Another Plant Has Started Growing On My Parsley Plant. What’s Going On? What Is This Plant? Is It Edible?

Question

The plant is a lot taller than normal parsley and has skinny flat leaves.

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Answer 1

That is the seed stalk. Parsley is biennial and lives two years. In its second year it sends up a flower stalk which will produce seed heads you can save to grow more parsley.
If your parsley is only in its first year and is producing a seed stalk, it is "bolting." That means it is producing seed early due to unfavorable conditions such as high heat outdoors.

https://www.gardeningknowhow.com/edible/herbs/parsley/tips-on-how-to-grow-parsley.htm