Rosemary trees, topiaries and potted arrangements (sometimes with sage or other herbs) are often sold this time of year. In my part of the world, except perhaps for a few varieties (and maybe not even then! I lost even the hardier sorts of rosemaries that wintered on my porch last year!) rosemary is not hardy unless it is somehow protected–grown in a cold frame, or wintered over indoors.
It’s the indoor wintering that can often prove to be problematic, however. Because while rosemary cannot grow outside in my region in the winter, it also doesn’t like to be indoors either. Most homes are too warm for it and it quickly develops diseases like powdery mildew (white fuzzy stuff on the leaves)
While powdery mildew is not likely to kill the plant unless it is a really severe case, it will eventually blacken the branches and it certainly makes the fragrant leaves (some call them “needles”) unsuitable for cooking. Who wants to cook with a mildewed plant?
So there are a couple of ways to avoid this: first, try not to crowd the rosemary (impossible at my house).
Next, never over-water. (Rarely a problem at my house). But I water from the bottom in an attempt to ensure this is even less of a problem.
Also make sure the plant has the sunniest window possible. South is ideal.
Finally, if your south window is in a cool room, that’s even better.
And if your rosemary does come down with the “fuzzies,” a spray of 50/50% solution of milk (any sort of milk will do) and water should help cure that. Just try to correct any cultural issues (over-watering, too little sun) if possible at the same time.