It is impossible for verbs which have a continuous and proceeding action to come in imperative form with the word יהיה (will be); as for יצמח, יגדל, יפרה – it will not say יהיה צָמֵח or יהיה גָדֵל, but rather say יהיה גָדול (it will be great) - which establishes that it is already absolutely great.

And from this, they learn that when it is written ראשו יהיה פרוע (the hair of his head shall go loose)²³, it does not mean growth of hair, but rather it is talking about revealing the looseness and the hair.

footnotes: ²³ Leviticus 13:45.