The shelf life of dried herbs depends on whether the herbs where preserved whole or crushed. The lifespan also differs from species to species and can change drastically if the herbs are not stored properly. Whole dried herbs can be expected to last one to three years, although crushed and powdered herbs may only last six months.
Typically, dried herbs that are still viable for use smell potent. Even dried herbs which have lost their color and turned brown may still be good if they still smell good. If an herb is completely odorless, it is probably also tasteless, which means that it is too old for use.
Many store bought and prepackaged herbs have "sell by" or "use by" dates. These dates are typically not a good indicator of the freshness of the herb. In some cases, herbs may last much longer than their use by dates, and in other cases, they will not. A whole leaf can retain flavor better than a crushed leaf and lasts longer. Crushed herbs tend to lose their potency much faster than whole leaves.
Apart from the difference between species, properly stored herbs will last much longer than carelessly stored herbs. For the best retention of flavor and odor, dried herbs should be stored in glass, airtight containers away from the sun at about 68 degrees F (20 degrees C). Wooden containers can actually absorb the essential oils in herbs that give them bouquet and flavor. Sunlight can cause discoloration and greater fluctuations in temperature. If the temperature changes too often or too rapidly, it will cause condensation and ruin the herbs.
If herbs are not stored in an airtight container, they can absorb moisture. This compromises the flavor and texture of the herb and can also cause molding or rotting as well as discoloration. If herbs are stored in a container with a sifter lid, it is common for cooks to shake the herbs directly into a boiling pot or steaming roast pan. This allows moisture from the hot vapor to enter the remaining herbs, so this should be avoided for maximum shelf life.
In some cases, especially if the herbs were dried and packaged at home, there is some remaining moisture in the leaves. When placed in an airtight container, the moisture will rot the leaves, sometimes resulting in mold. If this happens, the herbs should be discarded. Although home drying herbs is a relatively easy and inexpensive way to gather a variety of seasonings, it is important to dry herbs thoroughly and store them appropriately.