For other uses, see marrow. A marrow is a vegetable, the mature fruit of certain Cucurbita pepo cultivars. The immature fruits of the same cultivars are called zucchini (in North America, Australia and Germany) or courgette (in the British Isles, the Netherlands and New Zealand).he fashion for eating immature marrows, called in Britain courgettes, is relatively recent in Britain. Sudell (1966) does not mention courgettes, although he has a section on vegetable marrow, noting both trailing (vining) and bush types and saying cut when young.