Cold Comfort
Jim Cable explores the advantages of the versatile coldframe and how it assists with both protection and propagation
Some autumn days are benign, and we can still enjoy the warmth of the weakening sun. But night frosts threaten, it can be windy and, on some days the rain lashes down. With winter just around the corner, we look to different ways of keeping cosy. Some of our plants need protection from the elements too, and a coldframe can be the simplest space-saving solution.
These low-level miniature greenhouses might have glazed, wooden or brick sides, but all are united in having a clear sloping cover that may be hinged or entirely removable. In sunshine the temperature inside a coldframe is typically 5-10°C above ambient. Plants nestled within are sheltered from wind and their watering can be controlled.
There is nothing new about growers protecting their plants in an attempt to cheat nature. The Romans made their coldframes using straw or stone walls topped with sheets of mica to let in light.
They were also the first to use glass for windows – albeit translucent rather than clear. Manufacturing methods slowly improved over the centuries, and by the early 17th century glass windows were common in England and the first greenhouses were being constructed. Glass manufacturing and construction techniques developed further during the 18th century, which led, in turn, to the glass and iron palaces of the Victorian era.
Cast iron frames then became available to Victorian gardeners of modest means, allowing them to emulate the grand estates and grow such exotics as cucumbers and melons and give Mediterranean vegetables a protected start in life. Mrs Beeton, perhaps more famous as a cookery writer, also wrote extensively about garden management during the 1860s. She gives detailed advice on choosing or constructing a garden frame, ‘a structure moveable at pleasure… for raising and protecting early crops
in a warm aspect in the spring, and for obtaining and saving lettuces, etc., throughout the winter’.
If you have been growing salad leaves and leafy herbs such as coriander and mint in pots on the patio, moving them into the shelter of a coldframe now will keep them cropping for a few weeks more. The first week of November (two weeks earlier in the north) is your last chance to start off winterhardy salad subjects such as claytonia, pak choi and mustard. Sow into modules of peat-free compost, water and place in a coldframe or, for speedier germination, on a windowsill indoors. You can then move them into the coldframe as young seedlings. It is important that the frame occupies the sunniest space you can give it – south-facing and as far from tall trees and buildings as you can manage. Shadows are long at this time of year and, of course, days are short. Light is at a premium and plant growth will be slow, so while you should not expect a large harvest it is satisfying to pick a few spicy or crunchy leaves to pep up a winter salad.
Coldframes provide the ideal environment for overwintering small stock plants, whether these are rooted cuttings or specimens bought at a show, nursery or garden open day. If you intend to overhaul a border during the winter months, these plants will be happy waiting in the wings in a coldframe until their permanent quarters are ready. Young plants are more vulnerable to waterlogging and cold, and even perfectly hardy species can suffer in an open bed over winter. If they are borderline hardy, such as the ‘Wish’ series of salvia or French lavenders, then some protection is essential.
Now is the ideal time to plant spring-flowering bulbs. If you put some in pots either in addition to flower beds or as your sole display, sheltering some inside a coldframe will help create a staggered display. The protected bulbs will get ahead in the
New Year and you can transfer them to create a focal point outdoors just before they flower. If you live in a high rainfall area, bulbs in pots can sometimes rot. In this case, placing them all under glass where you can control their watering makes sense. It will also help deter marauding squirrels.
A coldframe is ideal for raising alpine subjects such as Cyclamen graecum or ipheion species since it protects them from excessive wet and snow.
The ideal scenario is to grow alpines in pots plunged into fine-grade gravel in the base of the frame. This moderates both temperature and available moisture.
Commercial coldframes come in different heights. Consider the overall size of the plants – and their containers – that you want to protect. Materials vary, too. Most coldframes are light enough to be moved around your growing space, which can be useful, but beware of flimsy construction that will easily succumb to wind damage. Heavier frames are advisable for windy gardens, and aluminium and toughened glass models are strongest. Cheaper designs use wood and polycarbonate sheeting and, while not as sturdy, can still give several years of good service.
A north frame is a particular type of coldframe placed on the soil to the north of a building or other structure so that it is entirely shaded from the sun. While light is allowed to penetrate, the absence of direct sunlight and the enclosed nature of the frame keeps temperature fluctuations to a minimum.
This and high humidity inside the frame provide
the ideal conditions for rooting cuttings inserted directly into the soil. Semi-ripe cuttings of broadleaved evergreens such as box or euonymus can be propagated in this way, which can be useful if you need lots of them – to make a hedge for example. This is a job for next year in early- to mid-summer. Hardwood cuttings of shrubs such as dogwood, buddleja, willow and philadelphus can be taken from November through to February.
Coldframes need monitoring to provide the ideal conditions for their inhabitants. Ventilate by lifting the lid during the day unless the weather is very cold or wet. They can be kept frost-free by covering with hessian or a blanket on clear frosty nights. Water in the mornings, preferably only on sunny days. Your aim should always be to avoid shutting in plants with wet foliage, which will encourage fungal attack.
Perhaps you should add a coldframe to your Christmas wish list. Next spring you could be starting dahlias into early growth in order to take cuttings and increase your favourites, germinating seeds and gradually acclimatising tender plants before they fledge to the open garden.