Weeds - Weeds

Weeds are unwanted plants, especially herbaceous plants, that grow on land managed by humans and have a negative impact on the managed land. Weeds are sometimes confused with wild plants, but wild plants are plants that grow in wildlands.

In agriculture, weeds deprive crops of the moisture, nutrients, light, temperature, and oxygen they need, reducing crop yields, causing deterioration in the quality of produce, encouraging the proliferation of pests and diseases, making farm work more difficult, and increasing the amount of management labor and costs required for weed control, which in turn lowers land prices. Outside of agriculture, weeds in gardens and parks also impair the aesthetic appearance and encourage the outbreak of pests and diseases, while weeds in playgrounds and roads impede their functionality, both of which increase management costs. In agriculture, some weeds are wild plants that originally grew in the area and have invaded cultivated land, but there are often specific weeds that are ecologically similar to specific crops, and these are often invasive plants that have come along with the spread of crops. Therefore, weeds are well adapted to the cultivated environment, and some even have difficulty growing in natural environments other than cultivated land.

Roughly half of the cultivated land area in Japan is paddy field, where 191 species of plants from 43 families that grow in marshes and shallow waters are considered weeds. The main ones are barnyard grass, Cyperus nigricans, pine bait, Cyperus serotinus, Eleocharis kuroguwai, bulrush, Sagittaria pygmaea, Ardisia crenata, pondweed, Monochoria knapweed, Rhizome japonica, and water chickweed, and most of them are monocotyledonous plants. There are 302 species of weeds from 53 families in fields, with crabgrass being the most dominant weed, followed by barnyard grass, green foxtail, dayflower, purslane, black ragwort, and lamb's quarter. Troublesome weeds in grasslands include bracken and dock, and troublesome weeds in lawns include annual bluegrass. In forestry, not only undergrowth but also plants that cover trees, such as kudzu and wisteria, are treated as weeds.

Since ancient times, Japanese agriculture has been said to be a battle against weeds, as weeds proliferated so much in Japan and a great deal of effort was spent on eradicating them. Weed control methods include mainly ecological methods that suppress the emergence and growth of weeds through cultivation management methods, mechanical methods, and chemical methods that use herbicides. The main purpose of flooding rice fields in rice cultivation is to eliminate weeds, and rice transplanting cultivation is said to have begun with the main purpose of preventing weed damage. Field plowing also plays a large role in suppressing the emergence of weeds. The development of herbicides progressed after World War II, and they became rapidly popular in the 1950s, greatly changing the previous weed control system.

Weeds are not only harmful, but they also contribute to the development of crops. Primitive wheat, which began to be cultivated nearly 10,000 years ago, naturally hybridized with weeds growing in the fields over the course of subsequent cultivation, and the superior bread wheat of today was born through repeated incorporation of their genes. Weeds also seem to have played a major role in the history of corn. Some, like rye, were originally weeds in wheat fields, but their resistance to low temperatures and poor soil was recognized and they gradually became crops.

[Hoshikawa Kiyochika]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

人が管理している土地に生え、管理対象に悪影響を与える望まれない植物、とくに草本植物を雑草とよぶ。雑草はときに野草と混同されるが、野草は原野に生えるものをいう。

　農業においては、雑草は、作物が必要とする水分、養分、光、温度、酸素を奪って作物の収量を減少させ、生産物の品質の劣化をおこし、病害虫の繁殖を助長し、農作業を困難にし、雑草防除などの管理労力の増加や管理費用を増加させ、その結果、土地価格を低下させる。農業以外でも、庭園や公園などの雑草は美観を妨げ、病害虫の発生を助長するし、運動場、道路の雑草はその機能の障害となり、ともにその管理費用を増大させる。農業での雑草は、もともとその地域に自生していた野草が耕地に侵入したものもあるが、しばしば特定の作物に対してそれと生態的によく似た特定の雑草があり、それらは作物の伝播(でんぱ)に伴ってきたもので、外来の植物が多い。したがって、雑草は耕地の環境によく適応しており、そのなかには耕地以外の自然環境では生育が困難なものさえある。

　日本の耕地面積の約半分は水田で、そこでは43科191種に上る湿地や浅瀬に生える植物が雑草となっている。おもなものはタイヌビエ、タマガヤツリ、マツバイ、ミズガヤツリ、クログワイ、ホタルイ、ウリカワ、ヘラオモダカ、ヒルムシロ、コナギ、キカシグサ、ミゾハコベなどであり、単子葉植物が多い。畑地の雑草は53科302種に上り、メヒシバがもっとも優勢な雑草で、ほかにイヌビエ、エノコログサ、ツユクサ、スベリヒユ、イヌビユ、シロザなどが主要なものである。草地ではワラビやギシギシなど、芝生ではスズメノカタビラなどがやっかいな雑草である。林業では、下草ばかりでなく、クズやフジなど木を覆うような植物も雑草として扱われる。

　古来、日本の農業は雑草との闘いといわれるほど、日本は雑草の繁殖が盛んで、その駆除にかける労力が大きかった。除草法には、栽培管理の方法によって発生や生育を抑える主として生態的な方法や、機械的な方法、除草剤を用いる化学的な方法などがある。稲作において田に湛水(たんすい)する主目的は除草であり、田植栽培もまた雑草害の防除が主目的で始まったといわれる。田畑の耕起作業なども雑草の発生を抑える役割が大きい。除草剤は第二次世界大戦後から開発が進み、昭和30年代になって急速に普及し、それまでの除草体系を大きく変えた。

　雑草には有害な面ばかりでなく、作物の発達に貢献している面もある。1万年近く前から栽培され始めた原始的なコムギは、その後の栽培の過程で、その畑に生える雑草と自然交雑し、その遺伝子を組み入れることを繰り返してより優れた現在のパンコムギが生まれた。トウモロコシができた歴史にも雑草が大きく関与しているらしい。ライムギのように、初めはコムギ畑の雑草であったが、低温ややせ地に強い性質が認められて、しだいに作物となったものもある。

［星川清親］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例