Wicking Bed

A wicking bed is a growing bed with its water store underneath it instead of above. The base is sealed so nothing drains away, a shallow reservoir sits below the root zone, and the medium draws water up out of it the way a paper towel pulls up a spill - a movement called capillary action. Water goes in through a fill pipe rather than over the surface, so the bed can go several days between waterings instead of wanting attention each morning.

Two forms are described: the medium sits on a perforated platform just above the water, or a basket of the same medium dropped into the reservoir wicks for the rest. Either way the base has no drainage holes and there is always an overflow low down - at bucket scale about 5 cm (2 in) from the bottom - so water can never rise into the roots. A sealed liner and an overflow at the right height are what make this more of a build than a plain bed. The medium decides everything else: a capillary column only forms in a fine material such as compost or potting mix, and a coarse, chunky fill breaks the wick, leaving plants to die of drought above a full reservoir. The bed is also not self-watering. At bucket scale the crop still drinks 1 to 2 litres (1 to 2 quarts) a day at the height of the season: the reservoir changes how often somebody comes, not how much the plants use.

The quiet failure is salt. Water moves upward and leaves through the leaves, so whatever was dissolved in it stays behind. The bed grows well for a season or two, then declines, and a white crust appears at the surface. Rain on an open bed flushes it each wet season; under cover, in a dry climate, or on hard or reclaimed water, the surface has to be flooded before planting to carry the salt down. Other faults are mechanical: an overflow set too high or blocked drowns the roots, and an unbroken siphon in the overflow tube empties the reservoir while the bed looks full. It pairs well with a rainwater tank, since the fill pipe takes stored water without a pump, and with any compost supply - compost can be the whole medium.