Gardenias are wonderful, versatile plants and do very well in Canberra with just a bit of effort during the growing season. Great plants for dappled shade, or areas with morning sun, they are also useful as a low hedge in areas of semi-shade. They are not frost tender – this is a myth – and are actually quite hardy plants once established. The flowering period is from about October through to the end of February but the difficulty in Canberra is to get them to flower freely and maintain a good, deep green leaf colour. These problems are due to our dry climate but there are a number of simple solutions and the reward of fragrant white flowers is truly worth the small amount of extra effort.
A drip system along the root zone is a good idea as gardenias need moist soil to produce flowers.
Provide a thick layer of organic mulch, or use a pebble mulch. Place flat rocks around the root zone to stop evaporation and keep roots cool in very hot weather. Gardenias do need some humidity and use of mulch and/or rocks will help to provide this. They also do well when surrounded with paving as this keeps the roots cool (provided that they have some afternoon shade in such a position).
Dead head and use the old flowers around the roots as organic matter.
For potted gardenias, use a lime free potting mix (azalea and camellia mix is suitable).
In garden beds, avoid using lime as they prefer a neutral to acid soil. If the soil is alkaline (limey), then the plant is not able to take up sufficient iron from the soil so dark green veins will show but the rest of the leaf will be a paler colour.
Epsom salts is often recommended if foliage is yellowing, but don’t use excessive amounts. If used too frequently plants may show browning of the leaf tips so use of Epsom salts should be limited to only twice a year. Mix 1 dessertspoon of Epsom salts with 9 litres of water, add the recommended amount of Seasol and the recommended amount of chelated iron and water around the plants in October and again in early March. Brown or burnt tips and edges of leaves can also be due to dry conditions or severe frosts.
Some yellowing of older leaves is natural and more pronounced in spring as the weather warms and the plant produces new top growth while the soil is still cold.
Flowers falling before fully open may be due to cold nights following bud formation, or to inadequate soil moisture in hot conditions.
Bud drop or buds with small holes are usually caused by a species of weevil. Remove affected buds and spray with a pyrethrum product.
Water freely and use a liquid or slow release fertiliser regularly during the growing season, from October to March.
Gardenias are prone to scale but this is quite easy to control with pest oil. Watch for ants as they are often a sign that scale is present. If the infestation of scale is heavy, then prune the plant before spraying.
Prune each year in early March. It is important not to prune too late in Canberra as pruning will promote some soft new growth and this needs 6 to 8 weeks to harden off before our first frosts.