ANSWER: The Bible records Jesus gave what is known as the mustard seed parable on two separate occasions. The first time, recorded both in Matthew 13:31 - 32 and Mark 4:30 - 32, occurred sometime before Jesus celebrated the third Passover of his ministry (which happened in 29 A.D.). The second time it was given (Luke 13:18 - 19) occurred later, just before the winter of 29 A.D., when Jesus was making his way toward Jerusalem to keep the Feast of Dedication (Luke 13:22, John 10:22 - 23).
The 'mustard' parable is one of several 'Kingdom' related stories recorded in Matthew 13 given while the Lord preached from a ship to a large crowd of people on the shore. Many people have been taught that such stories were given so that listeners would understand what Jesus was teaching, however, the first part of Matthew 13 states otherwise (see verses 10 - 15).
31. Another parable He presented to them, saying, "The kingdom of heaven is compared to a tiny mustard seed which a man took and sowed in his field; 32. Which indeed is very small among all the seeds; but after it is grown, it is greater than all the herbs, and becomes a tree, so that the birds of heaven come and roost in its branches." (Matthew 13:31 - 32, HBFV throughout)
The world’s smallest seed is not the mustard but that of the orchid plant. Some people have jumped to the conclusion (especially after reading the King James Translation) that Jesus must have made an ERROR when he labeled the seed the smallest of all. Part of the problem lies in the KJV's translation which says it was "the least" (Matthew 31:32, KJV). A more accurate translation, shown above, states it was "very small" - meaning it was among the smallest but not literally the tiniest of all.
Additionally, Jesus was using a commonly known first century expression to state his point to the crowd.
" . . . it is sufficient to know that 'small as a grain of mustard seed' was a proverbial expression among the Jews for something exceedingly minute" (Basic Theology: A Popular Systematic Guide to Understanding Biblical Truth, section 14, Problem Passages).
"The rabbis called the smallest possible quantity 'the quantity of a grain of mustard,' and Mahomet uses the same expression in the Koran" (A Commentary on the Holy Bible: The One Volume Bible Commentary, comments on Matthew 13).
This parable contrasts the very small size of a seed with the tremendous results it produces. Likewise, the Kingdom of God will begin small, with just the few people now being called and converted in this age. However, it will grow to immense proportions as people are resurrected and learn to live God's way.
When Jesus finished giving his Kingdom teachings in Matthew chapter 13, he told his disciples the following.
51. Jesus said to them, "Have you understood all these things?" They said to Him, "Yes, Lord." 52. Then He said to them, "Therefore, every scribe who has been instructed in the kingdom of heaven is like a man who is a householder, who brings forth out of his treasure things new and old." (Matthew 13:51 - 52)
What is the treasure that brings forth things old and new? It is the truth of God’s word in both the Old and New Testaments. When a householder brings out some of his treasures, whatever they may be, he does not simply show them to his children, but distributes it to them. Jesus put the responsibility on his disciples to teach about the treasures of God’s kingdom to others.
Here is one final note regarding the mustard seed analogy given by Jesus. Do not waste time debating whether the plant in the parable produces a bush or a tree. In arguing such things, people miss the entire meaning of the teaching.