Oftentimes recipes will specify the use of flat-leaf or Italian parsley. Can you substitute curly leaves for flat, and vice versa? What's the difference beyond appearance?
Used to perk up dishes with its fresh green flavor and color, parsley can be much more than a garnish. The two main cultivars of this herb are curly parsley (Petroselinum crispum) with ruffled leaves and Italian parsley (Petroselinum crispum neapolitanum) with flat leaves.
In general, flat-leaf parsley has a more robust flavor, while the curly variety is associated with decoration. Some claim that curly-leaf parsley has no flavor or, conversely, that it tastes more bitter, but it really depends on the particular plant, its growing conditions, and age. Both kinds of parsley may be used in cooking and when substituting one for the other, taste to determine the flavor and adjust as desired. Think, too, about the texture that would work best in your dish.
Finally, don't discard the stems, which have a stronger flavor than the leaves and can be used in a bouquet garni and added to homemade stock or a pot of beans.
Related: Humble Herbs: In Praise of Parsley
(Images: Flickr member Alice Henneman licensed under Creative Commons; Flickr member Bruce Guenter licensed under Creative Commons)
Bacsac Bacsquare 04...

Oh this post in a few days late, I made a dish that called for parsley, and I stood in the grocery store holding both muttering to myself, "well what's the difference?!"... and got curly parsley. The recipe still tasted good, though!
I think the main difference is a sort of snobbery against curly parsley because it's been used as a garnish. I find the flavor much more assertive than flat leaf parsley, though I've found that the parsley I've grown, which is flat leaf, is more assertive and parsley-y than the flat leaf I've gotten in stores.
I agree with AndyPucko -- I find store variety curly parsley more assertive than flat. Usually, the flat leafed parsley I get from the store tastes like nothing. However, homegrown is lovely. (sometimes cilantro tastes like nothing, and sometimes it is awesome; it's pretty hit or miss)
Unless minced finely or cooked till soft, curly parsley sometimes tickles the throat. Other than that, I'm with others here and find no appreciable difference between the two.
I went to culinary school and I use curly parsley all the time. As people stated, if it's finely chopped, no one will ever know. Plus, it seems to last a very long time in the fridge wrapped in paper towel and put in a plastic bag.
Interesting, AnnL -- I tend to leave the curly parsley alone and cook the flat until it's a little less chewy, especially in a mostly uncooked dish. I agree that the difference in flavor between the varieties is negligible.
I grow both in my herb garden. Actually planted both originally to learn the dif. Year after year the curly wins hands down. Neither can be compared to what is available at the market which has no discernable taste to me.
The curly sort is sold in huge bunches where I live, but not the flat, so I frequently use curly parsley in all types of recipes. I used to think I didn't like the taste, but now I find the curly stuff to be refreshing and add a brightness. I find the flat leaf variety to taste a tiny bit minty (but perhaps that's just me).
There is not much difference in taste, and I have seen both types used in Italy. Flat may be better for Middle Eastern cuisine, where large amounts of raw herbs accompany each meal. Flat is less tickly.
Well I don't know about you but I love the taste of curly parsley and just HATE the taste of leaf/cilantro. I think curly has a fresh flavor that is very unique. I love it (so does my family) when you chop it fine and add to cut up red potatoes with melted butter. Fold the hot potatoes, butter and curly parley together and serve. Simple but delicious. you can add sour cream for those who like it. If you like cilantro (yuck lol) it should work the same but I think cilantro taste so bitter and wrecks the meal. Just my opinion :)
I just love the smell of fresh curly Parsley!! Two thumbs up for curly and two thumbs down for flat leaf Parsley.