At first glance, the leaves of Italian flat-leaved parsley (Petroselinum crispum) and coriander (Coriandrum sativum), also called cilantro or Chinese parsley, are similar to each other. If you look closely, however, you can tell them apart. Another way to distinguish between them is by smell and taste since each has different chemicals in its leaves. Both plants belong to the carrot family, but they have different growth habits. Coriander is an annual and parsley is a biennial, growing in USDA zones 5 through 9. There's also a curly-leaf parsley, but its tightly curled, dark green leaves are easy to tell from coriander leaves.
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Parsley and coriander both have divided leaves, with three leaflets connected together at the base. The leaves feature lobed edges. Parsley leaves look more jagged since the leaflets and their lobed edges have points. Coriander leaves are gently rounded, both the overall leaf and the lobes on the edges. The leaf texture and color also help to differentiate the two plants. Cilantro leaves are thinner and usually lighter green than the thicker, tougher parsley leaves. Parsley also usually produces more leaves per individual stem than cilantro.
A sure-fire way to tell the two plants apart is to rub one of the leaves and sniff it. Parsley gives off a fresh, almost grassy smell, with more pungent undertones. Cilantro, on the other hand, isn't always perceived to have a good smell. Some people liken cilantro's aroma to that of bedbugs, soap or lotion. To most people, cilantro has a pleasant -- but very different -- smell from parsley. Those who find it unpleasant may be genetically predisposed to sensitivity to the aldehyde chemicals found in cilantro.
Parsley leaves are generally thought to have a pleasant taste and contain the organic chemical compounds myristicin, limonene and menthatriene. Cilantro contains very different chemicals, about 82 percent of them in the aldehyde class. People who find cilantro's smell unpleasant also don't like the taste, but this perception can change over time so the effect may be learned as well as genetic. Harold McGee, writing in the New York Times, suggests that people who are sensitive to cilantro's taste crush it to make it into pesto, which allows leaf enzymes to convert the aldehydes to mild-tasting substances. Parsley leaves dry well for use in cooking, but cilantro leaves don't retain their taste well after drying.
Parsley and cilantro contain different proportions of possibly beneficial compounds. Parsley has larger amounts of vitamin C and beta-carotene (a form of vitamin A), than cilantro does. On the other hand, cilantro has about twice the amount of antioxidant compounds than does parsley. Both contain small amounts of potassium, iron and folate, with parsley having more of these than cilantro.