The weeds of farmland are a diverse and economically valuable group of plants that have formed a durable botanical thread since the land was first opened for cereals and pasture over 5000 years ago.

Weeds are an economic burden when they take resources from crops, interfere with harvesting or contaminate yield. But only a few plant species cause economic loss at any time, and those few change over the years in response to trends in cropping.

Weeds are not all burden: many species provide food and shelter for insects, some beneficial to agriculture. They mop up fertilisers missed by the crop, so reduce pollution. Leguminous weeds can fix nitrogen from the air, so reduce the need for mineral fertiliser. Many weed species have nutritional or pharmaceutical properties that are still valued in cultures throughout the world.

Fifty years of chemical weedkillers have not exterminated weeds in north west Europe. The beneficials have been reduced, some to extinction, but the aggressive few have simply adapted. The future lies in limiting the economic burden of weeds, encouraging their beneficial aspects and discovering new economic products in their biochemistry and structure.

{5000 Years – Plants/Weeds is being moved over from the old site. New images and text to be added 19 September 2015.]

Content of the weeds pages

Introduction, global view

Of all the organic things that interfere with growing crops, the in-field weeds are probably the most important worldwide, more important economically than pathogenic diseases and the insects. Because many of the weeds are what are called generalists, they readily invade and adapt to disturbed cropped habitats anywhere.

They are weeds because they do one or more of three things: deny the crop resources of solar energy, water and nutrients; interfere with field operations by dragging down the crops or jamming up machinery; contaminate the harvest. Unlike many pathogens and insects that are specialists, weeds regard fields, whether in cereals, legumes, oilseeds or vegetables, as just another opportunity. Some weed species occur throughout vast tracts of land. Others may be a rare wild plant in one region but spead aggressively in another.

The agricultural weeds come from many different plant families. They tend to release great quantities of pollen, either directly to the air or on the bodies of insects. They reproduce variously by seed and by vegetative materials. The most successful are highly variable genetically, occurring in different forms that – in an evolutionary sense – continually outwit their adversaries.

However, it’s not just the damaging weeds that cohabit with crops in fields. Many other plant species have taken up residence there that are a negligible threat to crops and more importantly are a benefit both the working of the agro-ecosystem and to human existence through their value as a source of medicines, oils and other substances. Many a market in eastern europe will have a good selection for sale of what we call weeds. Chinese medicine has taken the use of herbs, some of them weeds, to very high levels. The dye plants such as indigo, woad and weld could all be classed as weeds in some circumstances.

Modern intense agriculture barely distinguishes the economic threats from the rest, and it’s the neutral or beneficial ones that have suffered the greatest declines in the last 50 years. Some are now more common in gardens than in fields. Yet many of them are still around in fields, as we shall see, and have still to reveal their economic potential.