are divided into biological, chemical, cultural, and physical/mechanical control. Physical/mechanical methods Coverings In a domestic gardens, methods of weed control include covering an area of ground with a material that creates an unsuitable environment for weed growth, known as a weed mat. For example, several layers of wet newspaper prevent light from reaching plants beneath, which kills them. In the case of black plastic, the greenhouse effect kills the plants. Although the black plastic sheet is effective at preventing weeds that it covers, it is difficult to achieve complete coverage. Eradicating persistent perennials may require the sheets to be left in place for at least two seasons. Some plants are said to produce root exudates that suppress herbaceous weeds. Tagetes minuta is claimed to be effective against couch and ground elder, whilst a border of comfrey is also said to act as a barrier against the invasion of some weeds including couch. A layer of wood chip mulch prevents some weeds from sprouting. Gravel can serve as an inorganic mulch. Irrigation is sometimes used as a weed control measure such as in the case of paddy fields to kill any plant other than the water-tolerant rice crop. Manual removal Many gardeners still remove weeds by manually pulling them out of the ground, making sure to include the roots that would otherwise allow some to re-sprout. Hoeing off weed leaves and stems as soon as they appear can eventually weaken and kill perennials, although this will require persistence in the case of plants such as bindweed. Nettle infestations can be tackled by cutting back at least three times a year, repeated over a three-year period. Bramble can be dealt with in a similar way. A highly successful, mostly manual, removal programme of weed control in natural bush land has been the control of sea spurge by Sea Spurge Remote Area Teams in Tasmania. Tillage Ploughing includes tilling of soil, intercultural ploughing and summer ploughing. Ploughing uproots weeds, causing them to die. Summer ploughing also helps in killing pests. Mechanical tilling with various types of cultivators can remove weeds around crop plants at various points in the growing process. An Aquamog can be used to remove weeds covering a body of water. Thermal Several thermal methods can control weeds. uses a flame several centimetres/inches away from the weeds to singe them, giving them a sudden and severe heating. The goal of flame