A polytunnel structure comprises a cover support member 10, a cover 12 supported, in use, by the cover support member, the cover being movable between an unrolled condition in which it forms a roof for the polytunnel structure, and a rolled up condition, and a retainer 16 movably mounted to the cover support member and movable between a stowed position in which it does not obstruct the cover, allowing the cover to occupy its unrolled condition, and an in use position in which it holds the cover, in its rolled up condition against significant movement relative to the cover support member. The retainer may have first 16a and second 16b spaced apart arms. The retainer may be located beneath the cover member in its stowed condition and be pivotable relative to the cover support member about an axis substantially perpendicular to the axis of the adjacent part of the cover support member. Also disclosed is a retainer adapted for use in the structure.
