Most of the incidences of the introduction of plants primarily for hedge/windbreak purposes, and which later became invasive, occurred in the 1800s, before any risks were associated with these species. Characteristics of good hedge plants, the presence of spines or thorns, a dense multi-stemmed shrubby habit, and unpalatability or tolerance to browsing, are also shared by invasive species. The risks associated with international introduction of potentially invasive plants have been well highlighted in recent decades, though seed of many of these is still available from mail-order and internet-based companies. The main risks, however, are likely to concentrate on local spread, as farmers continue to plant live fences from plants already in the area.
In the Forestry Compendium (CABI, 2005), there are 524 tree species recorded as being used for either hedges, windbreaks or shelterbelts, and of these, 94 were either invasive, aggressive, or noted to have a tendency to become a weed. However, many of the characteristics of good windbreak trees (fast growth, tall, narrow crown, wind-firm, etc.) make them good plantation species, so it is rare that a species would have been introduced principally as a windbreak tree. For example, Casuarina spp. are commonly planted as windbreaks in the tropics as are Cupressus spp. in Mediterranean regions, but they are also frequently used as forestry and ornamental species. Hedge plants, however, generally have unique characteristics uncommon in forestry, such as having spines or thorns and a dense, low-growing form, and resistance or tolerance to browsing.
Of the 463 invasive plants in the Crop Protection Compendium (CABI, 2007), the following 23 species (almost 5% of the total) are (or have been) used for hedging or as a hedge plant: Acacia karroo, Caesalpinia decapetala, Casuarina glauca, Dichrostachys cinerea, Genista monspessulana, Gliricidia sepium, Hakea drupacea, H. gibbosa, H. sericea, Jatropha curcas, Lantana camara, Ligustrum robustrum, Lycium ferocissimum, Opuntia stricta, Parkinsonia aculeata, Pithecellobium dulce, Psidium cattleianum, Rhamnus cathartica, R. frangula, Rubus fruticosus, Tamarix aphylla, T. chinensis and Ulex europaeus. Most are multi-purpose species, but providing a stock-proof or ornamental hedging is recorded as the main reason for long-distance introduction of just over half of these; highlighted below.
Caesalpinia decapetala was widely introduced to form a security barrier against people or wild animals, and is also regarded as a striking ornamental.
Dichrostachys cinerea has a number of environmental uses for example in agroforestry, soil improvement, revegetation, land reclamation, soil conservation and erosion control, and has been widely planted for hedging and live fencing.
Hakeadrupacea, H. gibbosa and H. sericea were introduced from Australia to South Africa primarily as a hedge or barrier plant, but also for sand stabilization and firewood production (Shaughnessy, 1986), and also to New Zealand for the same reasons.
Lantana camara has been introduced throughout the tropics and subtropics, often used as a hedge plant, and in East Africa, in locations where it is not weedy, it has been used effectively as a live fence (Howes, 1946).
Ligustrum robustrum was introduced as a hedge species in Mauritius in 1895 (Rouillard and Guého, 1990). It continues to be used as an ornamental species, and is commonly found as a hedge plant in La Réunion.
Lycium ferocissimum was widely introduced from South Africa to Australia as a hedge plant and had become naturalized by the mid 1800s (Parsons and Cuthbertson, 1992), and it was also introduced to New Zealand at the same time for the same reasons, and whereas it is no longer grown for this purpose, many of the non-coastal invasions have spread from old hedges.
Opuntia stricta has also been used as a hedge plant, and the showy flowers and attractive fruit also make it a popular pot and hedge plant and it is by this means that it probably arrived in Australia prior to 1839 (Mann, 1970).
Parkinsonia aculeata is widely used as a thorn hedge and as an ornamental, shade or shelterbelt tree (Howes, 1946; Troup and Joshi, 1983; Webb et al., 1984). It was promoted in Australia as an evergreen hedge (von Mueller, 1888) and by British colonialists as an ornamental, for soil fixation and as a hedging plant (Streets, 1962; Troup and Joshi, 1983). It continues to be used and promoted for ornament, shade, windbreak and hedging purposes (Madany, 1991).
Rhamnus cathartica and R. frangula have been used extensively as a hedgerow and ornamental plant. By the 1900s they had become naturalized and widespread in parts of North America where they were used as a hedge, shelterbelt or ornamental.
