Exegesis:

kokkō sinapeōs (only here in Mark) ‘grain (seed) of mustard’: the plant is identified as the sinapsis nigra (cf. Lagrange). The mustard grows wild in Palestine: it is an annual plant, growing from seed, and, especially when cultivated, may reach a height of 10 to 12 feet. A garden herb (cf. lachanon ‘herb’ next verse), it is, not quite accurately, called a ‘tree’ (Lk. 13.19), due to its large size.

mikroteron on ‘being smaller (than)’: as is common, the comparative ‘smaller than’ is used for the superlative ‘smallest of.’ The neuter tense of the adjective and participle is probably due to the neuter spermatōn ‘seeds’ which follows.

on ‘being’: the participle is concessive ‘though it is.’

spermatōn (12.19, 20, 21, 22) ‘seeds’ (notice sporos ‘seed’ in v. 27, and kokkos ‘grain,’ ‘seed’ in this verse).

Translation:

It must be translated so as to refer to ‘the kingdom of God.’

Grain of mustard seed may be rendered as ‘a seed of a plant called mustard,’ employing a word borrowed from the dominant prestige language of the area (Central Tarahumara, Eastern Highland Otomi). Frequently, one can find a type of local mustard plant, which, though somewhat different, can still be used as a basis for the translation, e.g. ‘a seed of a kind of … plant’ (in which the appropriate close equivalent can be used; cf. Taungthu). Toraja-Sa’dan, Indonesian and Javanese use sawi, a sort of mustard plant (Brassica rubosa), the leaves of which are eaten as vegetables.

The constructions introduced by which, when are such that they frequently require some radical readjustments, e.g. ‘it is like the grains of a plant called mustard; when these seeds are sown in the earth, they are the smallest….’ In this rendering we have suggested the plural for singular since in a number of languages such generic statements must be regularly in the plural form. However, for the sake of the following verse, it is preferable, if at all possible, to employ the singular throughout. Note also the change of sown upon to ‘sown in,’ as in a number of languages.

Despite the fact that the statement smallest of all the seeds on earth cannot be taken in any absolute sense, one should nevertheless translate the text as it is.

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .