A retainer of generally U shape for a growth supporting medium, the retainer comprising two end panels (1,2) with convergent front and rear edges (4,3), a front panel (5) joining the front edges (4) of the two end panels (1,2) and lying at an angle to the plane occupied by the rear edges (3) of the end panels (1,2), a flange (6) on each rear edge (3) lying in a plane substantially at right angles to the plane of its associated end panel, the flanges (6) being adapted to respectively engage in support members whereby the retainer can be supported in an operative position.
