Exotic plants: The good, the bad and the ugly
"The exotic vegetation that replaces indigenous plant communities in urbanising regions, disassociates us from the rhythms and diversity of the native landscape and a sense of the place; and we are the poorer because of it."
Michael Hough, Professor of Landscape Architecture, York University, Canada
According to a spokesperson for Nursery & Garden Industry Australia (NGIA), nurseries and garden centres sell 94 percent exotic plants and 6 percent natives.
I hope to live long enough to see these figures reversed.
Two recent scientific reports reveal that 40 per cent of the most damaging weeds have escaped from Australian gardens, and that our love affair with exotic plants is creating diseases that can alter natural landscapes – forever.
When I was a student of my original career in osteopathy, we were reminded of the Hippocratic Oath: First, do no harm.
Applying the same rule to the environment, I ask:
- Is the plant imported?
- Does it compete successfully with indigenous species, for sunlight, nutrients and water?
- Has it established, or has it the potential to establish, self-propagating populations in native vegetation outside its natural range?
- Does it prevent or reduce the establishment of native species?
- Does it change geo-morphological processes, fire regimes or hydrological cycles?
- Does it change the nutrient content of the soil?
Among a small minority of ornamental exotic trees that may deserve a place in your garden, one of the best is Magnolia grandiflora, Southern Magnolia.
Southern Magnolia, native to south-eastern USA. is a fire retardant, non-invasive and usually well behaved tree that needs a moist, relatively fertile soil. On account of its many desirable qualities, including its visual compatibility with much of Australia’s native vegetation, I view this beautiful tree as an ‘honorary native’.
Southern Magnolia has large glossy green leaves, and produces large, fragrant, cream-coloured flowers in summer. Reaching around 25 metres or more, it is better suited to parks and large gardens. For small gardens, the dwarf cultivar Magnolia grandiflora ‘Little Gem’, that reaches only 7 or 8 metres, is the better choice.
Gardenia augusta and its cultivars Gardenia augusta ‘Magnifica’, ‘Florida’ and ‘Radicans’ are deservedly popular exotic shrubs. Gardenias all bear fragrant white flowers fading to cream. They are heavy feeders that need well-composted soil, warmth, sun, shelter, humidity and a fair amount of water.
Daphne odora, Daphne, is a compact plant that prefers morning sun or dappled shade. The small highly fragrant pink and white flowers open from mid-winter to late spring. It needs a moist, well composted alkaline or neutral soil.
Waterlilies (Nymphaea species and cultivars) are among the most beautiful of water plants. Of the seven species indigenous to Australia, most have very large leaves, including Nymphaea gigantea Giant Waterlily, that produces blue flowers from late spring to late autumn. Indigenous to Queensland and northern New South Wales, Giant Waterlily is too large for most garden ponds and seldom survives further south than Coffs Harbour NSW, the southernmost limit of its natural distribution.
Nymphaea capensis Cape Waterlily, introduced from South Africa, is hardier, equally beautiful, has smaller leaves and similar flowers, and is often seen in lagoons and dams, especially around the mid north coast of New South Wales.
With the notable exception of the invasive yellow-flowered Nymphaea mexicana, research indicates that Cape Waterlily and other exotic waterlilies have no adverse effects on the natural environment. Waterlilies all require at least eight hours of sunlight each day.
The above selection of exotic species is intended only as a guide. Gardeners with their own special favourites, need not hesitate to grow them, provided they present no threat to the environment and are closely monitored if necessary. (Tim Low, author of Feral Future, grows exotic buddleias in his Brisbane garden: It brings the butterflies and reminds him of the garden of his childhood.)