Pentas lanceolata have thrived in the summer heat and humidity

Happy New Year to iGarden readers! It seems ages since my last blog in December. So far, the very dry summer that was predicted for us by the Australian Bureau of Meteorology in September 2023 has not really materialised: we recorded almost 200 ml of rain in our gauge last week alone! And over the past few weeks, we have experienced days of muggy humidity that made me feel as if I were in Jakarta! We have always had humid summers, in Sydney, but this was something else again! Apparently very warm sea surface temperatures off the coast of NSW are causing more evaporation and hence more moisture-filled air, raising the 'dew point' to oppressive levels. The higher the dew point, the hotter we feel the apparent temperature to be, because it is harder for perspiration to evaporate from our skin to cool us down. Some studies also suggest high humidity can cause psychological distress as well. In severe cases, sunstroke and heat exhaustion can occur if someone is exposed to extreme humidity and heat combined.

Lavender is one of the Mediterranean plants that suffers in hot, humid weather

Lavender is one of the Mediterranean plants that suffers in hot, humid weather

What about our plants?? We humans often have the option of retreating to air-conditioned comfort, but the plants are marooned in the garden! Instead of perspiring, plants transpire: the process through which they draw water up through their roots, spreading moisture and nutrients through the stems, then releasing processed moisture into the air, primarily from their leaves. High humidity levels can slow down this process, as the surrounding air is already saturated with water vapour. This slowdown can affect the plant's ability to draw up nutrients from the soil, leading to potential deficiencies and reduced growth. High humidity can also interfere with the process of photosynthesis, by which plants use sunlight to turn carbon dioxide (from the atmosphere) and water (from the soil) into oxygen and the sugars that the plant cells can use for energy. Very high humidity also can impact a plant's ability to cool itself, a huge problem if it is a plant not suited to a hot climate. This may account for some mysterious overnight deaths of plants during humid times.

Cane begonias are enjoying the hot, humid weather

Cane begonias are enjoying the hot, humid weather

Plants that originate in hot, humid climates have various adaptations that enable them to cope, and indeed thrive, in these conditions - but plants from places in the world that have a cooler climate overall or have a dry heat will really struggle when the humidity on our hot summer days soars. This includes plants from Mediterranean areas, California, West Australia, Mexican deserts and low-humidity areas of South Africa - which over time, I have come to realise are not generally destined for a long life in a Sydney garden. I have found that it is plants from places like south-east Asia, India, South American countries (especially Brazil), Texas and the humid regions of Mexico and South Africa that flourish best in our climate, perhaps more so than ever now.

Lychnis coronaria afflicted with a fungal disease

Lychnis coronaria afflicted with a fungal disease

Hot and humid conditions also promote the growth of fungal diseases. Plants that come from areas of high heat and humidity have adaptations to allow moisture to flow off their leaves - such as a pointed end of the leaf called a 'drip tip'. But plants from low-humidity areas often have hairy or downy leaves in order to trap water vapour close to the leaf surface, making them more likely to succumb to fungal diseases when humidity is high. Examples of these include many silver-leaved plants such as lavender and Santolina, Stachys byzantina (lambs' ears) and Lychnis coronaria. Many traditional cottage garden plants that have a basal rosettes of foliage (such as Aquilegia, Geranium, Silene, Verbascum and herbaceous Salvia are often attacked by fungal diseases in times of high humidity as moisture collects under the foliage close to the ground. Succulents also suffer in times of high humidity due to their leaf adaptations to a hot, dry climate. Downy mildew, botrytis, blight fungus, sooty mould and rust are just some ghastly fungal diseases that thrive on damp leaves in high heat and humidity.

Dahlia affected by soil fungus

Dahlia affected by soil fungus

Fungal diseases are also more often to occur in the soil in times of high humidity and heat, and I have at times found patches of a nasty white fungus attacking my Dahlia, Japanese windflowers, bearded Iris and perennial Phlox.

What can we do? With immediate problems of fungal diseases, I have been drenching and spraying my problem plants with an organic fungicide based on potassium bicarbonate, which supposedly destroys the fungal growth and leaves a protective coating on the foliage to prevent more spores from germinating. The fungicide can be mixed with some horticultural oil to help it stick on to the foliage. Interestingly, some authorities also recommend the use of neem oil or horticultural oil for these problems as these also keep the spores from developing.

An old home remedy for black spot and rust is to dissolve a teaspoon of bicarb soda in a litre of water, adding a dash of horticultural or vegetable oil (to help the mixture stick), and spray this on affected foliage. It can also be used regularly as a preventative measure. Milk has also been touted as a black spot cure and powdery mildew: mix one cup of full cream milk with two cups of water, shake well and spray on the leaves. Some gardeners swear by an application of weak compost 'tea' sprayed on foliage as a cure for diseases. To combat sooty mould, a suggested recipe is two parts sunflower oil to one part dishwashing liquid.

Roses can be afflicted with black spot in hot, humid summers

Roses can be afflicted with black spot in hot, humid summers

Adding plenty of compost to your soil can encourage microorganisms that are helpful in fighting diseases. Regular application of a seaweed-based solution can help stimulate strong, healthy growth that might be more able to withstand diseases. Making sure you have good air circulation around your plants will also help reduce problems with fungal diseases.

Try not to water over foliage, as this can promote diseases if the moisture lingers, especially overnight. Remove and destroy fallen leaves, flowers or fruit that are afflicted with a disease - don't leave them there to potentially reinfect the plant and certainly don't add them to your compost heap: put them in the green refuse bin. Remove any badly affected leaves from the plant and put them in the green bin, too, before spraying anything on the plant. Disinfect tools with methylated spirits after using them on infected plant material to avoid cross-infection.

In the longer term, especially if higher levels of humidity are going to become more frequently experienced, plant choice is the most important consideration. Embracing plants that come from semitropical areas where high heat and humidity are the norm has been the way forward for me in my garden, heartbreaking as it was to give up the dream of an English cottage garden.