As soon as your plants have filled out and become well established, you can start using the fresh leaves of most species and continue harvesting them all summer long.
It is best to harvest herbs in the morning, once the dew has evaporated but before the sun's rays become too strong, as this is when they contain the most essential oils.
If you want to preserve the herbs to use them later, the best time to harvest them is before they flower, to obtain the maximum potency and flavour from the essential oils.
Dill, cumin, coriander and fennel seeds are harvested when fully ripe, usually at the end of the season.
Small-leafed and long-stemmed species are suitable for air-drying in bunches. After washing the stems, tie them up with an elastic band. You can also spread them out on racks or trays. Be sure to cover the racks or bunches with cheesecloth to protect them from dust.
Herbs should be dried in a warm, airy, dark place. Warmth will speed the drying process. Air circulation is important to prevent mould from forming as the moisture evaporates. Don't dry herbs in direct sunlight, or they will lose their flavour.
Faster drying methods, such as a conventional or microwave oven, should be used for large-leafed herbs. Watch carefully that they don't burn or become too dry. Heat your oven to 65°C/150°F, spread the herbs out on cookie sheets, leave the oven door slightly ajar and turn them several times while they are drying. The whole process will take several hours. If you decide to use a microwave oven, check the herbs every 20 seconds, as the leaves will dry very quickly.
To dry stems with seeds, hang them in a paper bag or over a cloth in an airy, warm, dry spot.
Dried herbs and seeds keep well in airtight, opaque glass containers. Some kinds, including parsley and chives, can be frozen immediately after harvesting.
Dried or frozen herbs should be used within six months, or they will lose much of their flavour.