Answer: There are many possible causes. If they are yellowing from the bottom of the plant first, it could be overwatering or poorly drained soil. (Most annuals like soil that is evenly moist like a wrung out sponge, not sopping wet/saturated and not dried out.) If they are yellowing all over, it might be due to transplant shock. If they are yellowing accompanied by spots on the foliage, it might be a disease problem. It could also be due to overfertilizing or to accidental herbicide exposure, for example. You might want to consult with your local county extension to see if they can help you diagnose the problem more specifically.
Q&A Library Searching Tips