A mineral wool batt for use as a plant growth medium, particularly in applications for growing vegetation (including plants) in which water retention and/or the avoidance of water run-off is of interest. The mineral wool batt has an absorbent layer which comprises needled mineral wool fibers; superabsorbent particles in the absorbent layer may be sandwiched between a denser upper and/or lower barrier layer(s) which assist in preventing their escape.