Weeds by Nina Edwards

Author:Nina Edwards
Language: eng
Format: epub, pdf
Publisher: Reaktion Books

A weed sprayer used on an urban railway in Yakima County, Washington.

This . . . requires special crops built to withstand these chemicals, which, naturally, results in special weeds which can resist as well . . . it changes . . . the spirit of weeding, where we carve out our little plot from the wilderness; it changes the game into a war. When modern chemical farms spray broad spectrum herbicides, they do so much damage, to all plants, that they are forced to find chemical solutions to problems that used to be solved by nature itself. Using mass amounts of chemical fertilizer, much richer in nutrients than normal old compost or composted manure, to restore the nutrients leached from the soil by harsh growing of monocultures in herbicide-sterilized land, is an example of the slippery chemical slope.17

It is hardly surprising that since weeds are weeds partly because of their ability to survive, then those for whom a successful herbicide has been found may find a way round the problem. The common groundsel, apparently vanquished by atrazine and simazine since the late 1950s, began to develop resistance, the first confirmed example of herbicide resistance. In Australia ryegrass has become immune to at least nine different herbicides. Stephen Powles of the University of Western Australia in Perth suggests that ‘herbicide resistance is a fantastic example of evolution in response to human-induced selection pressure’,18 with plants mutating in leaf shape and waxiness, and even changing their molecular structure.

Some plants inhibit the growth of others by themselves producing natural herbicides. These ‘allelopathic’ compounds are nothing new. The dodder plant found near a crop can be distracted away by tomato plants if they are grown nearby. Both tomato and wheat contain the chemical beta-myrcene, to which the dodder is attracted. However, the tomato plant also contains two other chemicals, and their combined bouquet is more attractive to dodder than beta-myrcene alone. Wheat also contains a chemical that repels dodder.19 Theophrastus mentions weeds being inhibited by chickpea crops around 300 BCE; Pliny the Elder in 1 CE has corn chickpea, barley and bitter vetch scorching corn.20 Weed scientists are investigating ways of using allelopathics to favour crops over weeds. Since weeds often gain ground by growing up before the crops have taken hold, one method is to sow a winter cover crop capable of inhibiting the main season’s crop weeds. They can also be used as a mulch during the cropping season, or the allelopathic compounds might be extracted and applied as a herbicide. Furthermore, synthetic herbicides have been derived from these allelopathic species.

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