Frost hardy plants and gardening tips

Plants can be safeguarded against frost, even those more suited to warmer climates. Here are some great suggestions for getting your garden through the big freeze.

What is frost?

Most frost damage occurs when sunlight hits the still-frozen plant tissue the following morning. It is the sudden change in temperature, and not the cold itself, which causes the damage, so plants shaded from early morning sunlight will be far less vulnerable. Another effective, albeit more labour-intensive method, is to go outside on frosty mornings with a watering can and use the water to melt the ice before the sunlight hits.

A more insidious hidden frost known as Black frost occurs when the air is generally dry and the temperatures drop below freezing point, internally freezing the plant from the inside and blackening the leaves over a few days.

Hoar frost is far more common in cooler zones in Australia and is recognizable by the appearance of thick white ice crystals developing on top of the plants surface. These crystals form when surface temperature on an object drops below zero and the surrounding water vapour in the air freezes on contact.

Nutrition is everything

You can boost your plants’ natural resistance to frost by applying a diluted solution of SeaMax Organic Fertiliser Liquid every 2 weeks, starting at least 3 weeks before the onset of severe cold. You can also help ‘toughen up’ plant tissue in preparation for frost by resisting overwatering or overfeeding from late autumn onward.

Get things covered

Young plants can be more susceptible to frost damage, so providing shelter by erecting shade cloth or wrapping hessian over the growing tips (until frosty conditions have passed) can get them through the most vulnerable stage. Even subtropical trees such as avocado, sapote and macadamia can be grown in areas with quite severe frosts if they are protected through the first two or three years of life.

How low can you go?

Frost tends to settle in lower-lying areas, which is a good thing to bear in mind when assessing which types of plants to put in different parts of the garden. Take note of which parts of the garden are most severely affected by frost, especially in undulating gardens. You don’t necessarily have to get outside on a cold morning and see the frost for yourself — just taking note of areas that have been frost damaged can be a good indication.

Mulching matters

Applying a thick blanket of mulch will help to insulate plant roots from temperature extremes and reduce the plant’s overall vulnerability to cold.

Once frost has bitten…

It may look unsightly, but it’s a good idea to leave any frost damaged plant tissue on the plant until spring has arrived. Dead foliage can shield the remaining plant from further frost damage.

A solution in a pot

Growing plants in pots is an excellent way to protect them from frost. In fact, doing this can allow gardeners, even in cold climates, to grow plants that require subtropical, or even tropical conditions. Allow your plants to reside happily in the garden through the warmer times of year but as soon as frosty temperatures threaten, move plants into a greenhouse or sunny indoor patio until the threat of frost has passed. Bring container-grown plants under cover at night during cold periods. If containers are too large to move, drape them with bubble-wrap or shade cloth at night.

Camellias, rhododendrons & hydrangeas

Many camellia varieties tolerate the cool frosty mornings but the most notable are the winter-spring flowering japonicas. Their glossy dark thick leaves make them suitable for frosty positions and are a gardener’s favourite for their big showy blooms and compact thick growing habit.

Rhododendrons grow best in cool and mountainous areas where frosts are typical. Their trusses of delicate flowers bloom in spring. There are over 800 different varieties including hybrids suitable for more warmer climates near the coast. Rhododendrons and azaleas are often grouped together and for very good reason. All flowering azaleas are classed under the genus of Rhododendrons umbrella. One common way to tell them apart is that Rhododendron flowers generally form on trusses. 

Hydrangeas grow well in full sun in cooler climates, tolerating frosts well and semi-shade in warmer areas. Pink, blue or white flowers appear in summer (in alkaline soils flowers will be pink; in acid soils flowers will be blue; white flowers are always white). Hydrangeas hate drying out so keep them moist at all times. Trim off spent flower heads as they wither. Prune back to two plump buds in autumn and cut out dead wood at the base. Suited to mixed beds and borders around the house; in containers on verandahs and patios or as an informal, low hedge. Hydrangeas make excellent, long-lasting cut flowers.

Frost happy plants

• Bacon and eggs (Pultenaea villosa) – This evergreen shrub grows to 1.5m. Needs full sun and a light, well-drained soil. Red and yellow pea-shaped flowers appear in spring. Lightly prune after flowering. Drought tolerant. Well suited to native gardens.

• Veronica (Hebe spp.) – This evergreen, summer flowering shrub needs full sun and a light, well-drained soil. Grows to 2.5m Cultivars produce white, blue, lavender, purple, violet and pink flowers. Prune lightly in autumn to prevent plants from becoming leggy. Mulch lightly in early summer to keep roots cool. Great in shrub borders and mixed plantings. Veronicas are tolerant of moderately windy and dry conditions.

• The deciduous mulberry tree is cold hardy and produces delicious fruit in spring.

• Conifers are renowned for their stamina in frosty conditions.

• Hellebores – the winter rose flowers heavily in winter. Loves the winter sun, protect from humidity.

• Deciduous trees and shrubs – acers, oaks, flowering cherries and dogwood.

• Many grevilleas and callistemons are frost tolerant. Try G. Deua Gold, G. Gold Cluster, G. Lady O and for heavily frosted areas try G. Fireworks, G.victoriae and G. rosemarinifolia.

• Rhaphiolepis indica.

• Lavender (Lavandula spp.) This small, evergreen shrub needs full sun and grows to 1.5m. Great in containers and can tolerate some wind in a preferably light, sandy, well-drained, slightly alkaline soil (pH 7.5). Flowers in spring, summer and intermittently throughout the year. Flowers in white and various shades of pinkish-mauve, purple and blue. Trim back lightly after the main flush of flowers has finished to prevent the bush from becoming straggly. Mulch around the roots in summer. Looks great in herb and cottage gardens. Attractive, pale grey foliage. Lavender flowers and foliage are superbly fragrant. The flowers are perfect for potpourri and dried arrangements. Lavender is easy to propagate from cuttings.

Frost hardy grasses

Many varieties of Liriopes are frost hardy and compliment gardens as border plants and mass planting situations. They are evergreen, compact and display dense clumps of strappy foliage and spikes of purple-pink bubbly flowers, depending on the variety, in summer and purple-blue berries in winter. Plant in a full sun to shaded locations in free draining soil.

Lomandra longifolia is very hardy with almost no maintenance. Suited to dry soils and dry climates, but will grow in almost any climate, soil, location or situation. Excellent plant choice to manage erosion control and weed suppression. Grows to about 1.2m.

‘Utopia’ Dianella features stunning contrasting purple and green strappy foliage which will add thrilling colours to any landscape or garden. Suitable for most soils and climates except for hottest parts in Australia. An elegant clumping, ornamental Australian flax growing 50cm wide, with graceful flower stems reaching over 1m in height.

Frost hardy tips

• Grow deciduous plants or perennials that die down completely in winter.

• Protect small plants grown outside with a shelter made of shade cloth, bracken or fern fronds.

• Taller plants prevent cold, frosty air from falling on lower plants, so grow a canopy or hedge near susceptible species.

• Grow vegetables in raised beds to lift them above cold, frosty air; if you are not on water restrictions, set the timer on the sprinkler system to come on before dawn to prevent frost damage.

• Select plants from cooler climates such as China, Japan, northern Europe and the northern regions of North America.