The concept of smallness and from appearance wise ‘nothing’, causing largeness of surprising results, is applied by the used of Sinapis Nigra (Black Mustard) in two ways in the gospels. First, as in Mathew 17:20 the smallness is faith and the surprising results are miracles. The idea is that faith can be small as its object is God not itself. Second, as in Luke13:18, the kingdom of God is compared to the mustard seed (σίναπι) because as such small beginnings of the mustard seed actually held the end result within it. Within a seed there is a kind of pledge that its surprisingly large outcome will certainly come to pass. This is true about the humble beginnings of Christ’s ministry and Christianity – it was predetermined to grow with certainty based on its seed.
In the OT mustard is not mentioned. Yet later Jewish lit. shows that it was well-known in Palestine (Hbr. חַרְדָּל, Aram. חַרְדְּלָא, Arab. chardal), → n. 10. Acc. to the Mishnah (in contrast to → line 10) it was not cultivated in gardens but in fields, Kil., 3, 2; cf. 2, 9. It was grown both for the grains (T. Maas., III, 7 ) and also the leaves, loc. cit.; T. Kil., II, 8 ). There is no ref. to medical use. The smallness of the seed was proverbial. In some rules of cleanness the slightest quantity defiles, “even as little as a grain of mustard-seed,” Nid., 5, 2; cf. b. Ber., 31a.…to-day the mustard does in fact grow to a height of 2 1/2–3 metres in the vicinity of Lake Gennesaret. (Vol. 7: Theological dictionary of the New Testament. 1964- (G. Kittel, G. W. Bromiley & G. Friedrich, Ed.)
Alfred Edersheim further vindicates that the mustard seed was not a tree but a garden herb that was quite large compared to the small size of its seed. Also that it was proverbial in Jewish culture:
The very idea of Parables implies, not strict scientific accuracy, but popular pictorialness. It is characteristic of them to present vivid sketches that appeal to the popular mind, and exhibit such analogies of higher truths as can be readily perceived by all. Those addressed were not to weigh every detail, either logically or scientifically, but at once to recognise the aptness of the illustration as presented to the popular mind. Thus, as regards the first of these two Parables, the seed of the mustard-plant passed in popular parlance as the smallest of seeds. In fact, the expression, ‘small as a mustard-seed,’ had become proverbial, and was used, not only by our Lord, but frequently by the Rabbis, to indicate the smallest amount, such as the least drop of blood, the least defilement, or the smallest remnant of sun-glow in the sky. ‘But when it is grown, it is greater than the garden-herbs.’ Indeed, it looks no longer like a large garden-herb or shrub, but ‘becomes,’ or rather, appears like, ‘a tree’—as St. Luke puts it, ‘a great tree,’ of course, not in comparison with other trees, but with garden-shrubs. Such growth of the mustard seed was also a fact well known at the time, and, indeed, still observed in the East. (The Life and Times of Jesus the Messiah)