Peak Herb Leaves (June–August): Mid-summer is the optimal time for harvesting almost all leafy herbs. Basil, oregano, marjoram, sage, spearmint, and tarragon reach their peak essential oils just before or as the first buds appear. Seed Harvesting (July–August): As summer progresses, cilantro and dill (as well as fennel) will bolt and produce seed heads. Harvest when the seeds dry and turn brown.