What exactly is the correlation of kingdom and mustard plant?
Pliny the Elder, who lived between 23 and 79 C.E., wrote about the mustardplant in his encyclopedia "Natural History":
With its pungent taste and fiery effect, mustard is extremely beneficial forthe health. It grows entirely wild, though it is improved by beingtransplanted: but on the other hand, when it has once been sown, it isscarcely possible to get the place free of it, as the seed when it fallsgerminates 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 itsdomesticated counterpart, but even when one deliberately cultivates thelatter for its medicinal or culinary properties, there is an ever-presentdanger that it will destroy the garden. And, apart from those domesticatedtypes, such as brassica nigra or sinapis alba, there is, as DouglasOakman 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 thegarden, dangerous and, as wild in the grain fields, deadly. The point isnot just that it starts small and ends big but that its bigness is notexactly 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 Scottcomments, it would be against the teaching of the Mishnah, which, aroundthe year 200 C.E. and precisely because of that tendency for mustard tointrude and mix with other plants, decrees that should not be planted in agarden but only in a larger field where it can be carefully segregated byitself (374, 380). I prefer, methodologically, to bracket both Luke and theMishnah and to conclude that the core image of the parable is of themustard plant, whether of the deliberately sown but still relativelydangerous domestic variety or of the intrusive and so absolutely dangerouswild variety. "It is hard," Douglas Oakman rightly concludes, "to escapethe 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 ofbirds attracted by the mustard plant would think immediately, as in 34 TheSower[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 aproverbially small seed and grows into a shrub of three or four feet, oreven higher, it is that it tends to take over where it is not wanted, thatit tends to get out of control, and that it tends to attract birds withincultivated areas where they are not particularly desired. And that, saidJesus, was what the Kingdom was like: not like the mighty cedar of Lebanonand not quite like a common weed, like a pungent shrub with dangeroustakeover properties. Something you would want in only small and carefullycontrolled doses-if you could control it.