Invasive or weedy plants are particularly successful at passive dispersal, whereby seeds or fruits attach to a passing animal or bird. Active dispersal occurs when a species has developed a means of ejecting its seeds or fruits and propelling them a short distance. The ripened fruit of the Impatiens, for example, splits open readily and hurls out its seeds when it is bumped. Wind, water, and even some alien ants play a part in dispersing non-native plants. The variety we enjoy in our gardens as a result of unrestricted importation of non-native plant species has come at great cost to our native forests.
Green waste is a large part of the problem, since many invasive plants can propagate asexually.
By carelessly disposing of unwanted greenery, gardeners unwittingly cause the spread of species through vegetative reproduction. Also cited as sources of uncontrolled plant propagation are the use of bulk wildflower seed mixtures sold by local stores, and contaminants found in gardening and agricultural products such as soil conditioners and bird seed mixtures. The authors conclude with an appeal to collectors and field biologists to document and report plant dispersal mechanisms they observe. Mary Young is a free-lance writer in Honolulu.