What exactly is the correlation of kingdom and mustard plant?
Pliny the Elder, who lived between 23 and 79 C.E., wrote about the mustard plant in his encyclopedia "Natural History":
With its pungent taste and fiery effect, mustard is extremely beneficial for the health. It grows entirely wild, though it is improved by being transplanted: but on the other hand, when it has once been sown, it is scarcely possible to get the place free of it, as the seed when it falls germinates at once. (Pliny, "Natural History" 19.170-171; Rackham et al. 5.528-529)
There is, in other words, a distinction between the wild mustard and its domesticated counterpart, but even when one deliberately cultivates the latter for its medicinal or culinary properties, there is an ever-present danger that it will destroy the garden. And, apart from those domesticated types, such as brassica nigra or sinapis alba, there is, as Douglas Oakman emphasizes, the wild mustard, charlock, or sinapis arvenis, whose "plants have from time immemorial been found as weeds in grain fields" (1986:124). The mustard plant, therefore, is, as domesticated in the garden, dangerous and, as wild in the grain fields, deadly. The point is not just that it starts small and ends big but that its bigness is not exactly a horticultural or agricultural desideratum.
In the three independent versions of Jesus' parable, only that in the "Sayings Gospel Q" refers to a domesticated mustard plant deliberately sown, for Matthew in a field, for Luke in a garden. The "Gospel of Thomas" and Mark seem to presume one that sows itself, takes over, and grows big enough to attract birds for shade. Within my own methodology, I prefer not to assume that Luke's garden is original, although if it was, as Brandon Scott comments, it would be against the teaching of the Mishnah, which, around the year 200 C.E. and precisely because of that tendency for mustard to intrude and mix with other plants, decrees that should not be planted in a garden but only in a larger field where it can be carefully segregated by itself (374, 380). I prefer, methodologically, to bracket both Luke and the Mishnah and to conclude that the core image of the parable is of the mustard plant, whether of the deliberately sown but still relatively dangerous domestic variety or of the intrusive and so absolutely dangerous wild variety. "It is hard," Douglas Oakman rightly concludes, "to escape the conclusion that Jesus deliberately likens the rule of God to a weed." And he is also surely correct that a peasant audience hearing Jesus speak of birds attracted by the mustard plant would think immediately, as in 34 The Sower[1/3] parable, "that birds are natural enemies of the sown" (1986:127).
The point, in other words, is not just that the mustard plant starts as a proverbially small seed and grows into a shrub of three or four feet, or even higher, it is that it tends to take over where it is not wanted, that it tends to get out of control, and that it tends to attract birds within cultivated areas where they are not particularly desired. And that, said Jesus, was what the Kingdom was like: not like the mighty cedar of Lebanon and not quite like a common weed, like a pungent shrub with dangerous takeover properties. Something you would want in only small and carefully controlled doses-if you could control it.