When it comes to greenhouses, you should buy the largest you can afford - you can never have too much space
"I'm thinking of buying a greenhouse. Can you give me any advice?" asks the occasional reader. Oh yes, I certainly can. The most important piece of counsel I can offer to anyone contemplating a greenhouse is to buy the biggest one you can afford - without it taking over your garden. Now, I'm not normally a fan of "big", but when it comes to somewhere for growing your plants under cover, you can never have too much space. There are just so many things that will benefit from the extra protection - and an awful lot of them you can eat.
At this time of the year, there are early salads, overwintered from last autumn, or sown just a few weeks ago: rocket (both wild and tame), oriental leaves, lettuce, land cress, radish, spinach, chard, spring onions. The extra heat trapped by the glass makes them shoot into activity, producing fresh growth that is bouncing with goodness. Then in a month or two, early beetroots, carrots, French beans, potatoes and strawberries are ready for harvest. And, come summertime, the greenhouse is the land of plenty, bursting its seams with courgettes, cucumbers, sweet peppers, chillies, aubergines, melons and tomatoes.
Tomatoes. If I could grow only one crop in the world it would be tomatoes. There is no taste as sweet, tangy and fragrant as a home-grown tomato, picked and eaten while still full of the sun's warmth. Just thinking about it makes me want to go and marvel at the baby plants, no bigger than my hand, that in a few months' time will become metres-long vines heavy with shining fruit: red, orange, yellow, purple and green.
There are hundreds of varieties of tomatoes, and it is my great regret that I have room for only five or six kinds each year. To be sure, you can grow them outdoors, in a sunny and sheltered position, but you won't get the depth of flavour as when they are grown in a glasshouse or polytunnel.
I like to eat most of what I grow in our greenhouse, but there is room also for a few inedibles: pelargoniums (tender geraniums), fuchsia and other pretties - and perhaps sometime in the future for exotics such as bougainvillea, plumbago and passionflower. Our little glass structure, no bigger than a single bedroom, is also a propagation factory, yielding scores of plants grown from seed and cuttings, for use in our garden, in the gardens of friends, or for plant sales. I can't imagine life without it - especially since it provides a warm and comfortable refuge on chilly days.
A greenhouse will set you back anything from a few hundred to several thousand euro. Polytunnels are considerably cheaper by volume, but may be less stable, are prone to ripping (or being clawed by cats), and the polythene covering needs to be replaced every few years. Nonetheless, they provide a vast amount of shelter at relatively little cost. If I had a larger garden, I'd have my own polytunnel, and I'd grow tomatoes in abundance - and much else besides - to my heart's content.
If you have a south- or a west-facing wall, you can have a lean-to glasshouse, with the warm wall providing a place to train an apricot, peach or fig, or an ornamental climber. If the structure is free-standing, give it a spot shielded from the wind, and away from overhanging branches. To make the most of the sun's rays, especially in winter, site it with the roof ridge running east to west; this allows the interior to get the greatest possible amount of heat and light from the south.
In our climate, there is no need for heating in a greenhouse, unless you are planning on growing seriously tropical plants. But, if you have the opportunity to put in electricity, do, so that you have the option of running a heated propagator (for certain spring seedlings and cuttings) and lighting. Needless to say, a qualified electrician should do the job, or there could be interesting results when you add that essential ingredient for plants growing under cover: water.
At the very least, you'll need a long hose to bring water to your greenhouse. In the heat of summer, it may need watering morning and night, and possibly a "damping down" in the middle of the day in hot weather. This latter operation just means whooshing the hose over the floor, and letting the evaporating moisture cool the air. If you plan on taking holidays during the growing season, and can't call in a favour from a friend or neighbour, then an automatic irrigation system will do the watering for you.
But don't be tempted to rely on such a system for everyday watering, and to leave the plants to get on with their business unattended. Your cosy, glassed-in space isn't just a haven for green things, it's also a paradise for pests - who will be getting on with their own business. And that includes eating your vegetation and reproducing with cheerful abandon. If you're watering daily, you'll be naturally vigilant, keeping an eye out for any interlopers, and seeing them off the premises fast.
Slugs and snails can be hand-picked at night, or lured to an alcoholic death in containers of cheap beer partially sunk into the soil. Aphids can be hosed off plants, or again, removed by hand. Whiteflies, which look like minuscule moths, congregate on the undersides of leaves and float into the air when disturbed. French marigolds (Tagetes) deter them, but you need a couple of plants per square metre of soil. The parasitic wasp, Encarsia formosa, can be introduced as a biological control. Red spider mite, which produces a mottled appearance on leaves, thrives in dry conditions, but regular damping down and misting keeps it in check.
That roll call of pests might sound as if one's greenhouse is under constant attack, but in fact, very little nipping in the bud is needed to keep the whole thing rolling along sweetly. Your greatest task will be a thorough, yearly clean of the glass, and all the surfaces - to remove algae, and any lurking pests.
My final word is this: ventilation. Most greenhouses are sold with too few opening windows. You'll have to pay extra, but make sure there are enough openable windows in both the roof and the sides, to allow cool air to flow in from below and hot air to exit.
After that, it only remains to be said: if you're thinking of buying a greenhouse, what's keeping you?