Rosemary is such a vigorous grower; it seldom needs fertilizer. But if growth is slow or stunted, or appears pale yellow, use fertilizer once in early spring before new growth begins.
You can use any all-purpose fertilizer, preferably a 5-10-5 or 10-10-10, in liquid or dry form. Be sure to apply the fertilizer to the root area, avoiding the plant itself as fertilizer can cause leaf burn.
You may also use a foliar spray in mid-season if you desire, to keep your plants healthy.
Alternately, you can use a top dressing of organic material in the spring instead of fertilizer.
Do you fertilize your rosemary plants? What type of fertilizer do you use? How often do you fertilize? Please tell us exactly how and when you fertilize your rosemary plants.