How to recognise, treat and prevent heat damage in plants
After months of extreme temperatures and scorching winds, many gardens have suffered so much heat damage that they look as if someone held a blowtorch over them. New growth is dry and brown, fruit may be cooked on the plant, and lawns have disappeared.
The natural reaction to seeing burnt leaves and stems is to get out the secateurs or pruners and cut everything back. This is not the best approach while hot conditions are still on the horizon. Pruning encourages new growth, which in turn will be burnt by hot and dry conditions.
The old burnt growth will continue to serve a purpose through the remaining hot weather, in that it will protect foliage and stems lower down the plant. By preventing those lower parts of the plant from heat damage, you're ensuring that when conditions improve, they can produce new shoots and help the plant to recover. Even dry lawns are giving protection to the root systems so that when the rain returns the grass can regrow.
Preventing more heat damage
The best way to protect susceptible plants from heat damage is to shade them. This can be in the form of temporary covers on extreme heatwave days, such as old sheets that are removed when the sun goes down, or more permanent shading from shadecloth screens or sails. Potted plants can be moved into a shaded spot on a verandah or patio, or otherwise placed under trees while heat-wave conditions persist.
It is also important to provide as much water to plants as possible. By providing shade, the plants are able to take up water to keep their leaves hydrated and cool throughout the day. While observing Sydney's current water restrictions, water plants that are likely to be heat-affected in the early morning and also in the evening wherever possible.
Persistent hot weather can also lead to hydrophobic soils – these are soils that resist water so that it runs off rather than soaking in. Always check that soil and potting mix is absorbing water and that the moisture is soaking to the root zone. Where water is not soaking in, apply a soil wetting agent. Those products that are mixed up in a watering can or applied as a hose-on application, should change the hydrophobic nature of the soil immediately but repeat applications may be needed.
Another source of heat that can burn plants is reflected heat from hard surfaces such as paving, paths, metal fences and the walls of buildings and garden structures. This type of heat can cause damage to low growth on plants. It can even heat up pots causing damage to the plant’s root system. Reflected heat can be extremely damaging, and often goes unnoticed until the damage appears.
To reduce damage from reflected heat, move potted plants away from hot surfaces. Cover surfaces near permanent plantings with cardboard (such as flattened boxes) or by laying down shadecloth or old carpeting. It may look strange, but it is a temporary strategy to reduce plant damage. Also place cardboard or shadecloth between a hot metal fence and the plants growing next to the fence.
Longer term, consider establishing climbing plants or a dense planting of shrubs along the fence to shade it and reduce reflected heat. Painting paths or hard surfaces and covering some areas of paving with groundcover plants (such as heat-loving, prostrate herbs) can also help reduce reflected heat in the future.
Un-mulched soils also absorb heat during warm and sunny conditions. Bare soils may be hotter than the air temperature by 10C or more. Simply feeling the surface of bare earth on a hot day will reveal how hot soil can become. This excess heating not only robs soils of moisture, it can also cook the root systems of plants growing in those soils leading to dieback and even plant death.
The solution is to cover soils with a 5cm layer of coarse mulch. In fire-prone areas select non-flammable materials such as pebbles, gravel or recycled glass.