Protect Plants From Frost: Methods by Plant Type and Zone

Horticultural fleece (also called row cover or frost cloth) is a lightweight spunbonded polypropylene fabric that traps radiant heat while remaining breathable. A standard 17–30 gsm fleece raises the temperature around plants by 2–4°C — enough to see most tender annuals through a light to moderate frost.

How to apply

• Drape loosely over plants — don't pull taut, as you need an air gap for the insulating effect to work.
• Secure edges with stones, tent pegs, or bury in shallow soil to prevent wind lifting it off overnight.
• Remove or open during the day when temperatures allow, to prevent overheating and to permit pollination.
• Double-layer for harder frosts: two layers of standard fleece outperform one layer of heavy fleece because the trapped air between layers does the insulating work.
• Do not leave plastic sheeting in contact with foliage — moisture condenses and causes rot. Fleece is breathable; plastic is not.

A cloche is a transparent cover — glass bell jars are the classic form, but modern versions are polycarbonate tunnels, plastic domes, or cut plastic bottles — that creates an enclosed microclimate around a single plant or a short row. A cold frame is a larger, permanent or semi-permanent structure: a bottomless box with a transparent lid (glass or polycarbonate) that sits over a bed and can be opened and closed.

Ventilate cloches and cold frames on mild days — sealed covers on a sunny winter day can overheat plants within an hour. Prop the lid open a few centimetres whenever the temperature inside exceeds 10°C.

Mulch does not protect foliage — it protects the soil. A thick layer of organic material insulates the ground, slowing temperature fluctuation so that soil stays above the lethal freezing point for the root systems beneath. This is the primary protection method for hardy bulbs, perennial crowns, and established shrubs.

Materials and depths

• Bark chips or wood chip mulch: 7–10 cm (3–4 in). Good for ornamental beds and shrub borders. Long-lasting.
• Straw: 10–15 cm (4–6 in) for tender crowns. Cheap, effective, but must be removed in spring before it traps slugs.
• Composted leaf mould: 7–10 cm (3–4 in). Excellent for perennials and woodland plants; improves soil structure as it breaks down.
• Compost or well-rotted manure: 5–8 cm (2–3 in). Feeds the soil while insulating. Use on established (not newly planted) beds.
• Gravel or grit: 5 cm (2 in). Best for drainage-sensitive plants (alpines, lavender, Mediterranean herbs) where organic mulch causes crown rot.

Always leave a 5–8 cm gap between mulch and the main stem or crown of a plant. Mulch pressed against stems traps moisture and encourages crown rot — the opposite of what you want.

Container plants are more frost-vulnerable than their in-ground equivalents because the roots have no deep soil mass to buffer temperature swings. The pot walls conduct cold directly to the rootball. Wrapping the pot — not the plant — in bubble wrap or hessian creates an insulating layer that can hold the rootball above freezing through all but the hardest sustained frosts.

• Wrap the outside of the pot (not the rim — leave drainage clear) in two layers of large-bubble bubble wrap and tape or tie securely.
• Cluster pots together against a sheltered wall — pots lose heat slower in a group than individually, and walls radiate stored daytime warmth.
• Raise pots off the ground on pot feet to prevent waterlogging, which freezes faster than drained soil.
• This method works for: hardy ferns, ornamental grasses, box, bay, hardy agapanthus, hardy fuchsia. It does NOT work for non-hardy tropicals — those need to come indoors.

Some plants cannot be protected in place. Citrus, bougainvillea, tree ferns, most succulents, bananas in containers, and tender perennials (dahlias in Zones 5 and below, cannas in Zones 6 and below) must be moved into a frost-free environment before temperatures drop below 5°C (41°F).

Threshold temperatures to act

For structural and tropical plants you cannot move — bananas (Musa), giant gunnera, hardy tree ferns (Dicksonia antarctica), and architectural exotics — packing the crown tightly with dry straw or bracken fronds provides the insulation they need to survive in Zones 7–8. The crown holds the growing point; if that survives, the plant will reshoot from the base even if all the foliage is lost.

• Cut back banana stems to 60–90 cm (2–3 ft) after the first frost blackens the foliage. Do not cut to the ground — the stem provides additional thermal mass.
• Stuff the hollow trunk with dry straw, then wrap the whole structure in several layers of horticultural fleece tied with twine.
• For gunnera, fold the large leaves over the crown before the first hard frost, then heap straw or bracken on top to at least 30 cm (12 in) depth.
• For Dicksonia tree ferns, pack the crown (the top of the trunk) with straw and wrap the trunk in fleece. Do not cover the fronds.
• Remove all packing by late spring once frost risk has passed — straw that stays wet promotes crown rot.