Because of the flavor differences, the seeds and leaves of the dill plant are not ideal replacements for each other; however, it is possible in a pinch. Keep in mind that you will need to use different amounts when substituting one for the other. Three heads of dill weed is roughly equivalent to a single tablespoon of the seeds. In addition, bear in mind that the seeds stand up to longer cooking times better than the leaves. This means that if you are using dill weed in place of the seeds, it is best to add them towards the end of the cooking time rather than at the beginning.
Fresh dill weed is a popular complement to fish but can also be a pleasant addition to potato salad. Like the dill seed, dill weed works well with legumes but it is also enjoyable in coleslaw and is useful for flavoring dips. You can even use the seeds and the leaves of the dill plant together in some salad dressings and vinegars.
Is dill weed an effective substitute for dill seed or vice versa?
When making substitutions, you should also consider the difference in appearance between the seeds and the leaves. Some people find the appearance of dill weed in pickle brine to be unappetizing. If you are using dill weed instead dill seeds to flavor your pickles, you may want to chop it finely to make it less noticeable.
In the United States, the most well known use of dill seeds is as the main flavoring in dill pickles; however, they are widely used in Indian, Eastern European and Scandinavian cuisines. Dill seeds are excellent when used in acidic dishes including pickled beets, carrots and even pickled fish. You can also add them to your lentil dal or use them with any other legume to aid digestion.
Like many herbs, the seeds and the leaves do have some similarities but they are not identical. The flavor of dill leaves is similar to that of parsley and anise with notes of lemon. While dill seeds do have the same notes of anise, they also have notes of caraway. The seeds’ flavor is more pungent and some cooks even consider it slightly bitter and reminiscent of camphor; on the other hand, the leaves’ flavor is more delicate. In addition to all that, dill seeds have a characteristic not found in dill weed: their flavor tends to become stronger when heated.
When fresh dill is being used to flavor a recipe (as it is in pickles, soups, and sauces), use fresh tarragon in its place. To make the proper substitution, use an equal amount of fresh tarragon for the fresh dill, or dried tarragon for the dried dill. You can also use dried tarragon as a stand-in for fresh dill weed, but you’ll need to adjust the quantities, as it has a more intense flavor. Use one teaspoon of dried tarragon for every tablespoon of fresh dill called for in a recipe. Tarragon works well as a substitute for dill in seafood dishes and in salad dressings.
Dill is incredibly easy to grow, so consider adding it to your garden. It’s an annual, but it reseeds readily. Just allow some of the flowers to go to seed at the end of the season, and it should come up on its own next year.
Substituting Other Herbs
If dill weed is being used as a garnish for a dish, use fennel fronds instead. They look very similar. Fresh parsley can also be used as a garnish. It looks a bit different, but will still add that pop of green. If you don’t have either, just leave the garnish off, or get creative with whatever you have on hand.
Dill seeds taste similar to dill weed, but they have a slightly bitter edge to them. They appear frequently in pickles, bread, salad dressing, and soup recipes. While you might be tempted to use dill weed as a substitute for dill seeds, you’ll get better results if you use caraway seeds or celery seeds in their place. Replace them measure for measure, and you should come close to the intended flavor.
Dill weed is sometimes also referred to as dill leaves. It’s the bright green, feathery fronds of the dill plant. It’s highly aromatic, and tastes of caraway or anise, with a bit of citrus thrown in.
Other canners will find that their store has a deal on cucumbers but there’s no fresh dill weed within miles to be found.
Or it’s the dead of winter, you are doing winter canning with imported cucumbers (despite the very remote odds of getting a crisp pickle with them), and the balcony where you grow your herbs in the summer has howling winds from Siberia whistling through it off of Lake Michigan.
Linda Ziedrich says that if a recipe calls for a fresh dill umbel and you don’t have one, use one teaspoon of dried dill seed instead.  Ziedrich, Linda. The Joy of Pickling. Boston, Massachusetts: The Harvard Common Press. 2009. Page 14.
(Note: an umbel is a whorl, a round circle spray of the plants buds or flowers.)
The National Center for Home Food Preservation says, “For each quart, try 3 heads of fresh dill or 1 to 2 tablespoons dill seed (dill weed = 2 tablespoons).”  National Center for Home Food Preservation. Frequently Asked Pickle Questions. Accessed March 2015. [Ed: It’s not clear what they mean by the dill weed = addition at the end there: perhaps it means OR 2 tbsp dill weed. ]