It’s a big investment, but you’ll reap the rewards if you sow and grow under glass, says our gardening expert
Every gardener dreams of one day owning a greenhouse. It is an inevitable path that starts with a bit of fleece and the miracles of growth that happen with just a few extra degrees of heat, and ends in lusting after luxury bespoke greenhouses online and fantasising about pineapples. Growing under protection changes the game: seedlings grow strong and robust in a way that never happens on a windowsill. You can extend the season and ensure heat-loving plants still bake in less-than-perfect weather.
For me, this has been a fantasy for years. I have built lean-to structures, fashioned from skip finds, old windows and used fish tanks, but they have all fallen apart. This year, as I trawled through online sales of fancy glasshouses, I did a cost-per-wear analysis. I realise that such metrics are usually reserved for handbags and expensive jackets, but my world is floated by happy green things and my windowsills are starting to buckle and warp from years of seedlings grown on them. If I bought this handsome, tall cold frame, it would cost just under £20 a week. Put another way, every tomato I eat this year will cost me a pound – but oh, how wonderful they will taste. A single plant can produce up to 200 fruits, so from five plants I could harvest 1,000 tomatoes.
Tall cold frames are tiny lean-to greenhouses, perfect for patios and small urban gardens. They take up 2-3ft of width and can sit happily against a house or along a fence, or make good use of the side return. Any lean-to greenhouse or cold frame needs to face west. A south-facing wall might be toasty in winter, but come summer it will fry everything inside, even with shading. The area must be flat and not exposed, particularly to cold winds. Even a relatively gentle breeze of 15mph will double the amount of heat drawn from inside the greenhouse/cold frame.
When buying one, look for a design with plenty of ventilation. Once the sun shines, the inside warms up quickly, the humidity changes and you get stale air – all of which is a recipe for breeding pests and diseases.
Ideally, you want roof and side ventilation, which creates a chimney effect: cool air is introduced from the side vents and hot air is expelled from the roof (which will be important come summer).
Finally, you’ll need shelving – the best sort is flexible, allowing you many shelves in spring for seedlings, but fewer in summer for large pot plants.