MUSTARD (σίναπι, G4983). Mustard is mentioned five times—twice in Matthew, once in Mark, and twice in Luke—always in reference to having faith like the grain of mustard seed. The tree known in Palestine as the mustard tree has minute seeds and yellow flowers. It was the Nicotiana glauca, which is grown widely in the Mediterranean and is a member of the Solanaceae family.
Most Bible students, however, agree that the plant is the black mustard, Brassica nigra. This is the plant grown for the production of the normal mustard, but in our Lord’s day it was grown possibly for its oil content. Plants, when isolated, may grow to a height of fifteen feet and have a thick main stem, with branches strong enough to bear the weight of a bird. L. H. Bailey, in his Manual of Cultivated Plants (1924), states that the black mustard could grow to a height of ten feet.
Because of its Greek name some believe that the plant must therefore be Sinapis alba, the white mustard, but this grows only two feet high. The Royal Horticultural Society prefers the evergreen, Salvadora persica, the Kilnel oil plant—sometimes called the mustard tree. The Salvadora grows on the sides of the Dead Sea. The Arabs give it the name of Khardal. This could be translated mustard tree.
The Salvadora does not, however, have tiny seeds, but small “stones,” smaller than those of the damson. Such “stones” would not have been broadcast by a farmer as described in the New Testament.
Mark 4:32 describes the plant as “greater than all herbs,” and the black mustard certainly fits this description.
International Standard Bible Encyclopedia (1915)
Among the rabbis a "grain of mustard" was a common expression for anything very minute, which explains our Lord’s phrase, "faith as a grain of mustard seed" (Mt 17:20; Lu 17:6).
The suggestion that thereferences may allude to a tall shrub Salvadora persica, which grows on the southern shores of the Dead Sea, rests solely upon the fact that this plant is sometimes called khardal by the Arabs, but it has no serious claim to be the sinapi of the Bible.