Herbicides and Plant Physiology. Andrew H. Cobb

Chapter 1 An Introduction to Weed Biology

      One year’s seed is seven year’s weed.

      A traditional rhyme

1.1 Introduction

      The human race has been farming for over 10,000 years. Weeds have been an unwelcome presence alongside crops ever since the first farmers saved and planted seeds in the region that is now present‐day Turkey and the Middle East. Indeed, when these early farmers noticed a different plant growing, decided they did not want it and pulled it up, they were carrying out a form a weed control that is still used today: hand roguing.

      But what are weeds? Weeds are all things to all people, depending on the viewpoint of the individual. To some they are plants growing where they are not wanted; to others they are plants growing in the wrong place, in the wrong quantity, at the wrong time; and to some they are regarded as plants whose virtues have yet to be fully discovered! The need to control weeds only arises when they interfere with the use of the land, and this is usually in the presence of a crop, such as in agriculture and horticulture. Weed control may also be necessary in other situations including amenity areas, such as parks and lawns, in water courses, or on paths and drives where the presence of plants may be regarded as unsightly. It should not be overlooked, however, that weeds contribute to the biodiversity of ecosystems and should only be removed when financial or practical implications make their presence unacceptable. With this in mind an appropriate definition of a weed is:

      Any plant adapted to man‐made habitats and causing interference of the use of those habitats. (Lampkin, 1990)

      Or

      A plant whose virtues remain to be discovered. (Emerson, 1912, see https://theysaidso.com/quote/ralph‐waldo‐emerson‐what‐is‐a‐weed‐a‐plant‐whose‐virtues‐have‐never‐been‐discovered)

1.2 Distribution

On a global basis only about 250 species are sufficiently troublesome to be termed weeds, representing approximately 0.1% of the world’s flora. Of these, 70% are found in 12 families, 40% alone being members of the Gramineae and Compositae. Interestingly, 12 crops from five families provide 75% of the world’s food and the same five families provide many of the worst weeds (Table 1.1). This implies that our major crops and weeds share certain characteristics and perhaps common origins.

Table 1.1 Important plant families which contain both the major crops and the worst weeds of the world.

      Source: Radosevich, S.R. and Holt, J.S. (1984) Weed Ecology: Implications for Vegetation Management. New York: Wiley. Reproduced with permission of John Wiley & Sons.

Number of species classified as the world’s worst weeds (%)	Family	Examples of major crops	Examples of major weeds	Common name
44	Gramineae	Barley, maize, millett, oats, rice, sorghum, sugar cane and wheat	Elytrigia repens (L.)	Couch
			Alopecurus myosuroides (L.)	Black‐grass
			Avena fatua (L.)	Wild oat
			Sorghum halepense (L.) Pers.	Johnson grass
			Echinochloa crusgalli (L.)	Barnyard grass
4	Solanaceae	White potato	Solanum nigrum (L.)	Black nightshade
			Datura stramonium (L.)	Jimsonweed
			Hyoscyamus niger (L.)	Henbane
5	Convolvulaceae	Sweet potato	Convolvulus arvensis (L.)	Field bindweed
			Cuscuta pentagona (Engelm)	Field dodder
			Ipomoea purpurea (L.) Roth	Tall morning glory
5	Euphorbiaceae	Cassava	Euphorbia maculata (L.)	Spotted spurge
			Euphorbia helioscopia (L.)	Sun spurge
			Mercurialis annua (L.)	Annual mercury
6	Leguminosae	Soybean	Cassia obtusifolia (L.)	Sicklepod
			Melilotus alba (Desc)	White sweetclover
			Trifolium repens (L.)	White clover

1.3 The importance of weeds

      Most plants grow in communities consisting of many individuals. If the resources available (such as space, water, nutrients and light) become limiting then each species will be forced to compete. Weeds are often naturally adapted to a given environment and so may grow faster than the crop, especially since the crop species has been selected primarily for high yield rather than competitive ability. A unit of land may therefore be regarded as having a finite potential biomass to be shared between crop and weeds, the final proportion being determined by their relative competitive ability.

1.4 Problems caused by weeds

The most obvious problem caused by weeds is the reduction of yield through direct competition for light, space, nutrients and water. Weeds can have many further effects on the use of land, as illustrated in Table 1.2.

Table 1.2 Problems caused by weeds.

      Source: Naylor, R.E.L. and Lutman, P.J. (2002) What is a weed? In: Naylor, R.E.L. (ed.) Weed Management Handbook, 9th edn. Oxford: Blackwell Publishing/BCPC. Reproduced with permission