Since the 17th century, when the first glass structures were built to grow the newly arrived oranges and other exotic fruits, greenhouses have, for many of us, served as secret gardens-within-the garden. Half an hour’s pottering in the greenhouse on a winter’s day can be as relaxing as soaking in a hot bath. And nothing soothes away the stresses of a hard day at work like going to the greenhouse on a June evening to pick the first tomatoes.
All work in a greenhouse is relaxing and easy: there’s no heavy digging to do, no weeds to battle with and no howling wind to freeze you. A greenhouse can be used in many ways: to propagate plants for the garden; to extend the season for growing fruit and vegetables; to grow exotic plants; or to protect plants that would not survive the winter outdoors. Even if you have a small greenhouse, or are new to greenhouse gardening, you can use it for all four of these things.
You don’t need complicated equipment. Artificial heat is not necessary for a useful and productive greenhouse. Even unheated greenhouses will keep plants protected from the worst of the weather and during the winter will be a few degrees warmer than outside. But adding a small heater that keeps the winter temperature a few degrees above freezing will dramatically increase the work you can do in the greenhouse and the types of plants you can grow.
Gazing into gardens on even a short train journey you can see that many greenhouses have been abandoned. They have become dank algae-covered stores for garden furniture, the paddling pool and broken bicycles or a dumping ground for old pots, used compost and dead plants. If this sounds like your greenhouse, clear it out and clean it up. With very little effort it can become an enjoyable, relaxing place in which to garden.
The next two months are usually the coldest, so making the greenhouse as snug as possible becomes a priority.
- If you’ve got a tap in the greenhouse make sure that it and the pipes leading to it are properly lagged.
- Snow on the roof gives a warm, comforting feeling inside the greenhouse but if it is too thick it may break the glass. Push it off gently with a sweeping brush.
- When very cold weather is predicted cover plants with horticultural fleece, but remove it the following morning. If you have a heater or heated propagator make sure that they are working, switched on and set correctly
- In a heated propagator make first sowings of French beans, peas, and carrots.
- Continue sowing herbaceous perennials.
- Pot up dahlia tubers to encourage new shoots that can be used as cuttings.
- Sow tomatoes, aubergines and peppers
- When forced bulbs have finished flowering, plant them out in the garden to make room for
- Temperatures and light levels start to rise during March. Keep the greenhouse well-ventilated and on very sunny days place netting over seedlings and cuttings to shade them from the heat of the sun.
- With warmth and light come pests. Aphids can be rubbed off and caterpillars picked off. Place yellow sticky cards near infected plants to attract and kill whitefly.
- Sow tomato seeds, early peas, French beans, broad beans and carrots to produce early crops. Sow melon seeds
- Sow seed of annuals such as cosmos, antirrhinums, nicotiana and marigolds.
- Prick out seedlings that you sowed earlier in the year.
- Pot up sweet peas and any other plants that were sown last autumn.
- If tender plants such as fucshia, pelargoniums and diascias start growing, use the new growth as cuttings.
- Begin watering and feeding the Agapanthus, cannas and other tender plants that are over-wintering under the benches.
- If the weather is good you will need to start shading the greenhouse. Paint white shade paint onto the outside of the glass. Just do the sunny side of the house initially and finish the job next month.
- Continue pricking-out and potting-on seedlings and young plants.
- If there is room to grow tomatoes in the greenhouse plant them now in large pots or grow-bags
- Plant up hanging baskets and hang them from the eves to grow on.
Temperatures rise rapidly in May and for the next four months keeping the greenhouse cool, shaded and well ventilated is important.
- This is the best time for the satisfying job of giving the greenhouse a thorough clean. If it’s not done every year the place will become an unhealthy environment for plants. Clean it on a bright sunny day because you inevitably end up getting wet. Empty the greenhouse, standing pots and trays of plants in a shady corner of the garden. Start by clearing away any moss that has started to appear around the glazing bars or in the eves and clean away any algae from the frame. If algae has started to grow in the overlap of glass panes you can wipe it away by gently sliding an old palette knife between the panes. Wash the inside, frame and glass, with warm soapy. You can also add a special greenhouse disinfectant to the water. Insects, slugs and little snails love to snuggle into aluminium frames so use a soft scrubbing brush to flush them out. It can be a messy job but will help keep the greenhouse free of pests and diseases. Scrub down benches, shelves and staging as thoroughly as you have the frame and glass. Once the inside has been cleaned and swept open the doors and vents to help it dry out.
- If you have painted shading solution on the outside of the glass, now is the time to wash it off. If you use a high-pressure washer don’t be too gung-ho with it: up close the pressure of the water can crack the glass. Check that the guttering is not blocked by leaves and debris.
- Now that the greenhouse is clean you don’t want to bring in any pests and diseases that may be clinging to pots and trays. As you put plants back into the greenhouse, wipe the pots and remove any slugs or snails that might be lurking underneath them. Look over the plants and take off any dead leaves. Empty pots and trays need to be washed before you put them back into the greenhouse. If you haven’t got time to clean them now, leave them outside and do it another day.
- Pot up bulbs to flower during the winter, either for taking into the house for growing in the greenhouse. As well as specially-prepared Paperwhite narcissus plant miniature daffodils, dwarf iris, muscari, snowdrops and crocus and Lily of the Valley. Stagger the potting over several weeks so that they don’t all flower at once.
- Take cuttings of tender perennials such as Salvias, Osteospermum to ensure that you will have plants next year if the plant in the garden dies during the winter.
In most parts of Britain the first frosts arrive during October and tender plants need to brought into the greenhouse.
- The foliage of Agapanthus, Cannas, and other bulbs should be cleaned up and their pots stored underneath the benches. Keep the compost very slightly moist during the winter.
- Cactus and succulents such as Agaves need light during the winter so give them space on the top of a bench.
- Cut back pelargoniums, tender perennials and fuchsias by a about half before storing them on the top of the benches .
- Bring pots of herbs such as chives, thyme and parsley into the greenhouse to use during the winter.
- If the weather is dry and sunny, open the greenhouse to ventilate it. Keep an eye out for dying leaves and remove them. In cold greenhouses during the winter plants can develop botrytis and other moulds. If leaves develop grey sooty mould, take them off and throw them away (outside the greenhouse).
- Take root cuttings of herbaceous perennials such as oriental poppies and Verbascums.
- Sow seeds of sweet peas and violas. Start sowing herbaceous perennials.
- Line the greenhouse with bubble wrap to insulate against the coming cold weather. Aluminium frames usually have slots in the glazing bars for plastic plugs that will hold the bubble wrap. For wood-framed greenhouses use specially-designed pins
- The first of the bulbs you potted in September will be flowering now. Enjoy them