Flood & Drain (or Ebb & Flow)

A feature common to many systems is that plants are fed from above the roots (i.e. referred to as “top-feed”). In flood & drain systems however, the nutrient is fed from beneath (Fig 1.2). This process floods the root zone to a pre-determined maximum level after which all the nutrient is allowed to drain back into the reservoir. This process is controlled by a pump, timer and a specialized ‘inlet’ and ‘overflow’ device.

‘Inlet’:  This also serves as the drain outlet. It is positioned as close as possible to the base of the flood & drain tray. Once pumping stops, the nutrient is able to fully drain into the reservoir via the inlet and pump. To allow reverse flow the pump must be of a ‘centrifugal’ type. Complete drainage is necessary to prevent water-logging.

‘Overflow’:  This is positioned at the maximum desired flood height. This means pumping can only flood to the height of the overflow. To maximize feeding and flushing of the root zone this valve should be set as high as possible. However, if a medium is used, avoid wetting its top surface otherwise excessive disease, pest and algal problems can occur.

Suitable media for flood & drain include expanded clay, perlite or even none at all. The water holding capacity of the medium determines the feeding frequency.

Advantages to Flood & Drain systems

● The flooding action helps prevent salt build-up in the root zone and achieves uniform root zone pH and conductivity.
● Flooding purges stale (oxygen deficient) air from the root zone. Drainage draws fresh air back into the medium.
● Flooding can enhance the penetration of disinfectants throughout the root zone.

Disadvantages to Flood & Drain systems

● If no medium is used, pump failure can result in plant death within a few hours, especially in hot weather.