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Hawaii's Most Invasive Horticultural Plants:
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Hawaii's Most Invasive Horticultural Plants:
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One of the major threats to Hawaii's native species and forests is the rampant spread of a large number of invasive alien plant species across the state. These plants displace Hawaii's distinctive native flora, resulting in the loss of diverse native forests that support a large array of native animals. Preservation of the array of native plants and animals that make Hawaii unique must confront the problem of invasive alien plant species.
The ornamental plant trade accounts for the majority of invasive plant introductions to Hawaii. This website is designed to educate the concerned public about why this has occurred and to inform them about those invasive alien plants that they should avoid using so as to reduce the threat these invasive species pose to Hawaii's native ecosystems.
| In considering how to stem the flood of plant invasions in Hawaii it is important to remember that the vast majority of these invasions don't "just happen" - they result from conscious choices made by humans to plant invasive species. Even though it is tempting to think that planting some beautiful tree or shrub in one's backyard, along the street, or in an agricultural lot is harmless, the fact that seeds of many of these plants are widely dispersed by birds or wind means that these seemingly inoccuous plantings can easily impact what remains of native Hawaii, even if that impact is not easily seen by the original planter. It is important to remember that the multi-million dollar control effort against miconia (Miconia calvescens) was necessitated by the actions of only one or a few individuals who decided to introduce this invasive pest to Hawaii. The prudent choice for the responsible horticulturist is to avoid such invasive plants and, instead, landscape or garden with either natives or non-invasive alien plants. At the very least, the plants depicted on this website should be avoided. |
| For more information on the latest invasive pest assessments: UH Botany: Weed Risk Assessment |
In addressing the invasive plant threat to our native ecosystems, it is important to bear in mind some important distinctions. First, the term
alien speciesrefers to a species transported or established outside its native range by the activities of humans, whether done so intentionally or not. This definition does not imply that human-mediated dispersal of species is inherently unnatural, but it recognizes that the rate at which humans are homogenizing the world's diverse biota is occurring at a scale previously absent in Earth's evolutionary history. For example, it has been estimated that the rate of new species establishment in the Hawaiian Island was approximately one new species every 35,000 years prior to human arrival in the islands; it is now on the order of 20-30 species/year, an approximately million-fold rate increase. Not all alien species, however, pose a threat to Hawaii's native forests and species; in fact, only a small fraction do. Those that are a problem are termed
invasive species, which are alien species that significantly disrupt the community structure or proper function of an ecosystem. Of the approximately 13,000 alien species of plants that have been introduced to Hawaii, only about 1% (130 species) of those have become invasive so far. Biological evidence suggests another 200-300 species already present in the state may become problems in the future. Given these distinctions, it must be emphasized that efforts to protect Hawaii's native ecosystems and species from destructive alien species is focussed only on invasive alien species and not all alien species
per se.
This is the most direct means of demonstrating invasiveness, but reliance on this method is of limited usefulness in protecting Hawaii because by the time evidence of invasiveness is locally available it is typically too late to effectively control the problem. Secondly, one may rely on evidence of the behavior of particular plant species in similar habitats elsewhere. For example, if a particular plant has been shown to be invasive in, say, Fiji, it is likely to be invasive in Hawaii as well because of the similar habitats in the two archipelagos. This method does not guarantee that a particular plant will be invasive in Hawaii but it does make it quite likely to be so. The strength of this line of reasoning is that it can be used proactively to entirely avoid introducing destructive plants to Hawaii or can be used to remove them at an early stage in the invasion process. For example, cogon grass (Imperata cylindrica) is widely destructive throughout the Old World tropics and in Florida and, consequently, is banned from importation into Hawaii. Similarly, Chinese privet (Ligustrum sinense) is highly invasive in the southeastern United States and in Australia, has started to form dense thickets in a small area of Kauai, and is, consequently, the target of a compaign to remove it from Kauai before it causes lasting damage.
By not planting these species we can spare some of our islands from suffering the ecological disruptions that have resulted from these species elsewhere in the state. An example is fountain grass (Pennisetum setaceum), which has been tremendously destructive so far only on Big Island because active control programs are working to keep it off or remove it from the other islands. For these species, it is early enough to stop them from becoming major ecological problems everywhere in Hawaii by discontinuing their planting and by removing known plants.
Because plants do not invade native ecosystems by simply pulling up their roots and moving there, it should be clear that plants disperse to new areas via movement of their seeds and spores. These propagules can disperse by a variety of mechanisms, but those propagules adapted to be spread by animals or wind can most easily move long distances. Hence, plants using animals or wind as dispersal mechanisms are capable of quickly invading native ecosystems in areas remote from where the adults themselves are planted. Because of this trait alone, many plants that have animal- or wind-dispersed propagules have high potential to be invasive in Hawaii. All such species, even if not listed on this webpage, should be viewed with caution as ornamentals. An exception to this rule is most orchids, which have small wind-dispersed seeds but have not usually been invasive because they generally require specialized pollinators, which are absent from Hawaii, to fertilize their seeds. Of course, if orchid pollinators were introduced to Hawaii this situation could change.
Animal-dispersed seeds are typically fleshy berries, relatively small in size, and variously colored red, orange, yellow, black, or bluish-black. The dispersers of greatest importance in Hawaii (as elsewhere) are fruit-eating birds, but some mammals, such as pigs, are also important dispersers of some alien plant fruits. Among the species illustrated on this webpage, a quick perusal will make it obvious how large a percentage of our invasive plants possess bird-dispersed fruits.
All else being equal, plants that produce many seeds per plant each year are far more capable of quickly invading native ecosystems than are those that produce relatively few seeds per year. For example, miconia (Miconia calvescens), which is the subject of a multi-year control effort by State, federal, and private organizations, is capable of producing several million seeds per year per plant, making its rate of population increase explosive and partially accounting for the great threat it poses to Hawaii's forests.
All else being equal, fast-growing plants that quickly reach maturity will be more invasive and harder to control than slower-growing plants. An outstanding example of the importance of this phenomenon is salvinia (Salvinia molesta), a floating aquatic fern, which under ideal conditions is capable of doubling its population size every 2-3 days, quickly choking out water bodies that it infests.