Wet, compact soil or overwatering can trigger a phosphorus deficiency to appear even when all other factors are perfect. So make sure you water your plants properly every time to help prevent a phosphorus deficiency.
It’s important to remember that some cannabis strains naturally grow with red or purple stems even when all their nutrient needs are being fulfilled, so red or purple stems is not a symptom to worry about on its own.
Sometimes you will get a cannabis phosphorus deficiency, and the stems do not appear red or purple at all, or the coloring may not be pronounced.
When you notice that stems are turning red or purple starting from underneath, it may be a sign of a phosphorus deficiency only if accompanied by other symptoms. If the only symptom shown by your plant is red or purple stems, and you are not seeing any other signs of splotches or unhealthy leaves, the red or purple stems are likely caused by the genetics of your plant. If that’s the case, you have nothing to worry about.
The progression of a cannabis phosphorus deficiency
When there is a phosphorus deficiency, the lower (oldest) leaves turn dark green. Leaves occasionally get a bluish or bronze tinge, and may thicken or curl downward before exhibiting dark gray, bronze or purplish splotches. Sometimes the stems of the affected leaves will turn bright red or purplish, usually starting from underneath.
A cannabis phosphorus deficiency usually appears with some or all of the following symptoms:
Please note that leaves which have been damaged by a phosphorus deficiency will probably never recover or turn green, so you want to pay attention to other leaves for signs of recovery.
Notice that the stems or veins never turned red or purple on this leaf, except for some parts that were actually affected by the phosphorus deficiency.
Phosphorus deficiencies cause dark splotches on leaves and can appear at the bottom of the plant on some of the oldest leaves. Learn to spot & fix the issue.