A frost forecast can be worrying when you've spent months growing plants, but not every cold night calls for panic. Hardy kale may barely notice a light frost, while a courgette can collapse after one freezing night. Understanding which plants are at risk is the first step towards protecting them properly. On a UK allotment, conditions can vary even across a short distance: exposed raised beds, sheltered walls, polytunnel corners and low-lying plots each behave differently. This guide explains how frost forms, which crops to move or cover and how to avoid the common mistakes that make winter damage worse.

Understand what frost does to plants

Frost occurs when plant surfaces fall to freezing temperatures, allowing ice to form. The precise damage depends on species, how acclimatised it is, how long cold lasts and whether the plant is actively growing. Tender cells can be injured when ice forms and draws water away from tissues. A leaf may look dark, water-soaked or limp after thawing. Hardy plants have adaptations that let them tolerate lower temperatures, especially once seasonal cooling has prepared them. A sudden early frost may therefore cause more damage than a similar temperature later in winter. Wind, wet soil and repeated freezing and thawing can add stress even when the air temperature does not sound especially severe.

Know the difference between hardy and tender

Tender vegetables include courgettes, runner beans, French beans, cucumbers, tomatoes, sweetcorn and most outdoor peppers. Many are badly damaged at or just below freezing. Kale, leeks, parsnips and established winter brassicas cope much better, although extreme conditions can still cause problems. Young seedlings may be less resilient than mature plants of the same crop. Container-grown plants are often more vulnerable because their roots are surrounded by air and a small volume of compost that cools rapidly. Labels such as 'hardy' are useful starting points, but consider the variety, plant age and your local winter weather before deciding something requires no protection at all.

Follow forecasts, but check your own plot

The published forecast is usually a general prediction, not a thermometer reading at soil level beside your lettuce. Clear skies and still air encourage radiational cooling, allowing frost to form in low spots even when a standard weather station reports a temperature slightly above zero. Cold air can gather at the bottom of a slope, while a sheltered urban courtyard may stay milder. Keep a minimum-and-maximum thermometer at plant height in a representative part of the garden. If you have a polytunnel, compare inside and outside temperatures over several nights. The record quickly reveals which area is your coldest. Watch for forecast frost, but make decisions using the actual conditions where plants are growing.

Decide what must be moved first

When a frost warning arrives, prioritise tender plants in pots: citrus, pelargoniums, young bedding plants, peppers and any late potatoes being grown in containers. Move them to a suitably protected place before the coldest part of the night. A greenhouse is useful, but an unheated one can still freeze during severe spells. If a plant is genuinely frost-sensitive, choose a frost-free indoor location or appropriate heated propagation space. Avoid bringing plants straight into an excessively hot room where they may dry out or suffer sudden changes. Check for pests before moving pots inside. Keep labels in place and allow air to circulate so damp foliage does not develop fungal trouble during storage.

How to use horticultural fleece

Fleece is useful for short cold spells because it reduces exposure and can help retain some of the warmth released by soil. Place it over hoops or supports so delicate leaves are not crushed, and secure all edges to prevent wind lifting it. Where suitable, a double layer may offer additional protection, but it also reduces light and airflow. Remove or open coverings when mild weather returns, particularly on sunny days when temperatures underneath may rise. Fleece is not a guarantee against prolonged severe frost. If the plant is too tender for your winter, a cover alone may not be sufficient. Use a breathable horticultural product rather than a random plastic sheet touching vulnerable leaves.

Plastic covers: use caution

Clear plastic can keep rain off and trap daytime warmth, but cold plastic in direct contact with foliage may worsen localised damage. If using a temporary plastic shelter, hold it above the plants on a frame, ventilate to control condensation and remove it when temperatures rise. Secure loose sheeting against wind, and make sure rainwater cannot collect heavily on the roof. Do not seal plants into a damp miniature greenhouse for weeks without checking them. Plastic covers are most helpful when well managed; poor ventilation can cause rot and fungal disease long before frost itself becomes the main problem. For tender plants, a proper protected growing space is often more reliable.

Protecting raised beds

Raised beds warm earlier in spring and drain well when properly built, but their exposed edges can also cool rapidly. Protect vulnerable crops with hoops and appropriate fleece, leaving enough clearance for leaves to develop. Mulch suitable established plants to moderate soil temperatures, but avoid piling wet material around crowns and stems. Check the bed after heavy rain because waterlogged compost and freezing weather are a difficult combination. Winter brassicas may need netting against pigeons more urgently than frost protection, so design covers that allow both forms of protection. Avoid repeatedly disturbing frozen soil: allow it to thaw naturally before carrying out cultivation or lifting delicate roots.

Looking after containers and hanging baskets

Pots lose heat more quickly than the ground, especially if they are small or exposed on all sides. Group containers in a sheltered place away from strong wind and raise them slightly to maintain drainage. Wrap vulnerable pots with horticultural fleece, bubble wrap around the outside of the container or another suitable insulating material, taking care not to block drainage holes. For particularly precious tender plants, move them into a more reliable protected environment. Hanging baskets can become very exposed overnight; relocate them if practical. Water sparingly in cold weather, checking the compost rather than following a summer watering schedule. Roots can rot in freezing, saturated compost even when the leaves appear dormant.

