Weed facts for kids

Weeds growing in the cracks of a concrete staircase

A weed is a plant considered undesirable in a particular situation, growing where it is not wanted. The concept of weeds is particularly significant in agriculture, where the aim is growing crops or pastures of a single species, or a mixture of a few desired species. In such environments, other plant species are considered undesirable and therefore weeds. Besides, some weeds have undesirable characteristics making them a plant pest in most human settings.

Examples of weeds are plants unwanted in human-controlled settings, such as farm land, orchards, gardens, lawns, parks, residential and industrial areas. Taxonomically though, the term "weed" has no botanical significance, because a plant that is a weed in one context, is not a weed when growing in a situation where it is wanted. In the same way, volunteer plants are regarded as weeds in a subsequent crop. Some plants that are widely regarded as weeds are intentionally grown in gardens and other cultivated settings, in which case they are sometimes called beneficial weeds. The term weed is also applied to any plant that grows or reproduces aggressively, or is invasive outside its native habitat.

More broadly, the term "weed" is occasionally applied pejoratively to species outside the plant kingdom, species that can survive in diverse environments and reproduce quickly; in this sense it has even been applied to humans.

Weed control is important in agriculture and horticulture. Methods include hand cultivation with hoes, powered cultivation with cultivators, smothering with mulch or soil solarization, lethal wilting with high heat, burning, or chemical attack with herbicides and cultural methods such as crop rotation and fallowing land to reduce the weed population.

Benefits of weed species

While the term "weed" generally has a negative connotation, many plants known as weeds can have beneficial properties. A number of weeds, such as the dandelion (Taraxacum) and lamb's quarter, are edible, and their leaves or roots may be used for food or herbal medicine. Burdock is common over much of the world, and is sometimes used to make soup and medicine in East Asia. Some weeds attract beneficial insects, which in turn can protect crops from harmful pests. Weeds can also prevent pest insects from finding a crop, because their presence disrupts the incidence of positive cues which pests use to locate their food. Weeds may also act as a "living mulch", providing ground cover that reduces moisture loss and prevents erosion. Weeds may also improve soil fertility; dandelions, for example, bring up nutrients like calcium and nitrogen from deep in the soil with their tap root, and clover hosts nitrogen-fixing bacteria in its roots, fertilizing the soil directly. The dandelion is also one of several species which break up hardpan in overly-cultivated fields, helping crops grow deeper root systems. Some garden flowers originated as weeds in cultivated fields and have been selectively bred for their garden-worthy flowers or foliage. An example of a crop weed that is grown in gardens is the corncockle, (Agrostemma githago), which was a common weed in European wheat fields, but is now sometimes grown as a garden plant.

Plants often considered to be weeds

White clover

White clover is considered by some to be a weed in lawns, but in many other situations is a desirable source of fodder, honey and soil nitrogen.

A short list of some plants that often are considered to be weeds follows:

• Amaranth – ("pigweed") annual with copious long-lasting seeds, also a highly edible and resilient food source
• Bermuda grass – perennial, spreading by runners, rhizomes and seeds.
• Bindweed
• Broadleaf plantain – perennial, spreads by seeds that persist in the soil for many years
• Burdock – biennial
• Common lambsquarters – annual
• Cogongrass - Imperata cylindrica - One of the most damaging pest weeds in the world, infesting vast areas in the tropics.
• Creeping charlie – perennial, fast-spreading plants with long creeping stems
• Dandelion – perennial, wind-spread, fast-growing, and drought-tolerant
• Goldenrod – perennial
• Japanese knotweed
• Kudzu – perennial
• Leafy spurge – perennial, with underground stems
• Milk thistle – annual or biennial
• Poison ivy – perennial
• Ragweed – annual
• Sorrel – annual or perennial
• Striga
• St John's wort – perennial
• Sumac – woody perennial
• Tree of heaven – woody perennial
• Wild carrot – biennial
• Wood sorrel – perennial
• Yellow nutsedge – perennial

Many invasive weeds were introduced deliberately in the first place, and may have not been considered nuisances at the time, but rather beneficial.

Weed control

A field of beets being weeded in Colorado, United States, in 1972

Weeds are plants that some people view as undesirable in a particular place. Throughout the long human history of horticulture, people have worked to control weeds for many reasons. Weed control is a highly developed field of knowledge.

Weed control methods vary according to the growth habit of the weeds in question, as well as the context. For example, different methods of weed control may be used on a food crop versus a fiber crop or a golf course, because there is often more concern about health effects of chemicals used on food crops.

Weeds can be categorized by their life habit. They can generally either be grouped as annuals or perennials. An annual weed grows from the seeds dropped in the previous growing season. Perennial weeds regrow from previously established roots, dormant stolons, tubers, rhizomes, as well as the seed.

Understanding the habit of weeds is also important for non-chemical methods of weed control, such as plowing, surface scuffling, promotion of more beneficial cover crops, and prevention of seed accumulation in fields. For example, amaranth is an edible plant that is considered a weed by mainstream modern agriculture. It produces copious seeds (up to 1 million per plant) that last many years, and is an early-emergent fast grower. Those seeking to control amaranth quote the mantra "This year’s seeds become next year’s weeds!". However, another view of amaranth values the plant as a resilient food source.

Some people have appreciated weeds for their tenacity, their wildness and even the work and connection to nature they provide. As Christopher Lloyd wrote in The Well-Tempered Garden:

Many gardeners will agree that hand-weeding is not the terrible drudgery that it is often made out to be. Some people find in it a kind of soothing monotony. It leaves their minds free to develop the plot for their next novel or to perfect the brilliant repartee with which they should have encountered a relative's latest example of unreasonableness.

Images for kids

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  A dandelion is a common plant all over the world, especially in Europe, Asia, and the Americas. It is a well-known example of a plant that is considered a weed in some contexts (such as lawns) but not a weed in others (such as when it is used as a leaf vegetable or herbal medicine).

See also

In Spanish: Maleza para niños