Dill's green, frilly appearance makes an attractive garnish, while its flavor echoes that of anise or caraway. Fennel fronds provide an easy stand-in if you're going for the look of fresh dill weed. If you need to substitute for dill in a recipe, what you choose depends on whether the ingredient list calls for dill weed or dill seed.
Dill weed is the feathery frond of the herb and has a much more pronounced flavor than dill seed. Fresh tarragon can stand in for dill but will impart a slightly more licorice flavor. Dried tarragon may also be used in place of dried dill weed. Dried dill weed can substitute for fresh; use 1 teaspoon of dried herb for every tablespoon of fresh.
Dill seeds taste slightly bitter and also hint at the flavors of caraway and anise. The seeds are usually used in pickling brines, as well as casseroles and roasts. Caraway seeds or celery seed provide a similar bitter flavor.