Choose your substitute by what you’re trying to preserve: tarragon’s licorice/anise aroma (fennel, chervil) or its bright herbal lift (basil, dill).
Start with the listed ratio (or less if you’re using a concentrated swap like ground star anise), then taste after the first simmer or mix.
If the substitute is bitter or too intense, balance with acid (lemon juice or a small splash of vinegar) and a little salt.
For hot dishes, bloom dried anise-like herbs in warm butter/olive oil for 20–30 seconds to reduce harshness.
For creamy sauces/dressings, stir in after heat is off so the aroma stays fresh.