Ulex europaeus has been introduced intentionally to many temperate parts of the world as a hedge plant to contain grazing animals, as fodder, and even to assuage the nostalgia of European colonists (Richardson and Hill, 1998). It was intentionally introduced to New Zealand as a hedge plant, naturalized quickly, and was declared a noxious weed in 1900 (MacCarter and Gaynor, 1980). It was introduced to Australia for the same reasons before 1845, naturalized by 1889, and was a recognized weed by 1909 (Richardson and Hill, 1998).
In addition to these examples, two species are notable for their popular use a hedge plant, which is likely to be a common reason for their spread locally: Gliricidia sepium is mainly valued as a fodder and shade tree, but as cuttings root extremely easily (‘quick stick’ being one of its common names), it is often planted as a hedge. Also, Jatrophacurcas is mainly valued for the oil produced from its seeds, but being unpalatable and the wood being valueless, it is often grown as a hedge (‘hedge castor oil plant’ being one of its common names).
Species are introduced via seed, and only very rarely as plants or parts of plants (such as probably occurred with Opuntia spp., where the cladodes, or leaf pads, are the main means of propagation). Seed of very many invasive trees are still available from mail order companies and can be purchased via the internet.
Shipping routes in the 1800s provided the main routes for introduction of hedging plants, between countries of similar climates, e.g. South Africa and Australia in both directions, from Europe to North America and New Zealand, and from Central America pan-tropically. In recent years, seeds of invasive ornamental hedging plants are sent by airmail and thus arrive at airports and not sea ports as before.
All introductions of trees and shrubs for use as hedges or windbreaks were intentional.
Many of the early introductions of hedging plants in the 1800s escaped and became invasive only a few years after they were first planted, thus limiting further introductions of similar species in the same areas, as farmers and landowners had learnt from their earlier mistakes. Nonetheless, other introductions were still made in the 1900s, but with increasing awareness of the risks, educational campaigns began, aimed directly at the ornamental plant industry. These have had an impact in some developed countries, notably the USA, Canada, South Africa, Australia and New Zealand, as well as some small island states. The costs of control of escaped environmental weeds have been shown to far outweigh any economic benefits from the trade of such species, and there were even moves to introduce a ‘polluter pays’ principle.
Regarding mail-order purchases of seed of invasive hedging species, the only possible control is through the checking of incoming post, as occurs in New Zealand to good effect. However, this is generally limited in effectiveness, and education via public and corporate awareness campaigns is more likely to have an impact and at lower overall cost. Regarding further spread of such species already introduced into an area, targeting plant fairs and markets, and individual suppliers have been shown to reveal the extent of supply of invasive species. In the aforementioned countries, there is also a move to have plant suppliers agree to a voluntary charter whereby they aim to use native species as alternatives where possible.
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Madany MH, 1991. Living fences: Somali farmers adopt an agroforestry technology. Agroforestry Today, 3(1):4-7.
Mann J, 1970. Cacti naturalized in Australia and their control. Cacti naturalized in Australia and their control. Brisbane, Australia: Department of Lands.
Mueller F von, 1881. Select extra tropical plants, ready eligible for industrial culture and naturalisation. 7th edition. Melbourne, Australia: Government Printer.
Richardson RG; Hill RL, 1998. The biology of Australian weeds. 34. Ulex europaeus L. Plant Protection Quarterly, 13(2):46-58; 5 pp. of ref.
Rouillard G; Guého J, 1990. [English title not available]. (Le Jardin Botanique de Curepipe.) Le Jardin Botanique de Curepipe. Curepipe, Mauritius.
Shaughnessy GL, 1986. A case study of some woody plant introductions to the Cape Town area. In: The ecology and management of biological invasions in southern Africa [ed. by Macdonald, I. A. W.\Kruger, F. J.\Ferrar, A. A.]. Cape Town, South Africa: Oxford University Press, 37-43.
Troup RS; Joshi HB, 1983. Vol IV. Leguminosae. Troup's the silviculture of Indian trees. Delhi, India: Controller of Publications.
Webb DB; Wood PJ; Smith JP; Henman GS, 1984. A guide to species selection for tropical and sub-tropical plantations. Tropical Forestry Papers, No. 15. Oxford, UK: Commonwealth Forestry Institute, University of Oxford.