In the twining vine Ipomoea purpurea we experimentally assessed the effect of support availability (a vertical stake), maternal genotype (family), and maternal environment (presence or absence of support in mother plants) on morphological traits, accounting for differences in initial seed size. While there was no effect of the maternal environment on seed size at the family level, families within each maternal environment did differ in seed size. Seeds from families of supported mothers showed higher germination than those from families of unsupported mothers. The maternal environment did not influence shoot traits but affected phenotypic plasticity in the number of leaves in response to support. Thus, whereas progeny plants from unsupported mothers did not show a response to support availability in the number of leaves, progeny from supported mother plants had a greater number of leaves once support was provided. The maternal genotype only affected the number of leaves.