Wally in Toronto, Ontario, asks:
When I bring my rosemary plants into the house for the winter, they get a powdery mildew that kills new growth. How do I treat this?
Good air circulation is important to prevent or at least control powdery mildew. Don’t crowd your rosemaries with other houseplants and keep them away from humid locations. Consider directing a small electric fan on your plants a few hours a day to increase air circulation. Remove infected leaves as they appear.
Unlike most herbs, rosemary doesn’t like completely dry conditions. Water when the top of the soil is dry, but avoid waterlogging the soil. There’s no need to fertilize in winter, but begin feeding every other week in early spring with a balanced formula, such as 20-20-20.