What does a mustard seed grow into!

Question

“How shall we picture the kingdom of God, or by what parable shall we present it? It is like a mustard seed, which, when sown upon the soil, though it is smaller than all the seeds that are upon the soil, yet when it is sown, it grows up and becomes larger than all the garden plants and forms large branches; so that the birds of the air can come and nest under its shade.” Mark 4:30-32

Are mustard plants in Israel larger than here. I've seen on-line everything from a small bush to a large tree used to depict what the mustard seed grows into. Commentary says it is 15ft tall. So what is this really.

Answer 1

It may likely grow into the Sinapis Nigra (Black Mustard). It can grow to eight feet tall, so it could actually be literally used by small birds to nest on its branches. However parables are not to be taken so literally and the image may be a slight exaggeration as part in parcel with the point of the passage.

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 [84]) and also the leaves, loc. cit.; T. Kil., II, 8 [75]). 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)

The main point of the parable is that under such small beginnings the kingdom of God in Christ will expand disproportionately to its original 'small' size. History has borne the truthfulness of the parable out quite evidentially.

Answer 2

Meaning of κόκκῳ σινάπεως

This is more or less just some additional information, Mike's answer is good. According to the IVP NT Commentary series:

Scholars still dispute what plant is meant by the “mustard seed.” Nevertheless, by no conjecture is it the smallest of all seeds that Jesus’ listeners could have known (the orchid seed is smaller); the point is that it was recognized as very small and yet yielded a large shrub. Around the Sea of Galilee, it can reach a height of ten feet and has sometimes reached fifteen feet. Its usual height, however, is about four feet; because it would grow anew each year, birds could not nest in it when they built nests in early spring. The hyperbole Jesus applies to the best image of growth from tiny to large he had available does not change the point, however; the kingdom might begin in obscurity, but it would culminate in glory. Even if birds could not nest in the mustard plant, they could perch in it (Matthew’s term here was sometimes used that way); Matthew’s language here alludes to Daniel 4:12, the splendor of another ruler’s kingdom. Sources suggest that Palestinian custom relegated mustard seeds to fields rather than gardens; one may thus contrast Matthew 13:31 with Luke 13:19, each adjusting the image for their respective readerships.¹

James Brooks affirms this assertion and also refutes an alternate interpretation of this parable:

Some have questioned, however, whether the growth depicted by the parable is desirable. They have claimed that an abnormally large herb with its branches filled with birds (sometimes symbols of evil) represents an overgrown, apostate, institutional church. Such an interpretation is, however, completely at odds with the meaning of the parables of the soil and the seed and the meaning of the term “kingdom of God.” Furthermore, growth is not the main emphasis; contrast between the beginning and the end is the main point of comparison. It is barely possible that the birds represent the inclusion of the Gentiles. In Dan 4:10–12 and Ezek 31:3–14 a tree symbolizes a foreign empire; and birds, those who enjoy its protection. In Ezek 17:22–24 the tree symbolizes restored Israel, and the birds symbolize those who enjoy its blessing.²

¹ _{Craig S. Keener, The IVP Bible Background Commentary: New Testament (Downers Grove, IL: InterVarsity Press, 1993), Mt 13:31–32.}

² _{James A. Brooks, vol. 23, Mark, The New American Commentary (Nashville: Broadman & Holman Publishers, 1991), 85-86.}