I'm new at it to, so I realy don't know what i'm talking about but, I think if the leaves go all thin and ferny, it means your cilantro is botling. Bolting means it will stop growing the normle cilantro and start growing flowers and seeds and the thin kinde of leaves, wich I hear don't taste so good.
I have been reading this thread wich tought me alot.

Here is a link that might be useful: cilantro help

Hello 2001 from NC. We're neighbors ( Virginia ).
Regarding Cilantro...here is our limited experience:
We bought cilantro from a nursery and very soon it did that fern thing....then we had flowers and seeds. All the cilantro died back except for the seeds...some green, some brown, depending on age. The seeds are great for use in cooking and we are using them but we wanted cilantro. We started some cilantro plants from seed and they were doing good but now they are getting ferny. We are thinking of trimming off the ferny part to prevent seed formation in an effort to keep cilantro. What may work is starting some new cilantro. Maybe next year we'll try planting seeds in a pot every 3 weeks or so to keep things going. Hope this helps. Good luck.

If you keep harvesting the leaves often you can keep it from going to seed somewhat. It is best to harvest often but also start seeds every month or so. The best way I have found is to scatter seeds and cover with a paper towel and keep it moist by spraying gently with water. Most seeds germinte this way and the paper disintegrates easily.
Hlily

Cilantro ( the common name for the leaves of the herb Coriander ) is a cool weather plant. Being an annual,one of the main triggers for it to go to seed is the nightime temperatures. If they average above 50 degrees you can expect the plant to go to seed and there is not much you can do to prevent it. If you can find "Culantro" (Eryngium foetidum )in your area, you will have the great taste of Cilantro in a perennial plant that can handle the summers.

Here is a link that might be useful: The Herb Garden

I visited this forum for the first time yesterday and finally have time to post a message. It just so happens that I was looking through Baker Creek's catalog yesterday as well and saw a Cilantro variety they have named Slo-Bolt Cilanto, although I'm guessing they may have just spelled Cilantro wrong. Probably a typo. Their description of it: "This is a slow-bolting version of Cilantro/Coriander that produces flavorful leaves over a longer period." Their website is RareSeeds.com. Hope this helps, Jeff

There is a ferny type of Cilantro called "Delfino". It produces ferny leaves (looks a lot like dill). Just as tasty as the broader leaf kind and quite beautiful.