Purple stems usually are a sign that there is a lack of phosphorus to the plant. This can be from a nute lockout situation or a lack of it being supplied to the plant in an another manner such as poor ferts.
Phosphorus Second to nitrogen as a limiting factor in most soils, phosphorus is essential for the production of a compound called ATP (adenosine triphosphate) that plays a critical role in turning light energy into the energy of chemical bonds. Phosphorus is also part of several proteins, enzymes and nucleic acids that are vital to all plants. Phosphorus deficiency symptoms show first in the more mature leaves, which turn dark green. At maturity, deficient plants are stunted and often show tints of red or purple colors that are caused by buildup of pigments known as anthocyanins. This condition slows flowering in the plant.
Abundant supplies of phosphorus speed maturity and make large flowers. If excess phosphorus is applied, root growth is stimulated, so if you have a short season, risk overdosing with phosphorus rather than underfeeding.
Sources of phosphorus include rock phosphate, colloidal phosphate, bone meal and super-phosphate fertilizer. I prefer nutrients in their organic form because you have a wider margin of error; you are much less likely to damage your plants with organics than with chemicals. In the case of phosphorus, organic forms are relatively slow to go into solution, the soluble forms that can be absorbed by the roots of plants. Colloidal phosphate becomes available to root more quickly than rock phosphate; steamed bone meal dissolves faster than raw bone meal. If you think you need instantly available phosphorus, then youll have to get super-phosphate; one commonly available form is Blood Food, a 0-10-10 mixture. (By law, the three NPK numbers on a fertilizer label indicate concentrations of Nitrogen, Phosphorus and Potassium)
not an indoor grower but have to alter the PH in soil sometimes. the product that i use is PH down. IT accomplishes this with phosphoric acid. 1quater teaspoon to a gallon drops 7 to 6. Dont know how critical you're ph factor is but phos is essential period , so phosphoric acid would give fast responce. other remedies are foliar sprays good luck Rzb
Purple stems mean very little usually, people always freak out about purple stems, DON'T SWEAT IT!!! your leaves will tell you when there's a problem and lots of breeds grow with purple stems, WW, AK-47, White Rhino, Nebula (both strains), Somango, Chitralah, Bhuddas Sister, S.A.G.E., Black Domina, etc., all have purple stems.
it can also be caused by low night temps, don't worry, it'll be all right......
I forgot, also Biddy Early and Warlock do the purp stem thing.....