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Your definition of a weed may depend on your closeness to the soil. To a gardener a weed is any plant that he/she didn't plant. To an ecologist weeds are plants that are adapted to disturbed habitat.
There are several characteristics that are common to weedy species. Plants that are able to germinate and reproduce in open areas where conditions are most extreme are more likely to become weeds. Annuals are favored because of their rapid production of offspring. Plants which produce large numbers of seeds will have the advantage of rapid spread into recently disturbed areas.
A few of our most aggressive weeds ie. polkweed (Phytolacca americana), ragweed (Ambrosia trifida) and Poison Ivy (Rhus radicans) are native species. Some of our western weeds are native to the Eastern U.S. but were inadvertently carried west with the pioneers. Such plants as evening primrose (Oenothera biennis), horseweed (Erigeron canadensis), black-eyed susan (Rudbeckia hirta), and the grass (Panicum capillare) have their origins in the Eastern U.S.
But, the majority of our weeds are alien species. Before the Europeanizing of America there was little disturbed habitat. The Eastern U.S. was well forested and the relatively sparse population of Indians cleared only small patches of land in rivers bottoms. With the coming of the Pilgrims forests were cleared, fields plowed, road built and small villages began to dot the landscape. All this Europeanization greatly increased the amount of disturbed ground.
Few American plants were pre-adapted for this ecological niche. But, with the seed and feed brought from the old world were the seeds for pre-adapting European weeds. Other weeds were intentionally introduces as agricultural crops or horticultural cultivars. Whitney states that 218 species of eastern weeds have escaped from medicinal or horticultural gardens and 40 species are agricultural escapees. Glechoma hederacea (Gill-over-the ground/creeping Charlie), Prunella vulgaris (heal all), Achillea millefolilum (yarrow), doc (Rumex sp.) Linearea vulgaris (butter-and-eggs),Chrysanthemum leucanthemum (ox-eye daisy) are just a few example of horticultural escapees.
Foreign ships with light loads would fill their holds with dirt and gravel from near by wetlands to act as ballast. Before loading with raw materials from the new world the unwanted ballast would be dumped near dock side frequently inoculating the new world with old world seed and spore. Loosestrife (Lythrum salicaria), currently spreading throughout the wetlands of the northeast out competing our native wetland species, is believed to have been such a ballast propagated species.
Raw wool from the old world was ripe with seeds. The weedy seeds from the cleaning of the wool were yet another means of introducing old world weeds.
Seymore's (1969 as quoted by Whitney, 1994) Flora of New England lists 877 foreign species which comprise approximately 30% of the flora of this region. Twenty percent of the Midwestern species are alien and the percentage is still increasing.
Pathogens too have come in with these plants. Anthranose (Discula destructiva) the fungi which is currently infecting our dogwoods is believed to have been imported from China on infected nursery stock of kousa dogwoods. The fungi almost simultaneously began to affect native dogwoods in Portland, Oregon and New York in the late 1970s. Both ports received shipments for the oriental dogwoods. Chestnut blight (Cryphonectria parasitica) and Dutch elm disease (Ceratocystis ulmi) all too well known as disastrous parasites inadvertently added to the North American flora.
With all the beautiful native plants it is time to curb the use of introduced species and cultivate/propagate are native species. These cultivars world be healthier, require less fertilization, less water and would eliminated the danger of the introduction of noxious weeds and pathogens.
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