Two views of the kind of mustard seed Jesus compares the kingdom of heaven to:
Jesus means the seed of the black mustard, the Sinapis nigra—the size of a pinhead, one of the smallest visible to the naked eye: an apt image for something very tiny (see Matthew 17:20).
The Sinapis nigra, also called Brassica nigra, is about 0.95-1.6 millimeters and weighs roughly 1 mg—among the tiniest of common garden seeds. The black mustard, an annual that shoots up rapidly and branches profusely, often reaches 2.5-3 meters in height, with a stem that is woody enough to speak of it as tree-like. Since its oil-rich seeds attract finches (goldfinches and linnets), it also becomes a home for birds. These traits (smallness of seed; swift, extensive growth, and provision for birdlife) paint a vivid picture of how something tiny can become something large.
Interpretation 1:
Jesus speaks of the seed of the black mustard, the Sinapis nigra. The seed is no larger than a pinhead—among the smallest visible to the naked eye—which makes it an apt image for something very small (see Matthew 17:20).1
Interpretation 2:
Botanically the Sinapis nigra is also identified as Brassica nigra, and these mustard seeds measure about 0.95-1.6 millimeters and weigh only roughly 1 mg, arguably the tiniest of common garden seeds. The black mustard is an annual that shoots up rapidly and branches profusely, often reaching 2.5-3 meters in height; its stem becomes sufficiently woody that it may be spoken of as tree-like. Because its oil-rich seeds attract finches (goldfinches and linnets), the plant also becomes a favoured refuge for birds. These traits—extreme smallness of seed, swift and extensive growth, and provision for birdlife—make the mustard plant an especially vivid picture of how the very small can develop quickly into something large.2
31 He put another parable before them, saying, “The kingdom of heaven is like a grain of mustard seed that a man took and sowed in his field.