It is best to prune Parsley throughout the growing season. If you try and let the leaves grow big, they will "go into seed" and you have a chance of losing the nice, healthy green leaves. Once the plant goes to seed, the stems become woody and tough. The best way to keep fresh Parsley all season outdoors, is to trim the largest and tallest stems every other week or so (depending on their own personal growth). The same applies indoors, if you use containers for year round culinary herbs.
I grow quite a few Parsley plants, some tend to die out faster then others due to the humid weather here in NJ. I always have enough Parsley to last me throughout the summer and enough to dry throughout the winter.
You can use the whole stem of Parsley when cooking, the leaves are the most tender but the stems have a good flavor too. You can simply chop it all together after you harvest it. If you prefer only the leaves, you can follow the same instructions as the previous post for Cilantro.
Happy Harvesting and Green Blessings . . .