Boyd’s Bible Dictionary:
(must). The black mustard of the East grows quite large and strong (Matt. 13:31-32; 17:20; Mark 4:31-32; Luke 17:6).
Concise Bible Dictionary:
This is mentioned as something very small to which the faith of the apostles did not rise; and the kingdom of God is spoken of as being like this small seed, which nevertheless grows into a tree large enough for the birds to lodge in its branches. Various surmises have been made as to what tree is alluded to. The one that best answers to the description is the sinapis nigra, a common mustard plant in Palestine. Its seeds are very small, and it grows to a tree of some ten feet high, quite large enough for the smaller birds to lodge in its branches. There is also the salvadora persica, which grows still higher. The teaching of the parable is that the kingdom of God would become elevated in the earth and suitable for emissaries of Satan to find protection under its shadow (Matt. 13:31; Matt. 17:20; Mark 4:31; Luke 13:19; Luke 17:6). In Matthew 13 it says that, “when it is grown, it is the greatest among herbs,” yet it “becometh a tree.” This reads as if its normal class was that of a large herb, but by some mysterious means it became a tree.
Salvadora Persica