Frost tolerant plants for cold Australian gardens

The damage turns up at breakfast. Growth that looked fine at dusk is limp and black by nine, one side of the shrub is cooked and the other untouched, and every camellia bud facing the driveway has browned at the edges. The district map says mild, your back corner disagrees, and the plant that died was sold as frost hardy. All of that can be true at once, because frost is a site problem before it is a plant problem.

What frost actually does to a plant

On a frosty night water freezes in the spaces between a plant's cells before it freezes inside them. Those crystals draw water out of the living cells, and the cell dehydrates until its membrane collapses. Hardy plants spend autumn loading their cells with sugars that lower the freezing point and hold water in place. Tender ones do not, so the tissue goes limp, then black.

Tissue that thaws slowly often recovers. Tissue that thaws fast under direct morning sun usually does not, because the cells cannot take water back in as quickly as the sun drives it out. That is why the burnt face is nearly always the east or north-east one, and why the same plant against a west wall comes through clean. Cold starts the damage and the morning sun finishes it.

Frost pockets and cold air drainage

Cold air behaves like water: denser than warm air, running downhill, pooling wherever it cannot keep moving. On a sloping block the bottom of the fall is the coldest ground you own, colder than soil a short way up the same slope.

What turns a slope into a frost pocket is usually something you built. A solid paling fence across the fall line, a retaining wall or a dense clipped hedge dams the flow and backs cold air up behind it. If the worst damage sits in a band above one fence, open the base of that fence at the low end and let the air keep running.

Radiation is the other mechanism: on clear still nights plants lose heat straight to the open sky, so frost bites hardest in the open while a plant under a canopy or an eave escapes.

Why a hardy plant still dies in its first winter

A hardiness rating describes a settled plant: dormant, in the ground, roots spread through soil that buffers the cold. A new plant has none of that.

Roots are the tender part, less cold hardy than the wood above them. That rarely matters once roots run deep, but in a pot, or in a root ball sitting proud with air gaps around it, roots freeze through while the top still looks healthy.

New growth has not hardened either. Woody plants spend autumn lignifying their stems, and one fed nitrogen late carries soft tips into winter that freeze first. A small root system also cannot replace lost water, so evergreens in their first winter often dry out rather than freeze. Crisp bronze foliage on a young conifer's windward side is that, not disease.

So in a cold district, plant in spring rather than late autumn, stop nitrogen after midsummer, and treat the first two winters as the ones needing help.

Protecting young plants through the first frosts

Water the soil in the afternoon before a forecast frost, because moist soil stores far more daytime heat than dry soil and releases it overnight. Keep mulch over the root zone but off the stem.

Cover with frost cloth, hessian or shade cloth on a frame of three stakes so the fabric never touches the foliage, since any leaf resting against it freezes anyway. Open the cover in the morning. If you do only one thing, screen the east side instead: shade cloth on two stakes that keeps the plant shaded until mid-morning deals with the thaw problem behind most visible damage.

Then leave the damage alone. Cutting blackened growth off in winter removes tissue sheltering the live wood behind it and pushes soft shoots out for the next frost. Wait until frosts finish, scratch the bark for green wood, and cut back to there.

Deciduous trees are the easy answer

A dormant deciduous tree gives frost almost nothing to damage: no soft leaves, no open flowers, wood that hardened through autumn. Most come from northern hemisphere winters well beyond anything an Australian garden produces, and autumn colour improves in the cold, since cold nights after sunny days drive the red pigments.

Silver Birch is the obvious pick, since heat and dry summers trouble it far more than cold does, and cold districts suit the birch range better than warm coastal ones. Among the maples, Acer x freemanii Autumn Blaze is a quick shade tree that colours dependably in the cold, with Norway maples and red maples hardier again. Japanese maples are frost hardy while dormant; what catches them is a late spring frost after the leaves open, which scorches new growth without killing the tree. Acer palmatum Sango Kaku pays its way twice here, because the coral bark colours strongest on bare stems in cold weather.

One flag for acreage: wilted red maple foliage, including the Freeman hybrids, is toxic to horses, so keep these trees off paddock fence lines.

The elms and zelkovas are as tough as anything grown, and English Elm ignores frost, poor soil and wind, though it suckers hard from the roots and has gone weedy in cooler southern districts, so keep it clear of paddock edges and bush boundaries, or take a zelkova instead. Buy from the advanced trees range for immediate height, but note that a large root ball planted in autumn sits high and dries out, which makes it the thing most at risk in its first winter. Our guide on when to plant trees and shrubs covers timing.

Evergreen structure for cold districts

Several plants in the conifer range come from climates far colder than any Australian garden. Norway Spruce and Oriental Spruce are true cold climate trees, better in the highlands than on a warm coast. Thuja occidentalis Smaragd gives a narrow soft cone, Juniperus chinensis Spartan a dense dark column that takes frost, heat and dry once settled, and Thuja plicata Fastigiata is the other dependable upright. Worth knowing in a family or acreage garden: thuja and juniper foliage is toxic to dogs, cats and horses if chewed.

Lower down, lilyturf and mondo grass hold foliage through a cold winter under deciduous trees where little else stays green. Westringia goes the other way: it shrugs off light frost, but it is a coastal shrub and a real frost hollow inland is not its country.

Camellias, magnolias and the browning flowers

Camellia wood and leaves handle frost without trouble. What browns is the flower, and usually not because it froze. A bud or open bloom that freezes overnight and thaws in direct morning sun goes brown at the petal edges within a couple of hours, while the same flower on the plant's shaded side opens clean. Site camellias where a wall, fence or high canopy keeps morning sun off them until mid-morning and the problem largely disappears.

Timing matters as much. Sasanquas flower through autumn and early winter, ahead of the hardest frosts, so in a cold district they give a steadier show than the japonicas, which open from late winter into spring right through the frost window. Camellia sasanqua Plantation Pink is a dependable one, with more in the camellia range, and our notes on camellia hedges and winter flowers cover the flowering seasons.

Deciduous magnolias behave the same way: buds swell in late winter, a frost catches them, and morning sun browns the petals. Site one where the sun arrives late and accept that some seasons the flowers will mark.

What not to do

Do not put tender plants at the bottom of a slope, or build a solid barrier across the fall line above them.

Do not hose frost off foliage in the morning or carry a frosted pot indoors to warm up, because fast thawing is the injury. Do not prune frost damage until spring, and do not feed a frosted plant, since nitrogen in cold soil only pushes soft growth towards the next frost. Do not read a label's hardiness figure as covering a plant in a pot either; pot roots are exposed on every side.

And leave some plants out of a cold district entirely. Norfolk Island Pine, Cook Pine, Hoop Pine and Bunya Pine are subtropical to warm temperate, and hard frost cuts them back or kills them while young. Kashmir Cypress is frost tender, and Illawarra Plum is a warm temperate rainforest tree that marks badly in heavy frost. Each belongs in a warmer district.

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