Root Crops – Herbs such as garlic, ginger, and galangal are best harvested in the fall when the leaves start to yellow and the plant is preparing for a dormant period.
Annual and Biennial Herbs – Basil, summer savory, parsley, and other annual or biennial herbs grown for their leaves can usually be harvested periodically during the growing season. In fact, basil benefits from regular harvesting, which will prevent it from going to seed and completing its life cycle. Those of you in colder climates should also remember that your shorter growing season and harsher winters make it necessary to treat some true perennials as if they were annuals.
Flowering Herbs – Edible flowers from herbs such as roses, lavender, borage, and pot marigold are best harvested when the flower has just opened. The flower petals, ovary, and calyx are all firm and at their maximum freshness at this point.
Seed Herbs – Herbs that produce seeds require the most precise timing for harvest. You must wait until the seeds are fully ripe, since no further ripening or improvement in flavor will take place after the seedhead is separated from the plant. Once maturity is reached, harvest immediately to maximize the amount of the seeds you capture. Delaying even a few days can result in loss of the seed crop to hungry birds or scattering of the seeds due to high winds or other weather conditions.
Perennial Leaf Herbs – Perennial herbs are the easiest to harvest. In general, you can harvest any time during the growing season when enough plant material is available. My only caution is to avoid harvesting in late fall. Late harvesting can stimulate growth of tender shoots that will not have time to harden before winter hits. It can also deprive the plant of its natural buffer zone against drying winter winds that cause “die back” and sometimes the death of the plant.