Growing Gardenias and Other Tropical Plants in Brisbane

Few plants have the appeal of a well grown Gardenia. Deep, dark green foliage that is usually high gloss is contrasted by a profusion of large sweetly scented flowers in Summer. In Brisbane these beauties have become synonymous with the classical and tropical fusion of heritage homes or modern landscapes. For some they thrive and for others… their appeal is confused as they simply cannot make the plant perform to its best. This coffee related shrub or small tree is native to tropical and sub-tropical areas of Africa, Asia, Madagascar and the Pacific islands and it is there that your mind must go.

When I think of cultivating a plant my first thought is to recall its natural ecology which ultimately it is best adapted to. Tropical soils are usually depleted and acidic. They can contain little nutrient (due largely to continuous leaching rains) and little organic matter. One thing that can be found in abundance is iron. While as gardeners we understand that enhanced soil structure through the application of organic matter can only aid in the growth of cultivated plants, it is the nutrient balance that many seem to get wrong with Gardenias and other similar tropical plants like Ixora. This leaves low iron levels or chlorosis as a common problem for Gardenias. This may leave your plant with weak yellowing growth which retains some green in the leaf venation (important to note). The pH of a soil dictates the availability of most nutrients and in acid soils iron becomes relatively more available. This understanding means that either soil pH will need to be slightly altered (pH 6.5 would suffice) or additional iron administered at key times.

There exists a relationship between iron, boron, the ecology and plant physiology that cannot be ignored in this instance. As iron is usually in an unavailable form to plants in that it can be stubbornly insoluble the plant roots must actively seek and capture iron in the soil. Boron is the nutrient responsible for many plants growth tips and a small yearly supply in the form of a trace element will enhance the growth tips of new roots to aid in the capture of iron. Iron is particularly insoluble during the winter months and is best applied as part of a full plant nutrition schedule in mid spring. Seasonal slow release feeding and composting will continue to boost warm season growth.

Another often confusing physiological response of Gardenias is leaf senescence. That is cool season growth of yellow leaves. The leaf venation is not clearly visible in leaves that have simply senesced (compared to chlorotic leaves) and they will be a solid yellow colour. To those that ask I first point out the difference between the types of yellowing leaves, one you can treat for a healthier plant, the other you can simply enjoy as a natural phenomena or pluck off if they offend you.

Yes, many other nutrient imbalances can cause the yellowing of growth in Gardenias but iron deficiency is the most complicated and most others will be catered for in a good general garden care model.

Aside from the yellowing issues which if approached in the course of a sound garden maintenance regime can be overcome with ease, Gardenias are quite low care plants. Occasionally scale or mealy bug may take hold. Treat these (and nearby vectors) with Neem. Apply potassium in mid spring for stronger flower growth. A light prune after flowering to keep shape. Dead heading to extend the flowering season and removal of weak, dead or diseased growth for more attractive healthier shrubs.