SOUR CITRUS

QUESTION I came across this citrus tree recently and thought it was a mandarin, but the fruit is really sour. They’re about the size of small mandarin and orange. Can you tell what they are and why it is so sour?

H. Smith, Tauranga

ANSWER This looks and sounds like Rangpur lime, Citrus × limonia. Calling them limes is not really an accurate description as they’re actually a hybrid between citron, C. medica, and mandarin, C. reticulata. They can be used in dishes, drinks and marmalade in much the same way as you’d use limes, except that Rangpur limes are highly acidic and quite tart in flavour. In parts of the USA they’re used as rootstock for grafting other citrus cultivars; here, Rangpur limes, along with most other citrus, are grafted onto ‘trifoliata’ rootstock, C. trifoliata, which suits the soils and environmental conditions of New Zealand best. It is thought this plant may have originated as a natural hybrid in India, where it’s still widely grown today, and where there have been several settlements named Rangpur, including the present-day town in Bangladesh.