Two or more rows planted in belts, with an alley in-between, could also be used. However, planting two rows rather than one will not allow the alley to be twice as wide. The major limitation on spacing is movement of water through soil adjacent to the trees. In most heavier soils, water will not move as rapidly sideways as it will vertically. Therefore, trees may lower the groundwater rapidly underneath the trees, but slowly at a distance from the trees.
Tree species Local experience with tree species is worth investigating.
However, there is a fairly restricted range of species that have been tried and found to be particularly useful. The usual reasons for choosing a tree in valley floor alley farming are that it has salt and waterlogging tolerance, is easy and reliable to establish, grows rapidly in the early years, has demonstrated tolerance to insect attack, and is unlikely to become a weed. Species that have met these criteria are shown below. Species Common name and notes Eucalyptus occidentalis Flat Topped Yate or Swamp Yate. Tested in many areas.