I have good luck with most herbs with the exception of rosemary. I cannot seem to get it right. It does okay for a short time, but then it gets gray and dies. I usually transplant into a clay pot using a good grade of potting soil, place in a sunny spot, and water only if it is dry. Rosemary is one of my favorites and I would like to figure out how to grow it successfully.
Kansas City, MO (zone 6)
Well, it certainly sounds like you are doing everything right. As a native of the Mediterranean, rosemary prefers a warm, sunny and dry environment which according to your question is exactly what you are providing.
Because you described the affliction as "gray" I’m suspicious that your rosemary had powdery mildew. It is a white, moldy looking fungus that is a common problem for rosemary. Powdery mildew tends to be more unattractive than fatal, but left unchecked it could kill a plant. Powdery mildew is most active during hot, humid weather. Placing your rosemary where it will get good air circulation will cut down on its occurrence. If it appears again, spray with a fungicide that is safe for edibles such as Garden Safe Fungicide 3. Continue to apply the fungicide every week until the problem is gone. I also recommend testing a small area first before you spray the entire plant.
Another thought is the potting soil is not draining well enough. This is a big concern with rosemary because too much moisture around the roots will cause the leaves and stems to turn brown and the plant will eventually die. Try a grittier mixture such as 1 part good quality potting soil, 1 part sand and 1 part humus.
Typically rosemary will not overwinter in your area. It freezes when temperatures dip below 25 degrees F. But it is an easy herb to grow indoors. Move it inside along with your houseplants in fall. Put it in a sunny window (south facing is ideal) and be careful not to overwater it. The roots can easily rot. An occasional misting can help if it gets too dry in your house.