Tarragon and rosemary are both very flavorful herbs with long histories in European cuisine. If you are a new cook with limited knowledge of classic herbs, see how they compare in the SPICEography Showdown below.
How does tarragon differ from rosemary?
Tarragon 
Rosemary
One key difference between tarragon and rosemary has to do with the botanical family from which these herbs come. Tarragon belongs to the sunflower family — it is related to such culinary and medicinal herbs as echinacea, chicory, and dandelion. Rosemary comes from the Lamiaceae family, along with mint and marjoram.
Tarragon is often dried and you may be able to find dried tarragon in your grocery store’s spice aisle. It is better than nothing but it is not the best form of the herb. Like cilantro and parsley, tarragon’s flavor profile is degraded by drying so you should use it fresh whenever you can find it. In comparison, rosemary handles being dried well. Instead of diminishing its flavor, drying tends to concentrate it. Dried and fresh rosemary have similar flavor profiles.
Flavor and aroma are the areas where tarragon and rosemary differ the most. Tarragon’s flavor is similar to those of anise and fennel and all three herbs are commonly likened to licorice. Rosemary’s flavor has elements of mint as well as of sage and tea. There are notes of pine resin in it as well but no licorice.
Tarragon leaves are delicate and tender. Rosemary leaves are firm and sharp enough to present a choking hazard.
Can you use tarragon in place of rosemary and vice versa?
How well herbs work as substitutes depends on how closely their flavors are to a perfect match. As noted above, tarragon and rosemary are not similar at all. Tarragon will not provide the minty or resinous qualities you want from rosemary.
Rosemary cannot give your food the strong licorice notes that tarragon would. There is no way to use one in place of the other without completely changing the flavor of the dish. If you are open to altering the recipe, both herbs should be interchangeable in many dishes. Tarragon and rosemary both work well in dishes that feature chicken and lamb. Along with proteins, both are used in vegetable dishes.
While you can preserve rosemary in vinegar, it is not necessary since the herb dries so well. Rosemary vinegar is not as well-known nor widely used as tarragon vinegar so rosemary is not a good substitute in this instance.
When should you use tarragon and when should you use rosemary?
Use tarragon in egg dishes like omelets, as well as in vinegar and in pickles. You can use it raw or cooked; add it to salads and to stews. Tarragon vinegar makes an excellent vinaigrette and it works in marinades too. Tarragon is one of the two main herbs in the classic French Bearnaise sauce and pairs nicely with grilled fish. If you want to get the most from your fresh tarragon add it at the end of the cooking time.
Rosemary offers an intense flavor that goes well with proteins that have strong flavors. Game meats along with lamb and mutton are some prime examples. When used in moderation, it can work with chicken and potatoes as well. Whether fresh or dried, you can add rosemary at the start of the cooking time.