When to harvest quince 'smyrna'

Depends on the crop — confirm which part is eaten before you harvest. Which part of quince 'smyrna' you harvest, and when, depends on the variety and on what you grow it for — and it is not something we assert from a name alone. Check a source that describes quince 'smyrna' specifically before you pick, and if any part of the plant is known to need cooking or preparation to be safe, follow that guidance.

Harvest gently with clean scissors or a knife rather than tearing, and take a little at first to see how the plant responds.

Keep it cool and use it fresh where possible; most home-grown crops lose flavour quickly once picked.

Assuming every part of quince 'smyrna' is edible. For a number of food plants, one part is the crop and another is not safe to eat — confirm which before you harvest.