The use of “weedy” species, together with the use of non-local species of unknown weediness, incites very heated debate in permaculture circles, inspiring a lot of name-calling and pseduo-science. My own view is that the promotion of known invasive, or potentially invasive, species as part of the permaculture approach is highly irresponsible and arguments for their use are often couched in language that approaches the mysticism or spirituality that is supposedly “banned” in serious permaculture. (You can see my views on the use of Leucaena here.)
Of course, the topic of invasive species overlaps with the consideration of “weeds” as an element of food productions activities. I’m not going to go into that here, but the eXtension website has an excellent article on weeds in agriculture (An Ecological Understanding of Weeds) that incorporates both the negative and positive aspects of weeds in agriculture. Well worth reading – much of it reads as if it was written by an well-informed permaculturist. In fact the author, Dr Mark Schonbeck, is credited by the the Virginia Association for Biological Farming as combining “deep scientific knowledge, practical farming technique and policy smarts”. I recommend googling his name – it will turn up a plethora of interesting and informative articles.
Dr Schonbeck’s article also deals with invasive species, under the heading of Imported Weeds and Invasive Exotic Plant Species. Among other things this part blows out of the water the permaculture argument that for a species to become invasive in an ecosystem there must have been a vacant niche in the ecosystem.
Below is the text of that part of the article (with what I think are the really telling points underlined by me), but I really recommend you read the whole article for its information about the place of weeds in food production systems. Remember that the weed/invasive species referred to are in relation to the US, though it is interesting how many are familiar to us in Australia.
Many of a region’s most problematic weeds are those that are not native to the region, or even the continent. These exotic plant species often grow more vigorously in their new habitat than they do in their area of origin, where certain soil organisms, herbivorous insects, climate patterns, and/or competing vegetation keep them in check. Kudzu (Pueraria thunbergiana), imported from Japan as a forage crop, is one dramatic example whose enormous vines can cover and kill large trees in the southeastern US. However, a small (4–18 inches) perennial weed called purple nutsedge (Cyperus rotundus), which has invaded the southern United States, causes much greater losses in cultivated crops (even sugarcane and coffee trees), and is considered the world’s worst weed (Holm et al., 1991).
Some of our major agricultural weeds were intentionally brought to the United States from overseas to provide food or forage. European colonists carried common lambsquarters (Chenopodium album) to the New World as a vital source of early season greens that prevented scurvy and other nutritional deficiency conditions. Common lambsquarters has spread around the globe and is now listed as the world’s 10th worst agricultural weed. Livestock farmers imported bermudagrass (Cynodon dactylon, 2nd worst weed), johnsongrass (Sorghum halapense, 6th worst), and quackgrass (Elytrigia repens, a major weed of vegetable crops in the northeastern US) to this country for their utility as forages. Other serious exotic weeds were first planted as flowers and other ornamentals, and subsequently spread from cultivated gardens into surrounding farmland and/or natural ecosystems. Still others arrived by accident as a seed contaminant in imported crop seed, feed grain, foods, bedding plants, or other materials.
Many exotic weeds have become “naturalized” over time, and are now part of a region’s agricultural weed flora that must be managed (not necessarily eradicated) to protect crop yields. However, some newly introduced plants growing in the absence of the natural enemies with which they evolved may spread unchecked, choking out native vegetation as well as invading pastures or cultivated fields. Imported weeds that threaten natural ecosystems and/or rangeland over wide geographic areas are designated invasive exotic plant species or invasive exotic weeds, and often become the focus of regional or nationwide coordinated eradication efforts. Examples include water hyacinth (Eichhornia crassipes) and purple loosestrife (Lythrum salicaria) in wetlands; Canada thistle (Cirsium arvense), musk thistle (Carduus nutans), spotted knapweed (Centaurea maculosa), and St. Johnswort (Hypericum perforatum) in rangeland; and autumn olive (Elaegnus umbellata) and tree-of-heaven (Ailanthus altissimus) in eastern deciduous woodlands. Classical biological control—the introduction of herbivorous insects or microbial pathogens that are natural enemies of these weeds in their native lands—has been used with considerable success to combat some invasive exotic weeds.
Arrival of a new invasive exotic weed on an organic farm is one instance that may justify efforts to eradicate the new arrival. Early detection—combined with an understanding of the ecology of the weed—is vital for successful elimination of the invader.