This variety, the kind most commonly used in cooking is not a long lived plant in my garden. A few years seems to be the max. They can get fungal diseases. Moisture seems to be key - not too wet and not too dry. My more decorative salvias have livesd for many years.

Yes, this happens to me (several times as well), and my deduction is much as above. A bit more moisture than it wants maybe three weeks or so before, and then sudden decline when the heat sets in. So sad, I LOVE sage as seasoning...but find it much easier to just buy the spice now :(