“smaller...becomes a tree.” The mustard seed is a very tiny seed but grows into a very tall plant, which Jesus hyperbolically calls a “tree.” Although there has historically been some disagreement about it, today scholars identify the “mustard” in the parable as the common black mustard (brassica nigra). In the parable, the man purposely grew this mustard in his garden, just as people still do today, and mustard was valued as a spice and for the oil it produced. Although these annual plants commonly grew to only 3 to 4 feet tall, much larger plants are regularly observed, some growing to 10-15 feet tall with a central stem as large as a man’s forearm, and especially in the fall as the lack of rain hardens the plant they are well able to support a bird’s nest (Plants in the Bible, p. 59, 60).
The context of Jesus’ statement about the size of the mustard seed is the man sowing seed in his garden, which is confirmed by the word “garden plants” (lachanon; #3001 λάχανον; a potable herb; a vegetable). This verse is not a botanical reference to the size of every seed known to man, but rather a comparison of the mustard seed to the other seeds a gardener would typically sow in his garden in the biblical world at the time of Christ. It is absurd, and a misuse and misunderstanding of how the Word of God is written, to try to prove an error in the Word of God by finding a seed smaller than a mustard seed. Harold and Alma Moldenke correctly point out, “Such statements as that concerning the size of the mustard seed must always be judged in the light of the knowledge of the time of the people involved” (Plants of the Bible; Dover Publications, 1952, p. 61). Furthermore, but less likely, Jesus may also have been using a natural hyperbole (exaggeration), a common figure of speech used in discourse, the same way many Westerners will say, “I am starving” when they are just hungry, or “I’m freezing” when they are just cold. The point of Jesus’ parable was that just as the mustard seed starts out very small but becomes very big, so too the Kingdom of Heaven seems to have a small start, but will one day fill the earth. When Jesus said the seed “becomes a tree,” we must remember that the word “tree” is flexible and can refer to both large and small trees, and many of the trees in the Middle East are quite small. Thus a mustard plant that grew to 10 or 15 feet (2 or 3 meters) could rightly be said to “become a tree.”
Matthew 17:20 and Luke 17:6 refer to “trust like a mustard seed.” The mustard seed is small, but it has complete trust that it can grow into the large garden herb. See commentary on Matthew 17:20.
“the birds of heaven.” The word translated as “heaven” is the standard Greek word that refers to heaven, the atmosphere, or the air. These are not “spiritual birds.”