Abstract: A low density (5-12 Kgs/M.sup.3), rigid, unicellular (i.e. closed cell) expanded polyurethane foam is perforated through at least part of its thickness by mechanical means, resulting in a hydrophilic, heterocellular (open cell) foam, whereby to facilitate the passage of emergent plant roots and also to provide groups of enlarged voids (in contrast to the fine cell structure of the unmodified foam) so as to facilitate water absorption and retention.