Know your first frost date — and your vulnerable plants

In most of the UK, the first proper ground frost arrives somewhere between late September in the north and east, and early November in milder coastal and city spots. But frost is a sneaky thing: a clear, still night with a light breeze can drop your patio thermometer to -2°C while the weather forecast is still promising a low of 4°C. Rather than relying on predictions alone, it pays to know which plants in your garden are genuinely at risk and to have your defences ready before the first forecast mentions the word "frost".

Tender plants fall into roughly three groups. There are the true tropicals — dahlias, cannas, pelargoniums, begonias and most salvias — which blacken at the first hint of ice. Then there are the half-hardy sorts, such as fuchsias, agapanthus and hardy palms, which survive a light touch of frost but sulk badly after a hard one. Finally there are the borderline hardy plants: bay trees, olives, tree ferns and young specimens of anything woody, whose roots and growing tips are far more vulnerable than their mature counterparts. Knowing which group you are dealing with tells you how much protection is really needed.

Fleece: the gardener's first line of defence

Horticultural fleece is the simplest and most versatile tool in your frost kit. It works by trapping a layer of still air around the plant, which slows the rate at which the foliage loses heat to the night sky. A single layer of standard fleece will usually see you through a couple of degrees of frost; double it for anything more serious. When you are wrapping, remember that fleece insulates best when it is not squashed against the leaves — drape it over a few bamboo canes or a simple frame so there is an air gap, and anchor the edges with stones, pegs or, on pots, a wrap of twine.

For low-growing plants, a double thickness laid loosely over the top and held down at the corners is often enough. For taller shrubs, it is worth building a temporary cage of canes and wrapping the fleece around the whole structure, leaving the top open on mild days so the plant does not cook in unseasonal sunshine. Remove fleece during the day whenever temperatures climb above about 5°C, and always take it off once the risk has passed, as light starvation and trapped damp do more damage than a light frost would.

Cloches and bell jars for individual plants

For precious individual specimens — a young olive, a favourite agapanthus in a pot, or a row of winter salads — a cloche gives more focused protection than fleece. Traditional glass bell jars are beautiful but heavy and pricey; modern polythene cloches, rigid plastic tunnels and even upside-down clear storage boxes all do the same job at a fraction of the cost. The principle is simple: they trap heat during the day and release it slowly overnight, giving you perhaps three to five degrees of extra warmth.

The one rule with any cloche is ventilation. On bright, still mornings, temperatures inside a sealed cloche can rocket, cooking leaves and encouraging mould. Lift the cover, or slide it to one side, for an hour or two each day. If you are using cloches over winter salads or herbs, keep them slightly ajar at one end and check the soil isn't drying out underneath, as rain will no longer reach the plants.

Mulch deeply to protect the roots

Frost damage to top growth is obvious, but it is the roots that matter most for long-term survival. A thick, dry mulch applied in late autumn insulates the soil, keeping root temperatures steadier and preventing the freeze-thaw cycle that lifts shallow-rooted plants clean out of the ground. Good materials include well-rotted bark, composted woodchip, straw, leafmould and spent mushroom compost. Aim for a layer 7-10cm deep, spread over damp soil after the ground has cooled but before it freezes hard.

• Keep mulch clear of stems and trunks — a collar of bare soil prevents rot.
• Do not mulch dry soil; water thoroughly first if autumn has been dry.
• For pots, wrap the container itself in bubble wrap or hessian, or plunge it into a larger pot of bark.
• Move terracotta and glazed pots onto feet or bricks so they don't freeze to the patio.
• Raise pots against a south-facing wall, which radiates warmth long after sunset.

Extra safeguards for the coldest nights

When a hard frost is forecast, a few last-minute tricks can make all the difference. Water your vulnerable plants the morning before — moist soil holds heat far better than dry soil. Move pots into a porch, greenhouse or unheated garage if you can, and group them together so they shelter one another. A sheet of newspaper laid over the top of a fleece-covered frame adds a valuable extra layer on the very worst nights.

For plants in the ground that cannot be moved, consider a temporary windbreak of horticultural netting on the cold side of the border. Cold damage in the UK is often driven as much by drying wind as by the actual temperature, and a simple screen can reduce it dramatically. Check ties and stakes on climbers too, as frozen ground loosens anchors.

What to do after the cold passes

Resist the urge to cut back damaged foliage straight away. Blackened leaves may look sad, but they shelter the stems and buds beneath, and they will drop off naturally in spring. Wait until the risk of hard frost has passed — usually late April in the south, mid-May further north — before pruning properly and feeding. Then stand back: with fleece, cloches and a generous mulch, most tender plants will reward you with a vigorous growing season once the warmth returns.