Water Conservation

• Conserving water and delivering it to the roots of garden plants is of upmost importance to gardeners in arid climates. Sunken beds help collect rain water and hold it until it soaks into the soil rather than running off, according to the Aridlands Newsletter. Sunken bed gardening methods are traditional among the people of the American Southwest and in West Africa. The walls of a sunken garden bed can also help protect young seedlings from drying winds. Mulch between planting rows further improves water conservation in a sunken garden bed, and it helps maintain an even soil temperature to cool plant roots in hot desert summer conditions.
  
  

Water Drainage

• Both raised beds and sunken beds can aid in water drainage in different ways. Sunken bed rain gardens can help divert, collect and absorb rainwater in wet climates such as the Pacific Northwest, according to the Oregon State University Extension Service. Water-tolerant perennial species and shrubs help filter water runoff, helping to prevent warming and pollution in natural water bodies. Raised beds help to drain water away from the roots of water-sensitive vegetable and flower plants in rainy climates or during winter snow melt and spring precipitation, according to the Ohio State University Extension. Where compacted or clay soils preclude suitable water drainage for productive gardening, raised beds can be filled with sand, topsoil and organic matter to promote appropriate drainage.
  
  

Soil Fertility

• Raised garden beds that are narrow enough to work from outside the bed without stepping on the soil prevent soil compaction, allowing increased air and nutrient access around the plant roots. Fertile, organic matter such as compost and manure can also be incorporated into a raised bed with reduced risk of washing out or floating out of the soil. Keeping sunken beds narrow and adding thick layers of organic mulch on top of the soil surface will also improve soil fertility of sunken beds. Raised beds keep soil warmer than sunken beds, according to the Cochise County, Arizona, Master Gardener Program. This can be an advantage for heat-loving crops such as tomatoes and eggplants, but cool weather crops such as lettuce and spinach may be more productive in sunken beds, especially when grown in hot regions.
  
  

Ease of Harvest

• Raised beds have the advantage of keeping produce cleaner, though using clean mulch such as straw in a sunken bed can help prevent soil splash on leaves and vegetables. Paths between raised beds can be mulched or paved with stones to make it easier to work in the garden during rainy or muddy weather, according to the Ohio State University Extension. Gardeners with limited mobility may find it easier to reach plants in raised beds, and tall raised beds can make gardening accessible to people who use a wheelchair or walker. Benches built into the exterior of tall raised bed walls, or into the interior of deep sunken bed walls, can also improve accessibility and comfort for all gardeners.