The apparatus includes in particular a channel section (1) made from extruded plastic as a box structure including two parallel wings (flanges) (2, 3) joined by a base (9), the central part (5) of which is lowered in order to define a lower drainage channel, those portions of the base (6a, 6b) on each side of the channel (5) being inclined towards the channel and constituting a shoulder for supporting the growing substrate (7) possibly through the use of a removable partition (8). <IMAGE>
