Answer: I think the oil sprays are causing the leaves to suffocate and turn brown. Generally it is best to prune the plants back after they have finished flowering. If you are spraying oil to control insects I'd suggest simply hosing the plants off with plain water if you find insects on them. Otherwise, just clip off the spent flowers and some of the upper leaves when flowering is finished. If you stop using the oil, I think you'll see a big change in the appearance of your May Night Salvia.
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