Problem 1: Jesus said that the mustard seed was “the least of all the seeds.” However, today we know that the mustard seed is not the smallest seed of all. Some think Jesus was speaking of the black mustard seed. But, even this is not the smallest of all seeds.
Solution 1: Jesus was not referring to all the seeds in the world, but only those that a Palestinian farmer sowed in his field. This is made clear by the qualifying phrase “which a man took and sowed in his field” (v. 31). And it is a fact that the mustard seed was the smallest of all seeds which the 1st century Jewish farmer sowed in his field. So there is no contradiction here between science and Scripture. What Jesus said was literally true in the context in which He said it.
Problem 2: Some claim that the mustard seed cannot grow big enough to house birds, let alone grow to tree size.
Solution 2: But this is not so, because there is evidence that some mustard seeds grow into trees about ten feet tall. This would certainly provide enough branch space for a bird to build a nest (Matt. 13:32).