Raised-bed gardening is a form of gardening in which the soil is raised above ground level and usually enclosed in some way. Raised bed structures can be made of wood, rock, concrete or other materials, and can be of any size or shape. The soil is usually enriched with compost. Vegetables are grown in geometric patterns, much closer together than in conventional row gardening. The spacing is such that when the vegetables are fully grown, their leaves just barely touch each other, creating a microclimate in which weed growth is suppressed and moisture is conserved. Overview Raised beds lend themselves to the development of complex agriculture systems that utilize many of the principles and methods of permaculture. They can be used effectively to control erosion and recycle and conserve water and nutrients by building them along contour lines on slopes. This also makes more space available for intensive crop production. They can be created over large areas with the use of several commonly available tractor-drawn implements and efficiently maintained, planted and harvested using hand tools. This form of gardening is compatible with square foot gardening and companion planting. Circular raised beds with a path to the center (a slice of the circle cut out) are called keyhole gardens. Often the center has a chimney of sorts built with sticks and then lined with feedbags or grasses that allows water placed at the center to flow out into the soil and reach the plants' roots. A self watering raised bed known as a wicking bed is particularly beneficial in dry climates and are often made by converting Intermediate bulk container (IBC's). Materials and construction Lumber is the most common construction material for making raised beds. If using lumber treated with chromated copper arsenate or CCA (though uncommon since 2004 in the USA and Europe), it is recommended to use a plastic liner between the wood and soil if the raised bed is intended for growing edibles. Another material commonly used are railroad ties, also known as sleepers, joined with steel rods to hold them together. Another approach is to use concrete blocks, although less aesthetically pleasing, they are inexpensive to obtain and easy to use. On the market are also prefab raised garden bed solutions which are made from long lasting polyethylene that is UV stabilized and food grade so it will not leach undesirable chemicals into the soil or deteriorate