About weeds


Luzula nutans © Ita McCobb

About weeds

It is important to understand how weeds operate as species and use an understanding of weed biology to improve their management since eliminating all weeds from an ecosystem can destroy valuable habitat for natural enemies of pests. Many weeds produce flowers and seeds they are an integral part of an ecosystem, providing food and shelter for wildlife.

The definition of a weed is any plant growing in an area where it is unwanted. This could apply to a garden, a farm or any possible landscape where a plant may appear.

From an ecological point of view, “Weeds are plants that are especially successful at colonizing disturbed, but potentially productive sites and at maintaining their abundance under conditions of repeated disturbance.” (Liebman, Mohler and Staver, Ecological Management of Agricultural Weeds, 2001)

Like other plants, weeds have the same requirements for light, water and nutrients, and compete for these, often prevailing over native plants and impacting on environmental diversity, agriculture and the local community.

Weeds succeed because they have developed extremely efficient ways of either staying alive or reproducing. Some send down tenacious tap roots deep into the soil that regenerate whenever the upper parts of the plant are removed; others grow, flower and set seed all in a matter of weeks.

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Purple nutsedge Cyperus rotundus © Ita McCobb

Weed types

An annual weed is a plant that normally completes its full cycle of growth in one year, flowering and seeding in a single season and then dying e.g. Chickweed Stellaria media.

Their physical seed size is generally small, as they must produce many seeds in order to have some seedlings survive repeated disturbance. The seeds also have the ability to survive dormant for years in the soil, waiting for the perfect conditions to germinate.

A biennial weed completes its life cycle in two years, so it flowers and produces seeds in its second year e.g. Hemlock Conium maculatum.

In the first year a biennial weed grows leaves, stems and roots; in its second year it flowers, forms seeds and then dies.

A perennial weed is a plant with an indefinite life span of more than two years e.g. Broad-leaf dock Rumex obtusifolius. Some may be quite short-lived but trees can survive for centuries.

They produce seeds but also have highly developed long tap roots that can regenerate if the top section of the plant or even part of the root is removed. Others spread by horizontal shoots or stems on the surface (stolons/runners) or by underground shoots (rhizomes) from which young plants emerge.

An ephemeral weed is a plant that has several life cycles in a growing season and can increase rapidly e.g. Common groundsel Senecio vulgaris.

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False oat-grass Avenula pubescens © Ita McCobb

Weed families

Common weeds belong to one of three families: Broadleaf weeds have larger leaves and grow from tap roots or fibrous root systems e.g. Stinging nettle Urtica dioica. Narrow leaf or grasses have long narrow leaves and fibrous root systems e.g. Couch grass Elymus repens. Sedge weeds appear similar to grasses e.g. Purple nutsedge Cyperus rotundus.

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Weed seed dispersal

Weeds have developed many ways of spreading. Some produce parachute-like seeds that take advantage of the wind, others cling to animal fur or are eaten, re-emerging in animal droppings. Some are spread by floating in water while others shoot their seeds out like projectiles.

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Cogon grass Imperata cylindrica © Ita McCobb

The pros & cons of weeds

Benefits of weeds

Weeds are a healing response to land that has been exposed by fire, flood, landslide, deforestation, soil cultivation or other disturbance.

Weeds offer important benefits not only for the soil but also for wildlife and humanity.

Many plants that we call weeds today were essential food in medieval times. Our medieval ancestors actively encouraged weeds in their vegetable plots, leaving no bare soil and using many as food during the gap between the end of Winter crops and the beginning of Summer ones. The green covering kept the soil moist and was dug in as fertilizer, improving the soil structure.

If the soil loses the nourishing organic compounds from living plant roots it also loses fertility. Weeds absorb, conserve and recycle soluble nutrients that would otherwise leach away.

Weeds can perform vital ecosystem services by protecting and restoring exposed or degraded soil by rapidly covering it, thereby protecting it from erosion and replenishing organic matter while feeding and restoring soil life.

Weeds also provide habitat for beneficial organisms and therefore contribute significantly to natural and biological control of some insect pests.

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Cock's-foot Dactylis glomerata © Ita McCobb

Negative impact of weeds

Weeds have evolved and adapted to be among the most efficient, successful and tenacious species in the plant kingdom and therefore have the potential to adversely alter ecosystem function.

Weeds compete directly with other plants for light, nutrients, moisture and space.

They can physically hinder plant or crop growth and development, reducing productivity as well as displacing native species.

Weeds are genetically designed to germinate, grow and propagate faster than most more desirable plants.

Weeds can release substances that are toxic to plants or crops in a phenomenon called allelopathy.

Weeds impact negatively particularly when they are introduced into conditions for which they are well adapted, with the absence of their normal natural restraints.

Many of a region’s most problematic weeds are those that are not native to the region.

Note: It should always be remembered that some plants that are cultivated in one place are often considered as weeds in another!

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References: Mark Schonbeck, Virginia Association for Biological Farming, An Ecological Understanding of Weeds; Cornell University; gardeningstepbystep.com; bbc.co.uk; metmuseum.org/cloisters gardens; University of Hull; The Telegraph, 27 April 2014, Val Bourne, Weeds and wisdom in the Middle Ages.