Answer: Spring is the best time to prune your rosemary back. It sounds as though you may have overdone your last pruning attempt if your rosemary died. Instead of allowing it to grow very large and then wacking it back, try to keep it under control by pruning a little all summer. If your plant is in need of a major overhaul, do it in stages rather than all at once and it won't go into shock and die. The general rule of thumb is to remove no more than one-third of the live material at any one time. So, if your rosemary is really overgrown, remove one-third now and then give it time to recover. You can remove another third in early July and then prune it back again in September.
Best wishes with your garden!
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