Building a cold frame for hardy rare species

Why a cold frame earns its place in a rare plant garden

If you grow rare or hard-to-find species, you quickly learn that the British winter is not one problem but several. It is not simply the cold that does the damage. It is the combination of sharp frost, driving rain, waterlogged compost and the freeze-thaw cycle that heaves roots out of the ground. A cold frame addresses all of these at once. It is, in essence, a low, unheated box with a glazed lid, and it can lift the effective hardiness of a plant by a whole category.

I have kept a cold frame in some form for over twenty years, and I would not be without one. It has carried dainty alpines through sodden Januaries, hardened off young plants raised from scarce seed, and given me somewhere to overwinter potted specimens that would otherwise sulk or rot outdoors. The best part is that a serviceable frame costs almost nothing if you are willing to work with reclaimed timber and a sheet of old glazing.

Siting and sizing your frame

Position matters more than construction. Choose a spot that gets full sun from mid-morning to mid-afternoon, but is sheltered from the worst of the prevailing wind. A south-facing wall or fence is ideal, especially one that holds a little warmth. Avoid the base of a downpipe or anywhere that water pools, as standing moisture under the frame is the quickest route to crown rot in rare cushion plants.

For size, think about what you actually need to protect. A frame 1.2 metres long, 60 centimetres deep and 40 centimetres high at the back, sloping to 25 centimetres at the front, suits most small collections. The slope is essential: it sheds rain, lets in more low winter light and allows the lid to sit open at an angle for ventilation. If you grow taller specimens such as rare shrubby daphnes or young Meconopsis, raise the back to 50 centimetres.

• Frame back height: 40–50 cm
• Frame front height: 25–30 cm
• Width: 60 cm is easy to reach across
• Orientation: slope facing south or south-west

Building one from recycled materials

Scaffold boards are excellent for the sides, being thick, tanalised and often available second-hand. Failing that, old decking boards or reclaimed floor joists work well, though softwood will need a coat of linseed oil or a plant-safe wood preserver. Do not use creosote near plants; the fumes linger and can scorch foliage. Line the inside with a length of old pond liner or damp-proof course membrane to slow rot, but leave the base open to the soil so worms and drainage can do their work.

For the lid, an old double-glazed unit or a sheet of polycarbonate gives the best insulation. Single panes of horticultural glass are traditional and admit more light, but they are fragile and dangerous around pets and children. Polycarbonate twin-wall sheeting is light, tough and surprisingly warm, and it diffuses light in a way that suits shade-loving rarities. Cut it to overhang the frame by two centimetres on every side so rain runs clear of the timber.

Hinge the lid at the back using two stout galvanised tee hinges, and add a simple wooden prop or an automatic vent opener. Automatic openers are worth every penny: they use a wax-filled cylinder that expands with heat and lifts the lid before you have even had breakfast. This single addition prevents the most common cold frame disaster, which is cooking your plants on a bright February afternoon.

Ventilation: the skill that separates success from soggy failure

Rare species often come from specialised habitats, and most of them hate stagnant, humid air. Damping-off, botrytis and crown rot all flourish in a closed frame on a mild day. The rule I follow is simple: if the temperature outside is above 5°C and the sun is out, the lid comes open. On genuinely mild days in late winter, open it fully. On cold but sunny days, crack it a few centimetres.

• Open the lid by 5–10 cm when daytime temperatures exceed 5°C
• Ventilate fully on mild, sunny days, closing again mid-afternoon
• Leave a permanent 1 cm gap at the top in autumn to prevent condensation
• Water sparingly in winter, and only in the morning
• Remove fallen leaves promptly so they do not harbour slugs and rot

Watering is where many growers go wrong. Rare alpines and Mediterranean species would rather be dry than damp in winter. Water only when the compost is visibly dry a centimetre down, and always before midday so any surplus can evaporate. A frame that stays permanently wet is worse than no frame at all.

Making the most of your frame through the year

A cold frame is not just a winter tool. In spring it becomes a hardening-off station for seedlings and rooted cuttings, gently acclimatising them to the open garden over ten to fourteen days. In summer you can shade it with a piece of old net curtain or greenhouse shading and use it for rare ferns, woodland wildflowers or tender succulents that need protection from harsh midday sun.

Come autumn, scrub the glazing inside and out, check the hinges and prop, and clear the soil beneath of slugs and vine weevil grubs. A tidy frame in October saves a great deal of heartache in January. If you keep a small notebook of what went in, when the lid was opened and how plants fared, you will quickly build up a picture of which rare species genuinely benefit and which would rather take their chances outdoors.

Start with one frame, sited well and ventilated attentively. It is the most modest structure in the garden, and very often the one that saves the plants you could not bear to lose.