Ok, I admit, my answer was not good. So, if you allow, I try again. So, let’s look first what the Bible said:
Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field: Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof.
Matt. 13:31-32
It says “least”. Thing can be least in many meanings. In this case it the mustard tree seed was not appreciated as high as the other plants and so was least of the seeds, low in rank. But it can grow as said there and as can be seen from the images here:
https://www.google.fi/search?q=must...=0.95#imgrc=04-HVlUpCToJyM:&spf=1577391708442
Sorry, I should have been more focused.