When to Harvest
Herbs should be harvested when the oils responsible for their flavor and aroma are at their highest. Proper timing depends on the plant part you are harvesting and how you plan to use it. Herbs can be harvested when the plant has enough foliage to maintain growth. With annual herbs, they can be cut back 50-75 percent and still recover. With perennial herbs, remove about one-third of the growth at any one time. Use a sharp knife or pruners to make clean cuts.
Try to harvest early in the day after plants dry off and before it gets hot. Herbs are best harvested before they start to flower otherwise leaf production declines. Deadheading or removal of flowers as they appear will result in the continued production of new leaves suitable for harvest.
When harvesting foliage from herbs, be certain that the plants have not been sprayed with pesticides. There are a variety of products to control insects and diseases but many of them are not cleared for use on herbs harvested for foliage to be eaten.