Answer: The leaves could be yellowing for a number of reasons. Too much or too little water can cause yellowing. If they are new transplants, they could be suffering some shock from the transplanting, and from adjusting to their new home. If the plants are in containers or in a new garden, they could be suffering from lack of nutrients, if you haven't fertilized them and/or if you have poor soil. (How are the other plants in your garden doing?) If the weather has been very cool it could be a reaction to that. Finally, it could be due to disease.
If the older leaves are turning bright yellow, then I would check to see that you are keeping the plants evenly moist. If they have gray or brownish spots, you may have some type of disease problem. Inspect the plant carefully, to see if you can find any particular symptoms that might help us determine the exact problem. Feel free to submit another question via the web site, describing the symptoms as detailed as possible. If other plants aren't thriving in the garden, you may want to have your soil tested for pH and nutrient levels.
Sorry I couldn't help more--but yellow leaves are such a common symptom I can't pinpoint the problem.
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