Frost is produced as ground temperatures drop overnight to 0ºC or below and moisture in the surrounding air turns to ice. Even if temperatures fall to 2ºC, expect the ground to be colder and a frost to be likely if the night sky is clear and the air is still. These conditions are most likely away from the coast.

In Australia, we mostly experience a white or hoar frost, where the ground is covered with ice and grass can become crunchy underfoot. In a heavy frost, car windscreens can be icy, the water in the hose can freeze, and ponds and even the pets’ water containers can develop a frozen crust.

As the sun rises and temperature warms, the frost quickly disappears - but for any frost-sensitive plants, the damage is done. Frost damages plants by freezing their sap. The frozen sap expands and destroys plant tissue. Plants that have been frost damaged may collapse, blacken or appear burnt.

Frost-sensitive plants include many evergreen sappy plants from tropical or subtropical areas such as hibiscus, gardenias and frangipani, along with new plantings and potted plants.

Deciduous plants that are dormant are usually frost tolerant. Many winter vegetables and annuals (including pansies) are frost tolerant (often described as ‘hardy’).

As frost damage can kill sensitive plants, it is important to have frost protection in place before the first frosts occur. Frost can also cause some containers to split further exposing plants to frost damage.

If a plant is frost damaged but not killed, leave the damage in place to protect the rest of the plant from further damage. Prune away frost damaged growth at the end of winter. Pruning during winter may also encourage new growth, which itself may be frost sensitive.


Protective Products

Anti-transpirant products such as Drought Shield and Envy, which are used during transplanting and for heat protection, also provide frost protection if sprayed over the leaves of frost-sensitive plants before frosts occur.

Physical barriers also protect plants from mild frosts. Place plastic, cardboard or sheeting over plants overnight, or use a frost blanket. Remove coverings in the morning.

The graft area of some plants such as citrus can also be damaged by frost. Wrapping the lower trunk with cardboard can help to protect this vulnerable part of a plant from frost damage.

Protecting plants from frost damage is one approach, however, for longer-term management grow frost-sensitive plants in protected parts of the garden or in containers that can be moved to a sheltered place during winter.

Observe the garden over winter to see where frosts occur and where they don’t and plant accordingly. The area under a spreading tree may be frost-free (even under the bare branches of a deciduous tree), and garden beds adjacent to north-facing stone or brick walls may also be slightly warmer than surrounding areas. Lower areas (often described as ‘frosty hollows’) or exposed lawns or paddocks may also see frosts regularly, while more elevated parts of the garden may be rarely frost-affected.

Cold winds can also cause plant damage in winter. Shadecloth, hessian and straw bales can be used to block out the cold and help cold-sensitive plants survive winter chills.