What to grow in a greenhouse in winter If you feel less bold, check the classifieds in your local paper. You might chance upon a "free to dismantler" or pick up a bargain for a fraction of the price of a new one. Or spread your net by signing up for your local Freecycle or Freegle group. The best bit about acquiring a preloved, second-hand greenhouse isn't just the bargain element. By reusing it, you're reinvigorating all the energy used to make it in the first place. And if you're concerned about locating spares and doing repairs, an online search for "greenhouse spares" reveals a cornucopia of suppliers for every bit and bob imaginable. What, no propagator? When I gave up heating my greenhouse I retired my electric propagator, too. Many seeds germinate quite readily, albeit more slowly, in the fluctuating temperatures of a sun-powered greenhouse. My "propagator" these days is a collection of clean, inside-out plastic compost bags. To turn it on, I place pots and trays inside a bag and put them on a sunlit piece of staging. If you sow a lot of seeds, split the bags open and lay them over the pots, black side up (they also prevent compost from drying out too quickly). Seeds that need a warm and constant temperature can come indoors for a bit of pampering. A windowsill in your most inhabited room or your boiler/airing cupboard is ideal (evict those towels). Check regularly for signs of life then get them into the light straight away. Freeze and thaw To guarantee fresh pickings in deep midwinter, you need to stay ahead of the frost game. Once a spell of prolonged freezing weather sets in, and crops have turned dark, sagged and the compost/soil is frozen solid, it's impossible to harvest them until they thaw (they'll snap if you try). If falling temperatures are forecast, cut some of what you want and store it in the fridge, or move pots of winter salad leaves to a frost-free place indoors. Covering crops with multiple layers of horticultural fleece (two or three at least) will give extra frost protection, but remove it straightaway once a thaw sets in. During milder spells, harvest at will, but keep the vents open and the air moving to deter fungal diseases like grey mould (botrytis). Keep crops on the dry side and only water, sparingly, when they just begin to wilt. Late winter isn't just for gazing at seed and plant catalogues. The following are best sown in pots or trays and, unless the weather's mild, brought indoors to germinate. There's no need to thoroughly soak the compost after sowing (which we tend to do for spring and summer sowings), just moisten it lightly. Once germinated, move seeds back to the greenhouse, only bringing them indoors on very cold nights. Don't expect them to romp away as they would in spring; growth, depending on the weather, will be slow but steady. Salad leaves I aim to sow at least one 12cm (5in) pot of salad leaves every week and get two or maybe three cuts per pot. Right now, the hardier oriental brassicas, green in the snow, komatsuna, mizuna and red mustard can be germinated indoors and will be ready, depending on the weather, in around six weeks. Mangetout peas 'Oregon Sugar Pod" stands low temperatures better than others. Sow four seeds to a 10cm (4in) pot. Pot on into 30cm (12in) pots when around 15cm (6in) tall and support with 1.2m (4ft) long twiggy sticks, or plant in soil beds. Sweet peas I never get around to sowing in October, but I've often caught up by sowing in November, December or January/February. You'll still get flowers weeks ahead of spring-sown plants. Broad beans Sowing now produces young plants for early spring, planting either in beds (near the door so bees will find and pollinate them) or outdoors. Sow one seed to an 8cm (3in) pot. Aquadulce Claudia" is reliable. Early-pull carrots 'Amsterdam Forcing" and 'Early Nantes" do well from late-winter sowings in a 30cm (12in) pot. By spring you'll have tender young roots to pull. Winter Bring pot-grown strawberries into the greenhouse in January (they flower and fruit best after being frosted). Remove dead leaves and repot in fresh peat-free compost. You are their pollinator: dust pollen between the flowers with a small brush, then enjoy the rewards from early May. Spring It's all systems sow as your greenhouse captures increasing amounts of sunshine. Slugs will thrive in ever-wetter summers, so start plants in pots or multi-celled trays to give them a head start when planted outdoors, aided by some organic slug pellets. Summer Make the most of warmth-loving crops. Indoor tomatoes, peppers, aubergines, cucumbers and melons all need the raised temperatures of a greenhouse to succeed. Autumn Use the warm, rain-free space of your greenhouse to store/ripen onions. Lay trays in full sun so skins bake and ripen fully. Ditto with pumpkins and winter squash. John Walker won the Garden Media Guild Environmental Award 2012. Visit his website Earth-friendly Gardener. 'How to Create an Eco Garden: The Practical Guide to Greener, Planet-friendly Gardening', by John Walker (Aquamarine, £14.99), is available from Telegraph Books (0844 871 1514) at £12.99 + £1.35 p&p. Helen Yemm returns with Thorny Problems on March 9