mustard tree
/ˈmʌstəd triː/noun
Any of various trees; specifically (a) black mustard, Brassica nigra, frequently with allusion to the New Testament parable (Matthew 13:31) which describes its growth into a tree; (b) the toothbrush tree, Salvadora persica; (c) Australian the native poplar, Codonocarpus cotinifolius; (d) Australian = mustard bush.
Origin
Early 17th century; earliest use found in Helpe to Discourse. From mustard + tree.