Canberra and the ACT region regularly record negative overnight temperatures between May and November. For an established tree with thick bark and deep roots, that’s manageable. For a tree planted in the last one or two years, it can be fatal — not because of the cold itself, but because of what the cold does to cells, moisture and bark. Here’s some information on the biology, and what you can do to help young trees:
A young beech tree hit by frost
- When temperatures drop below freezing, the water inside plant cells begins to form ice crystals. Those crystals expand, puncturing cell walls from the inside. When the plant thaws, those ruptured cells can no longer function — they collapse, turn brown and die. This is why frost-damaged foliage looks water-soaked and dark immediately after a freeze, then turns papery and brown as it dries. The damage is done not by cold itself, but by the physical destruction of cells as ice forms and expands.
- Young trees are most vulnerable because established trees have thick, corky bark that insulates the living cambium layer beneath it from rapid temperature swings. Young trees have thin, smooth bark with little insulating capacity. Their vascular tissue — the cambium just under the bark — is close to the surface and highly susceptible to freezing. Lose the cambium and you lose the tree’s ability to move water and nutrients. A frost that barely affects a mature tree can destroy the cambium of a young one entirely.
- Sunscald — the freeze-thaw problem. On cold but sunny winter days, the bark of a young tree can warm significantly in the sun, causing the dormant cells beneath to briefly activate. When the sun disappears and temperature plummets, those newly active cells freeze and rupture. The result is a vertical split or sunken patch of dead bark — usually on the sunniest side of the trunk. Wrapping the lower trunk with hessian or a commercial tree guard from May to late November helps prevent this temperature whiplash entirely.
- Roots are even less cold-tolerant than stems and branches. When the soil freezes around a young tree’s shallow root system, roots can be physically damaged and lose the ability to take up water. This is why a well-mulched root zone — 5 to 8cm of coarse organic material extending to the drip line — is so important. It doesn’t stop the ground cooling, but it significantly slows it, buffering roots against the sharp overnight drops that cause the most damage.
- New growth is the highest risk. The worst frost damage happens when a young tree pushes new growth in autumn or spring — tender new shoots and leaves that have none of the cold-hardening of older tissue. This is why you should avoid fertilising trees in late summer or early autumn, which can trigger a flush of soft new growth heading into the coldest months.
- If your tree is frost-damaged. Don’t prune damaged wood immediately — it’s tempting, but the dead material actually provides some protection to the live tissue beneath it through winter. Wait until late August or September when new growth begins to show you exactly where the live wood ends, then prune back to healthy tissue just above a bud.
- Beware of late frosts. Last year the ACT region recorded frosts after mid November. This can be fatal to young trees.
Signs of frost damage include: blackened tips, split bark, sparse spring flush. Some frost-damaged trees recover well with the right aftercare. Others are better replaced.