The plant has a small seed which is round (not all seeds, by any means, are round!) The round nature of the seed represents wholeness; and the seed is small because intentions, when we first begin inner work, are small and weak. They seem like nothing to us; and yet it is the intention, the purposefulness, of placing this seed in our inner garden that matters.
We plant a small, even tiny, intention. The nature of intention is an inward tending; that is, by placing this intention in our inner garden, this intention towards God, and by tending it—that is, inwardly nourishing it—the Kingdom of Heaven can grow in us. Its growth isn't assured; the gardener must be diligent. Yet as long as diligence is applied, the plant (like all plants) knows how to grow itself. It must be cared for;, but the growth of it must at the same time be left to itself. This is in the nature of gardens and gardening; one must intentionally arrange and care for, but the actual growth of the plant must be up to the plant itself. It doesn't need coaching.
The mustard plant is, in full size and bloom, an exuberant plant. It spreads it branches prolifically, and produces a beautiful and expansive effect, appearing to reach into everything within view of appreciation. It is this reaching into everything that becomes so appropriate to the analogy, once it is understood; one simply need see the plant in full size and bloom to understand. It is greater than all the herbs; that is, many common herbs have savor, but none quite the sheer scope and size of the mustard plant. So of all the things that have the potential to flavor our lives, the inner Kingdom of Heaven is by far the greatest.
At the same time, the plant produces a sharp and bitter seed; and it produces these seeds in great abundance.
This is the paradox of the plant; it is bittersweet. The flowers are lovely; the seeds are sharp and biting, yet sharp and biting in a way that we find good to the senses. That is to say, it challenges and exhilarates at the same time.
In the same way, inner impressions that come to us brokered by the Kingdom of Heaven carry the same sharpness, the same challenge, and the same beauty. They are nearly infinite in nature, as abundant as the seeds of the mustard plant; and we savor them even in their sharpness.
The abundance of the plant doesn't just extend to its flowers or seeds; the overall, and overwhelming, quality of the plant to anyone familiar with it is its sheer fecundity.
The mustard plant begets a seemingly endless number of seeds and seedlings; again, the selection of the plant as an analogy is perfect. We most certainly are reminded here of Meister Eckhart's overarching premise of the enormous fecundity of the Lord:
God is in all things as being, as activity, as power. But He is fecund in the soul alone, for though every creature is a vestige of God, the soul is the natural image of God. This image must be adorned and perfected in this birth. No creature but the soul alone is receptive to this act, this birth. Indeed, such perfection as enters the soul, whether it be divine undivided light, grace, or bliss, must enter the soul through this birth, and in no other way.
—ME, The Complete Mystical Works, Sermon 2, page 39
God gives Himself in fecundity, for the noblest work of God is giving birth (as far as one can say that one work of God is nobler than another): for God takes the greatest delight in giving birth.—Ibid, Sermon 71, page 363
In other words, when Christ chooses the mustard plant as the representation of the Kingdom of Heaven, he refers not just to the size or scale of the plant; all its properties are appropriate to His message.
Yet this perfect analogy, in the end, meets an entirely paradoxical breakdown: when He says the birds come to roost in it; well, of course this is not true, because in no way can birds roost in a mustard plant.
This point—the point at which the analogy departs from the known—introduces the mystery of a higher level. The birds of the sky Christ refers to here must be lighter than air—else they would find no support. They are, indubitably, birds of the soul.
The meaning of the parable is unmistakable, and has nothing to do with outer events or influences: tending this inner seed issues an invitation to higher forces to come and find a home in me.