Flood
& Drain (or Ebb & Flow):
A feature common to most systems is that plants are fed from above the
medium (i.e. referred to as “top-feed”). In flood & drain however, the
nutrient is fed from beneath (Fig
5.2). This process floods the root
zone / medium to a pre-determined maximum level after which 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:
+ The
‘inlet’ 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 to 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 and the associated disease
problems.
+ The
‘overflow’ is positioned at the maximum desired flood height. Hence
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, wetting of the medium’s surface should be avoided
otherwise disease, pest and algal problems can occur.
Suitable
mediums for flood & drain include expanded clay, perlite or even no media
at all. The medium’s water holding capacity determines the feeding
frequency.
Advantages:
+ The
flooding action helps minimize local salt build-up in the root zone and
achievement of satisfactory root zone pH and conductivity.
+
Flooding expels potentially oxygen depleted air from root zone.
Subsequent drainage draws fresh air back into the medium.
+
Flooding and subsequent draining improves the penetration of sterilizing
agents (etc) across the entire root zone.
Disadvantages:
+
Unable to inspect root mat – if medium is used.
