GRUESOME GARDENIA

Question We have three gardenias in the garden bed of a shady patio that did quite well for a few years, though they did tend to grow out through the trellis and they also got black stuff on the leaves. Now some of the main branches seem to have died, but there are new leaves appearing from below. What should I do? They’re watered by an irrigation system that also does the rest of the garden.

D Baker, Sydney Answer

It sounds like it’s probably a bit too shady for them. Gardenias do best in light shade or partial sun, rather than deep shade. If there’s not enough light their growth becomes weak and leggy and is more susceptible to serious infestations of scale insects, which feed by sucking the plant sap, and their excrement causes the black mouldy growth you saw on the leaves.

New growth from the base of the plant is a good sign, so I’d cut back the obviously dead shoots close to the new growth – you could leave a few of the healthier ones growing through the trellis as the leaves on them will provide some energy to the rest of the plant. Once the new ones are growing well you could cut those lanky ones back if you want. Get some fertiliser for acid-loving plants, such as Yates Thrive Camellia, Gardenia & Blueberry Plant Food, and mix it in to the top of the soil and water in.

To control scale, oil sprays are the best, and there are several brands from which to choose. You can mix a little (follow the instructions on the container label) in a small hand sprayer and spray over and under the leaves and the stems thoroughly. The oil smothers the scale so you need to get good coverage. Don’t worry if some of it drips onto the soil. It’s safe to use and non-toxic (unless you actually drink it, and even then it will probably only have a laxative effect).

If you need to spray leaves growing in full sunshine, leave it until the cool of the evening so the spray is fully dry by the time the sun comes up the next day. If you spray it in the heat of a sunny day you can end up with leaf damage.

As your gardenias are in an enclosed bed, try to control the watering so the soil isn’t soaking wet all the time, to avoid the risk of root disease. It might even pay to water that area by hand rather than rely on an irrigation system, which may be programmed to water according to the needs of the rest of the garden in full the sun.