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Problem: Cannabis leaves and roots show abnormal and slowed growth when showing a boron defiency. The first signs of a cannabis boron deficiency is abnormal or thick growth tips. New leaves may display chlorosis (yellowing of leaves). Stems may become rough or hollow. The plant may look like it has a calcium deficiency because boron is needed for the plant to properly use calcium. New growth is affected the most, and may look like it's been burnt or scorched.
Solution: Boron is not absorbed will without enough potassium and nitrogen, or if there isn't enough moisture. You plant may also exhibit signs of a boron deficiency if the pH is too high or low. Flush your system with clean, pH'd water that contains about half the nutrients needed for the plant and watch to make sure that the problem clears up for new growth.