Understanding citrus hardiness

What does citrus hardiness mean?

Hardiness is the lowest temperature a mature, healthy, properly acclimatised citrus can withstand during an occasional frost. It is not an ideal growing temperature, nor an absolute guarantee of survival. Above all, it is not a fixed value: hardiness is dynamic. The same tree, in the same spot, can come through -10 °C without a damaged leaf one winter and suffer at -7 °C another. What counts is the state of the tree when the cold arrives, and how the cold comes and goes.

Before, during and after the frost

• Before: gradual cooling over several weeks hardens the tree. A sudden frost after a mild spell catches it in full growth, far more vulnerable.
• During: duration matters as much as the minimum. As long as water is freezing in and around the tissues, it releases heat that holds them near 0 °C, so a short frost does less harm than a long one, even a slightly milder one. Wind during or just after the frost makes the damage much worse.
• After: the damage comes from the freeze-thaw cycle at cell level. A rapid thaw, in full morning sun, is the most destructive; a slow thaw, in shade or under cloud, gives the tissues time to rehydrate.

Symptoms often only show a few weeks later: leaves yellowing then dropping, bark splitting, shoots drying out. Before pruning a frosted tree, wait for spring growth to see what comes back.

What raises or lowers resistance

Raises resistance: gradual hardening, moderate vigour going into winter, free-draining soil, a favourable microclimate (south-facing slope, shelter of a wall, windbreak hedge, vegetation around the plant).

Lowers resistance: a mild spell just before the frost, waterlogged tissues (heavy rain, late irrigation, nitrogen applied in autumn), a sudden frost followed by a rapid thaw, repeated freeze-thaw cycles.

Prevention rather than cure

• Choosing the spot matters most. Watch your ground on frosty mornings: plant where it thaws first, and avoid hollows where cold air collects.
• The rootstock: prefer one that goes properly dormant in winter and does not restart too early in spring, such as Poncirus. See how to choose a rootstock.
• The kind of citrus: all else being equal, a satsuma is clearly more resistant than an orange or a classic mandarin. Our varieties rated -10 °C and lower are gathered in hardy citrus.
• End-of-season care: stop nitrogen in autumn, control watering at the end of the season, avoid hard pruning before winter and keep the foliage volume.
• The soil: covered ground (mulch, ground cover) limits heat loss during the night.

Young plants and pot culture

The values given apply to mature, acclimatised plants. A young plant, a recently planted one or a citrus grown in a pot must be protected earlier. In a pot, the roots are more exposed to cold than in open ground.

How to use the filter

Select the lowest temperature usually recorded in your garden. The filter shows the varieties rated to withstand that temperature or lower, for example -10 °C and below. Hardiness ratings are reviewed every year from our observations. In an exposed spot or if in doubt, keep a safety margin of a few degrees.