In the tropics, hedges are a bit difficult. However in these days of closely built suburban developments, a sound and sight screen is often desirable. There are not a lot of plants really suitable for a classic hedge, and things tend to grow so fast that you are forever clipping them to keep that nice clean line, so beloved of maze builders, parterre planters and topiarists. However, it is not impossible, and really it is up to you how much work you want to put in, and how much untidiness you can tolerate. Topiary is usually fairly basic in tropical gardens, but geometric shapes can be achieved.
First, you select your variety; decisions like, how tall, flowering, variegated, etc. need to be considered. Traditionally, hedges are not flowering plants, but cypress and box, but they don’t do well in the humidity and heat. However, there are many flowering tropical shrubs which do quite well as hedges and screening plants, and which last for many years of clipping to shape. What you are basically looking for is a densely growing plant, which has reasonably small leaves and will tolerate tip cutting without dieback.
For instance, there are two sizes of Ixora – the larger King Ixoras which grow up to 3 metres or more, and come with red, orange, pink and apricot flowers. There are also miniature Ixoras which are more suited to a 1 metre hedge, although with time, they will grow taller. There is also Duranta erecta, which comes in green, gold variegated white and variegated yellow, with white, pink or blue flowers. The dwarf ones make lovely low borders around beds, the standards can be shaped into quite tall (2 metres) hedges. Hibiscus, in all their many foliage types and flower colours, can be pruned to a hedge, as can crotons (Codiaeum variegatum), but it should be noted that the broader leafed varieties are more successful. Plumbago capensis also makes a pretty low hedge, with blue flowers.
If you want a cypress hedge, then there’s the top end native Cypress Pine (Callitris intratropica) which can be pruned to shape for a few years, but not as a long term proposition.
If you want to use native plants, some of these are quite suitable as well – Satinash (Syzygium fibrosum) grows to about 3 metres and has bright red new leaves. Phaleria clerondendron has strongly scented white flowers, Phyllanthus cuscutiflorus has pretty clusters of tiny flowers as well as pink new leaves.
Many other shrubs will make beautiful screens, even if you don’t get to trim them severely to shape – Gardenia, Acalypha, Leea, Grevillea and Phyllanthus.
Trimming hedges needs to be done fairly regularly during the growing season, since if it gets too unruly, the radical pruning then needed can cause dieback of a branch. Flowering plants should be pruned after a flowering flush, since most flower on the tips; trimming them back will encourage more flowers. It is perhaps better to ease back on the trimming during the cool/dry time, since it will take some time then for the new growth to fill back in.
Phalaria clerodendron is a tree, multitrunked to about 3-5 m, fast growing, with smallish shiny leaves and flowering 2-3 times a year with masses of scented, white tubular flowers, from the bark of the older branches.
Leea has a few forms, but basically a multiple leaf, sometimes quilted, sometimes shiny, sometimes green, sometimes deep claret purple, and heads of pink flowers, which are not that impressive, but the purple one is quite pretty. Phyllanthus is can be a ground cover for a while but then grows about a metre. It has tiny leaves, arching horizontal branches and insignificant flowers. All three are pretty tropical although Leea does OK as a pot plant in Brisbane, and maybe some Phyllanthus species too.