Dried herbs give less aroma than fresh ones because the volatile oils that create their aroma largely evaporate during drying. These oils are what make an herb taste fresh in the first place, and they are quite sensitive to heat and exposure to air.
Which herbs hold up better to drying?
Soft-leaved herbs like basil, parsley and dill draw most of their aroma from thin, delicate oil cells, which are easily damaged during drying and lose their oil. That is why dried basil ends up quite muted compared to fresh.
Woodier herbs like thyme, rosemary and bay leaf have thicker, sturdier oil glands; these hold onto their aroma much better when dried, and some cooks even prefer the dried version of these herbs in certain dishes because the flavour comes across more concentrated.
How is the lost aroma made up for?
It might seem like less dried herb would be needed than fresh, but the opposite is true; because the aroma is weaker, it generally takes about three times as much dried herb as fresh to get a similar effect.
Adding dried herbs early in cooking helps their oils slowly infuse into the dish and release their aroma. Fresh herbs, on the other hand, are usually added at the end, since their aroma burns off quickly with heat.
How dried herbs are stored also directly affects how much aroma is left; a herb kept in a sealed jar away from light holds its aroma far longer than one left in an open container or on a shelf exposed to light.
Dried herbs also have a limited shelf life; their aroma noticeably fades within about six months to a year, so a jar that has been open for a long time can be checked by rubbing a pinch between the fingers and smelling it.
The drying method also determines how much aroma survives; herbs processed by freeze-drying keep a much larger share of their volatile oils than herbs dried with hot air, since the process happens at low temperature. That is why some store products labelled as freeze-dried promise a stronger aroma.
Keeping the herb in whole leaf form and crumbling it only when needed also helps preserve aroma; herb that has already been crushed into a fine powder loses its aroma faster than whole leaves, since a powder exposes far more surface area to air. That is why some cooks prefer to rub dried herb between their fingers right before using it.
The climate and soil the herb grew in also affect its original oil content; herbs grown in sunny, dry regions generally build up more concentrated volatile oils, which leaves a relatively stronger aroma even after drying. That is why dried herb of the same type from different sources can taste noticeably different.