Thursday, September 10, 2009

Gardener’s guide to selecting non-invasive alien plants

Reproduced from the Tress Society of Zimbabwe - September Newsletter:

Do you want to make an environmentally responsible choice of plants for ornament, hedging, landscaping etc? Then consider the following guidelines in selecting species:
• DON’T select known alien species e.g. lantana (Lantana camara hort.).
• AVOID alien species that are similar to known invasive species—e.g. treat ALL species of alien acacias, inkberries (Cestrum spp.), cotoneasters, firethorns (Pyracantha spp.), lantanas etc. as potentially invasive. Substitution of a known invader with another plant having similar attributes will negate efforts to prevent further invasions.
• AVOID alien species that have fleshy fruits—many of the most troublesome invaders are spread by frugivorous birds, bats and other animals e.g. lantana, mulberry, guavas, privets, bugweed, syringa, prickly pears, queen of the night cactus, firethorns, cotoneasters, eglantine rose, brambles, ginger-lilies. Australian brush-cherry (Syzygium paniculatum is often sold as a ’good bird plant’ - and is on its way to becoming invasive!)
• AVOID alien aquatic species—all alien species that are climatically adapted have the potential to become invasive, e.g. sword plant (Echinodorus spp.)
• AVOID alien grasses—which are among the most invasive species in the world. Many spread prolifically from seed, underground stems (rhizomes) and runners (stolons). Fountain grass (Pennisteum setaceum), for example, is becoming increasingly invasive.
• AVOID alien climbers—which have a devastating effect by smothering the native vegetation; they are extremely difficult to control. English Ivy (Hedera helix) provides one good example. Cat’s claw (Macfadyena unguis-cati) is another.
• Avoid alien succulents—some of which have become the most costly and troublesome invasive species in the region, e.g. sour prickly pear (Opuntia stricta). Cacti are ALL alien in southern Africa (with the exception of a single small epiphytic forest species of Rhipsalis). The alien cacti are often confused with the indigenous euphorbias—don’t be fooled into purchasing a cactus that has been incorrectly labelled as euphorbia. Cacti have spines arising from specialized structures called areoles and the spines usually occur in clusters; they do not have a milky latex like the euphorbias. Agaves or American “aloes” with their tall flowering poles should not be confused with indigenous Aloe species with their showy, orange, red or yellow flowers.
Common misconceptions: cultivars and sterile plants
• Cultivars are OK because they can only be reproduced by cuttings, tissue culture etc. WRONG!—many cultivars e.g. variegated forms are able to reproduce by seed and their progeny revert to the original form, e.g. Chinese wax-leaved privet (Ligustrum lucidum) (there are two forms: normal and variegated). Queen of the night (Cereus jamacaru) has a monstrous form that sets seed which develops into the normal form.
• “Sterile” plants are OK because they do not reproduce from seed. WRONG!—some so-called “sterile” plants, e.g. pure yellow–and pure white-flowered, bushy lantana (Lantana camara hort.) produce fertile pollen that can cross-pollinate the normal fertile plants, as well as fertile embryo sacs that occasionally produce seed.
• The creeping, yellow Lantana cultivar Sundancer is marketed as “Lantana montevidensis”, “sterile” and “non-invasive”, however studies in South Africa and Australia indicate that it is partly L. camara, only sterile if planted in isolation, and inter-fertile with L. camara. Research in America indicates that a very similar cultivar, Goldrush, is probably a hybrid derived from the invasive Lantana camara complex and North American Lantana depressa. While there is no evidence that Sundancer and other yellow-flowered creeping lantanas are invasive, they are probably harmful in a much more subtle way—by swapping genes with the invasive lantana and adding the genes of L. depressa and possibly other Lantana species to the gene pool of the invasive, alien lantana complex, almost certainly increasing its genetic heterogeneity, hybrid vigour, resistance to biocontrol agents, invasiveness and suppression of indigenous biodiversity.
• Many sterile plants can reproduce vegetatively—from plant fragments e.g. jointed cactus, weeping willow, salvinia; others reproduce by suckering from the roots e.g. white and grey poplars; others from underground stems or rhizomes, e.g. giant reed, and from aerial tubers, e.g Madeira vine (Anredera cordifolia)
• Plants with only one sex present are OK because they can’t produce seed. WRONG! — purple pampas grass (Cortaderia jubata), for example, can produce seed without pollination (apomictic reproduction)

Reproduced (with minor editing) in the interests of science from SAPIA NEWS No. 10, January 2009.
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