Matthew 13:31; Mark 4:30; and Luke 13:19
“The kingdom of heaven is like a mustard seed, which a man took and sowed in his field; and this is smaller than all other seeds; but when it is full grown, it is larger than the garden plants, and becomes a tree, so that the birds of the air come and nest in its branches,” (Matt. 13:31; see also Mark 4:30; Luke 13:19).
No, the mustard seed is not the smallest of all seeds. Jesus was speaking proverbially. That is, he wasn't making a statement of absolute fact but using a proverbial style of communication.
There are different kinds of mustard trees in Israel and the mustard seed was the smallest of all the seeds known there and used by those in Israel. Also, notice that Jesus says that when it is full grown it is larger than the garden plants and becomes a tree so that the birds nest in it. There were many gardens in Israel with many types of plants, many of which were larger than the mustard plant. The olive tree for example, can grow to 20 feet or more. The mustard tree known as Salvadora persica has extremely small seeds and grows into a small bush. Brassica nigra is a mustard plant that grows to about 8 to 10 feet when mature and is probably the one Jesus was using for his illustration. Jesus would have known that it wasn't the largest of garden plants because of the prevalence of larger plants. Therefore, he was not making a botanical statement of fact. Instead, he was drawing attention to the comparison of the "smallest" to the "largest" and using it to illustrate how the Kingdom of heaven will expand in the world from a very small beginning to a huge presence.
Also, Jesus used the mustard seed elsewhere in a proverbial sense.
"And He said to them, “Because of the littleness of your faith; for truly I say to you, if you have faith as a mustard seed, you shall say to this mountain, ‘Move from here to there,’ and it shall move; and nothing shall be impossible to you," (Matt. 17:20; see also Luke 17:6).
So, we see that Jesus used the mustard seed in illustrations in the style of proverbs to illustrate a point and that he was not speaking in a scientifically accurate sense.