Choosing a substitute for parsley depends in large part on how you intend to use the herb, and whether you are looking for something that will mimic parsley in looks, in taste, or both. Parsley is generally a very mild herb, and carries almost no flavor. It is commonly used both fresh and dried in cooking, usually as a garnish. Cooks who cannot find or cannot eat ordinary parsley often choose other green herbs, particularly cilantro or chervil, as a substitute. Using other parsley varieties, particularly curly parsley, is often a good choice as well, though much depends on what is available, as well as your reason for avoiding parsley in the first place.
There are two main varieties of parsley, and they interchange with each other well. Italian parsley has flat dark green leaves, and is usually the most popular variety in commercial markets. Curly parsley, sometimes called “common parsley,” has a lighter color, tightly kinked leaves, and often a slightly bitter taste. Substituting one version of parsley for another, or even using dried parsley in place of fresh, is often the easiest way to compensate for not being able to find the sort of parsley you are after.
Parsley is rarely used for its flavor, which can make choosing a non-parsley substitute somewhat difficult. Chervil, a plant in the parsley family, is usually the best bet where taste-comparison is concerned. This herb has French origins and often tastes slightly like anise. It is not always easy to find chervil outside of gourmet grocers, however. Chervil is also not an acceptable substitute for parsley for people who suffer from parsley allergies.
If you are aiming more for the look of a parsley garnish, cilantro is one of the most popular choices. Cilantro leaves look almost identical to those of flat parsley. Taste is where things get different.
Cilantro has a very strong, very distinctive flavor that can be overpowering if used in great quantities. It is usually best to use just a few cilantro leaves to substitute for parsley unless you are sure the flavor will complement your dish. Cilantro is very popular in Latin American and Indian cuisine, but can clash with many Italian dishes, for instance.
Leafy herbs like basil and tarragon can also make a good substitute for parsley when look is the chief concern. Like cilantro, these herbs also have distinctive flavors. It is usually best to taste the herb before using it to get a sense of how it will blend with the flavors of your dish. This is particularly true when directly cooking with herbs, rather than using them as a garnish. Cooking with parsley is usually straightforward, as the leaves do not have much flavor to leech out. Other herbs will often intensify when warmed, however, which should be taken into account.
Green onion, though not an herb, can also be an effective substitute for parsley in some cases. The onion stalks can be sliced thin and sprinkled across the dish or cooked within it to add a pleasing green color with usually only a bit of flavor. A green onion's texture is much different from parsley's, but the look is often similar.
Finding a substitute for parsley in a garden setting usually requires completely different considerations. Many people start growing herbs like parsley for the sole purpose of deflecting predators and pests from other plants, particularly crops or food-bearing bushes. Parsley is a common companion plant for tomatoes and carrots. Basil, chive, and mint will also serve this purpose if parsley is either unavailable or fails to thrive.