Frost in the greenhouse

A greenhouse traps solar energy during daylight, but it loses heat after sunset. In an unheated greenhouse, overnight temperatures may approach those outside, especially during prolonged cold. Insulate selectively with suitable horticultural bubble wrap if it suits the structure, without blocking all ventilation. Keep the glazing clean to maximise scarce winter light. Stand sensitive pots on staging rather than directly on a very cold floor if appropriate, and use a thermometer to verify results. When heating is genuinely necessary, choose equipment designed for greenhouse use and follow electrical and fire-safety instructions. Never use an improvised open-flame heater in an enclosed space. Ventilate during milder periods to reduce condensation and disease.

Managing an unheated polytunnel

Polytunnels shelter crops from rain and wind and often warm rapidly on sunny autumn days, but nighttime temperatures can still fall below freezing. Position hardy salad crops and overwintering vegetables where they get the most available light. A low fleece tunnel inside the polytunnel can give an additional layer of short-term protection, provided it is securely fixed and checked frequently. Avoid growing tender crops well beyond their natural season just because they are under plastic. Moist soil can hold some warmth, but saturated conditions are harmful and should not be deliberately created. Monitor minimum temperatures, keep door fixings secure and inspect covers before storms. A sheltered structure works best as part of a sensible crop plan, not as a substitute for heating when a crop truly needs it.

Mulching around established plants

Mulch can help moderate soil-temperature swings and reduce the effects of drying winds on roots. Apply suitable organic matter around established hardy perennials and shrubs while leaving a clear gap next to vulnerable crowns and woody stems. A thick wet mound pressed against a stem can encourage rot. Mulching is particularly helpful where repeated freezing and thawing lifts small plants out of the ground. Where heavy rainfall leaves the soil saturated, first address drainage rather than adding more and more material. Keep mulch away from the leaves of low-growing winter salads so they receive light and airflow. The aim is to protect the root environment, not bury the plant.

Watering before and after frost

A well-hydrated, healthy plant often copes better with normal stress than one severely dried out, but this does not mean soaking pots before every cold night. Check moisture and water only when necessary, preferably early enough that foliage can dry. In winter, root activity and evaporation are much lower than in summer, so overwatering is a frequent problem. After a frosty night, let plants thaw naturally. Resist pouring hot water over leaves or rapidly warming a frozen pot. Wait until the damage is clear before cutting back large amounts of foliage: damaged upper growth can sometimes offer some shelter to lower shoots during further cold spells.

What to do with frost-damaged plants

First, check whether the roots and crown are still alive. A blackened courgette after a hard frost is unlikely to recover, but a hardy perennial may regenerate when warmer weather returns. Leave partly damaged plants for a short period if more frost is forecast, then remove obviously dead, rotting tissue when conditions improve. Keep tools clean to avoid transferring disease. Do not immediately apply strong fertiliser to force new growth in late autumn; fresh soft shoots may be even more vulnerable. Monitor plants over the following weeks. For valuable shrubs and climbers, patience is especially important because buds under the bark may remain alive long after leaves have been scorched.

Common frost-protection mistakes

A frequent mistake is assuming every cold-tolerant crop needs a heavy cover, while ignoring the truly tender pots beside it. Another is leaving plastic or fleece on through warm sunny days without ventilation. Growers may also forget that wind can lift unsecured covers, or that waterlogged pots can fail even when leaves are protected. Finally, protecting the top of a plant while leaving a small container fully exposed misses the roots. Use a short priority list: identify tender plants, choose reliable shelter, protect their roots and keep monitoring conditions. Good protection is responsive; it changes as the weather changes rather than being installed once and forgotten.

Frequently asked questions

Does frost kill garlic? Established garlic is generally cold hardy, though severe exposure and waterlogging can still cause problems. Will a polytunnel stop frost? Not reliably; unheated tunnels can freeze overnight. Can I cover plants with bedsheets? A clean lightweight fabric may offer temporary protection, but horticultural fleece is designed for this purpose and must be secured. Should I water frozen plants? Wait for thawing and assess the actual moisture requirement. Can I save frost-hit tomatoes? Severely frost-damaged tender growth rarely recovers well; harvest any sound fruit if safe and suitable before the next frost.

A simple frost action plan

The evening before a forecast frost, bring vulnerable pots under suitable shelter, cover delicate crops with secured fleece, check greenhouse and polytunnel doors and make sure containers can drain. The next morning, avoid disturbing frozen growth; let the plants warm gradually and ventilate covered structures when conditions allow. Keep observing local temperatures so you can refine your approach each year. Frost is a normal part of UK gardening, but thoughtful preparation helps protect the plants that need it most. Keep smiling, keep shining, keep gardening with Ben.