When people hear the word “weed” they usually think of some nasty, ugly herbaceous plant ruining their flowerbeds, such as bindweed or Mallow. Weeds are most commonly unwanted plants because they are deemed “ugly”. Actually any plant, wild or cultivated, is a weed if it is growing where it is unwanted. The worse types are those that are difficult to control. The very worst, are those that are virtually uncontrollable and as a result do tremendous damage not only to parks and gardens but to the local environment as well.
In this latter category are many ornamental garden plants, from herbaceous perennials to trees and shrubs. The problem is that they propagate themselves so vigorously, usually by prolific seed production and germination rates, that they not only sprout up everywhere within the garden, but are liable to escape into the surrounding countryside. Those species that establish themselves and start to spread are called invasive alien species. The affect on a local eco-system is liable to be catastrophic. Why is this so?
Non-native plants that are able to survive in the wild are likely to have a number of crucial advantages over native species. Over the millennia, complex associations develop between the mass of organisms, which control and regulate populations; predation and parasitism being an integral part of the eco-system. It often happens though, that alien species have no natural predators, or diseases and pests that keep their numbers down. Consequently, they start taking over a particular area, reducing and even eliminating the native species in the process. The once richly, varied habitat becomes the domain of a very limited number of plant species, which in turn devastates the fauna that had developed in association with the flora.
In Israel where I come from, a number of introduced plants have wreaked havoc with local habitats. Species of Acacia from Australia have taken over much of the Mediterranean coastline; Ailanthus altissima (Tree of Heaven) from China, is wiping out the native oak-pistachio communities in hilly country, while the decorative shrub, Lantana camara from America, is a major pest in gardens and beyond.
Similar problems are occurring all over the world. Apparently, California has also been “taken over” by the Tree of Heaven, while in many habitats in Australia, ornamental varieties of Melaleuca (splendid bushes!) are pushing out the native species of the same genus. Virtually all the culprits are escapees from agriculture or gardening.
The question is what can we, professional and home gardeners alike do about it? It is a question that we have to ask ourselves because unhappily, we are partly responsible for the problem. Modest steps in the right direction can be taken by refusing to plant those species that are suspected of being invasive. Make contact with local environmental groups for advice. We should also be reducing our appetite for new, exotic plants feature Articles, because it can take years before the invasive properties of a particular plant become apparent.