Purple stems are commonly caused by a Phosphorous (P) or Magnesium deficiency, if there is a P deficiency you may also notice symptoms such as brittle leaves or greyish spots. To assist the uptake of Phosphorous (as well as most other nutes) you should ensure the pH is slightly acidic– 6.0 will suffice.Click to see full answer. Also, why are my stems purple?Answer: When a cannabis plant or seedling has a purple stem without any other signs of problems, it is often the result of genetics. However, sometimes you’ll see red stems (pictured right) which can sometimes be caused by stress, environment or pH problems.Beside above, why is my sunflower stem red? The pigments that make the stems (and leaves) red are anthocyanins (red carotenoid pigments). When found in stems and leaves, these pigments protect these structures from becoming sunburned by the UV light. This is quite possibly what you are seeing in your sunflowers. Similarly, why has my sunflower stem split? Heavy rains after a drought or overfertilizing can lead to rapid, and possibly weak, growth. A strong wind might cause the plant to sway, splitting the stem.What is eating my sunflower stalks?The most common sunflower pests include the following: Sunflower Beetles – Sunflower beetles typically feed on the leaf foliage and in small numbers or older plants may seldom hurt the plants. Sunflower Borers – Sunflower borers and stem maggots burrow into the stems of sunflower plants to feed.