Protecting Garden Plants from Frost: What Works

Cover tender plants before dusk with a double layer of fleeceA lightweight, breathable fabric laid or draped over plants to trap a little warmth and keep off frost, wind and some flying pests. Full definition →, water dry soil first, take the cover off the next morning, and check the forecast's wind as well as its temperature. If you're looking at dahlia foliage gone black overnight, or a fuchsia's soft new tips hanging limp and water-soaked, that is frost damage. Ice has formed in or between the plant's cells, and as water freezes it expands and ruptures soft, non-hardy tissue. That is why the damage often shows as blackened, mushy or scorched growth, sometimes only once the tissue thaws. Frost-scorched evergreen leaves look bleached or bronzed because cold wind strips moisture from the foliage faster than frozen roots can replace it, which matters on an exposed Scottish plot, where wind is part of the damage.

That doesn't necessarily mean the plant is dead. Established, correctly hardy plants often recover: even blackened foliage may regrow from dormant buds at or below soil level, and the RHS suggests waiting until mid-summer before replacing something that looks dead. But most of this damage is preventable.

If it's dahlias or cannas that took the hit, the fix is made before the frost: once the first frost has blackened the foliage, lift and store the tubersA swollen underground stem or root a plant uses to store energy, which can also grow into a new plant, the classic example being a potato. Full definition → rather than leaving them for a second, harder frost that can reach the tuber itself.

Two kinds of frost

Radiative frost, the ordinary clear-night, frosty-morning kind, happens on clear, still nights. With no cloud to trap it, heat radiates from the soil and plants into open sky, and ground-level temperatures can fall several degrees below the air temperature reported for your area. Most protection advice is built around it: fleece, a clocheA small, portable clear cover placed over an individual plant or short row to trap warmth and keep off wind, rain and pests. Full definition → or even an old sheet traps the heat coming up from the ground, giving a warmer pocket of air around the plant.

Advective frost is a truly cold air mass blowing in on the wind, whatever the cloud cover. A light cover gives far less benefit, because the cold air displaces the warmth underneath rather than being kept out. Wind-driven cold calls for more solid, insulating protection, or for moving container plants to full shelter, not a single layer of fleece and hope. On an exposed Scottish site a windy cold snap can bring a hard frost too, not only still clear nights, so check the forecast's wind and cloud before deciding one fleece layer will do.

Why frost pools in parts of a garden

Cold air is denser than warm air, so it sinks and flows downhill like water, and pools wherever it can't drain away: hollows, the base of a slope, and anywhere a wall, dense hedge or solid fence blocks its path. These frost pocketsA low-lying spot where cold air drains and pools on still nights, making it noticeably more frost-prone than the surrounding area. Full definition → can be measurably colder than a slope twenty metres away on the same night. A gap in a hedge, open fencing instead of a solid wall at the low point, or a sloped bed all give cold air somewhere to keep draining. Two beds a few metres apart can have very different frost risk for no reason but where the cold air settles. Keep the hardiest plants in a known frost trap, and tender or newly planted ones elsewhere. Frost pockets vs suntraps has a checklist for finding your own.

On the night of a frost warning

• Cover before dusk, while the ground still holds the day's warmth. That trapped heat is what the cover keeps in overnight, so covering after the temperature has dropped gives up most of the benefit. Use a double layer of fleece, and keep sheets and blankets dry, since wet fabric cools rather than insulates.
• Water the soil beforehand if it's dry. Moist soil holds more heat and releases it more slowly overnight, warming the air immediately around the plant. Water in the morning or early afternoon, not at dusk, so foliage can dry before nightfall; wet leaves going into a freeze are more vulnerable.
• Take covers off once it warms the next day. Leaving fleece or cloches on into full daylight smothers growth and cuts light, and it stops the soil recharging its heat. A cover is a night-time tool.

Protection methods

• Fleece or cloches over tender bedding, vegetables and blossom: effective against radiative frost, less reliable in a cold, windy air mass.
• MulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → over the root zone of borderline-hardy shrubs and perennials, insulating the roots even if top growth is lost.
• Moving containers to shelter, against a house wall or into an unheated porch. Walls that absorbed daytime sun release some heat overnight, giving pots against them a milder microclimateThe local growing conditions in one specific spot that differ, sometimes substantially, from the wider area's average climate. Full definition →.
• Clustering pots so they shelter each other and expose less surface to heat loss.
• Wrapping the trunks or crownsThe base of a perennial plant where roots and shoots meet, often sold and planted as a ready-made dormant plant rather than grown from seed. Full definition → of cold-sensitive specimens, most relevant to tree ferns, cordylines and other borderline-exotic plants that milder, sheltered coastal gardens can sometimes grow outdoors year-round, provided the crown is protected through the hardest weeks.
• Delaying bedding until frost danger has passed.

Fleece grades differ in weight and warmth, and it is worth layeringPropagating a new plant by pinning a low, flexible stem into the soil while still attached to the parent, so it roots before being cut free. Full definition → a heavier grade over anything truly tender. The guide to garden fleece covers which weight suits which job.

Which plants need protection

Fully hardy, dormant, established plants need nothing for an ordinary frost. The RHS hardiness ratingThe RHS's H1a–H7 scale for how much cold a plant can survive outdoors, H1a being tender indoor-only and H7 hardy anywhere in the UK. Full definition → (H1a to H7) on a plant's label is the quickest check, and RHS hardiness zones for Scotland shows how the ratings map onto Scottish conditions. Protect:

• Tender and half-hardy plants not adapted to freezing at all: dahlias, cannas, pelargoniums, tender bedding.
• New, soft spring growth on plants that are hardy once mature, which is frost-tender at the shoot tip even on a tough shrub.
• Blossom on fruit trees and other spring flowerers, where a late frost can wipe out the fruit set though the tree is unharmed.
• Marginally hardy shrubs recently planted, before their roots are established enough to support strong regrowth.

The middle two matter more in Scotland than the calendar suggests. With the lowland last spring frost typically not clearing until April to early May, new growth and blossom are exposed well after some guides written for further south assume the risk has passed. Not every plant needs protecting, though: parsnips are better left in the ground until after the first heavy frost, which intensifies their sweetness and flavour. Avoid walking on frosted grass, which bruises the frozen blades and can leave brown footprints once it thaws.

Scotland's frost picture: altitude beats latitude

You'd expect further north to mean more frost, but altitude and exposure predict it better than compass position. Inverness, on the coast, averages fewer air-frost days a year than Edinburgh, further south. Cold air draining off the hills doesn't settle there, and the Moray Firth warms and cools more slowly than the land, moderating the coldest nights. Upland stations away from the coast see far harsher frost whichever region they sit in. (Air-frost days are days the air temperature itself, not just the ground, fell below freezing, the standard Met Office measure.)

It is the frost-pocket logic again at landscape scale, and it means your own site's altitude, exposure and cold-air drainage matter more for tonight's decision about covering the dahlias than which part of Scotland you're in. In most of lowland, inhabited Scotland the last spring frost typically falls somewhere from April to early May and the first autumn frost in October, shifting by region and altitude. Check your own area on the Scottish frost dates by region page, which splits lowland and upland.

Common questions

Sources