the warmth and moisture of the average greenhouse. And if, like me, you overwinter your greenhouse above freezing to protect tender plants, diseases won’t be killed off by winter cold, either.
So you have got several problems: the risk of disease that won’t be killed off in the winter, potential pests, and soil that may get a bit bare of nutrients, especially after a year or two of growing the same type of plants. But don’t despair, there are solutions to all of these.
Pests and diseases
One way to manage the pest and disease problem is to make sure that you wash and sterilise your greenhouse each year. It’s a good idea to apply a summer shade wash anyway, and you will need to wash this off for the winter, so that s a good excuse to wash inside and out with disinfectant. Light a sulphur candle to disinfect thoroughly. You will need to stay out for 12 hours, and it is best not to leave any plants in there, so pick a time when your tender plants can safely be left outside, or under cover somewhere else, and before you have started sowing seeds.
You can also minimise the risk of pests during the growing season by covering the door with a net curtain or similar to keep out butterflies (and hence caterpillars). Damping down, spraying water around in there during hot periods, will deter red spider mites. Also make sure that your greenhouse is well-ventilated to avoid problems with mould.
Spent soil is not an issue when growing in pots: you just empty them out at the end of the growing season and start again with new soil the next year. But you can’t do that with a greenhouse.
One way is to use grow bags to supplement the soil, either with holes cut in the bags for roots to go through, or just as they are. But somehow, for me that defeats the point of growing direct in the greenhouse soil. Instead, I buy compost or soil improver, and add it to the soil in the greenhouse. If you are worried about the level getting too high, you can dig out the top layer of soil and discard it, replacing with the compost. Suitable composts for this purpose include mushroom compost and mushroom and manure compost, both of which improve the structure of soils, and also add nutrients. Because both contain plenty of organic matter, they will also improve water retention if you keep forgetting to water. You can buy either of these from compost suppliers.