Where are you located? While it could be sunburn, it could also be a reaction to cold temps. Basil is a tropical plant. Tomato plants can also suffer from this purplish discoloration when temps are a bit too cold for comfort.Letendre wrote:My basil plants have been doing good; I recently started taking outside to harden off. Some of the leaves have been getting a purple discoloring. Any ideas? Perhaps it was a sunburn?