Chapter 29: Weeds and weed control
Ralph Waldo Emerson defined a weed as ‘a plant whose virtues have not yet been discovered’, but in fact a number of weeds were known to have ‘virtues’ and were spread by humans intentionally. Dyer’s woad is a significant pest in the western U.S. but was brought to North America intentionally for its use a source of blue dye (hence the common name). Many weedy plants, e.g. purple loosestrife (Fig. 1), were intentionally introduced by gardeners who found them attractive. A large number of weed organisms of all types are similarly the result of intentional introduction in new habitats (honeybees, earthworms, gypsy moth, starlings and many more).
Is there a better definition of weed? Ecologists consider organisms to be ‘weedy’ if they have a suite of characteristics (high reproductive potential, extensive dispersal abilities, rapid growth rate, ability to reproduce even under adverse conditions) that would make them likely to appear and survive in disturbed habitats. Disturbed habitats, sometimes called ruderal habitats, are areas that have once been populated but some disturbance, e.g. fire, has eliminated what once lived there. Significant for terrestrial habitats, they generally are sites with developed soils, as opposed to sites that were never vegetated and have no soil. So although ecological weeds are considered ‘pioneer species’, they are not pioneering onto previously uninhabited land with very young soils. Ecological weeds can colonize and quickly dominate a disturbed site but may not persist because they often are unable to compete with other species that eventually arrive at the site.
But ‘weed’ is not used just by ecologists and its definition is both utilitarian and subjective:
‘A plant that interferes with the management objectives at a particular location. It is a plant growing where it is not wanted. Under certain situations, the plant may not be totally undesirable.’
This definition focuses on plants but undesirable organisms come from the entire spectrum of life, including the non-plant organisms considered in this book: cyanobacteria, green algae, red algae, diatoms, dinoflagellates and fungi.
Weeds are ‘pests’, organisms that are undesirable for reasons that are specific to a particular situation and, as noted above, their undesirable nature depends upon situation.
- honeybees are pests when they nest in houses or perhaps when they outcompete native bee species
- earthworms are desirable in agricultural situations but are weeds in native habitats where they can alter conditions and disrupt the native community
- dandelions are weeds in lawns, where some consider them unsightly, but are useful to pollinators and to foragers who eat their leaves and flowers
- sandbur is highly undesirable on beaches because of its sharp fruits that are painful to step on and stick to clothing
- dyer’s woad clearly is (was) useful as a source of dye but if one is managing land for other purposes then it is a weed
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