Experts and BHG readers answer.
Parsley - when and how to pick
You can start to harvest parsley and basil as soon as the plants are well established and growing well. Particularly if you plan to use only a few leaves at a time, it's easiest just to snip off a few of the oldest leaves, keeping the new growth intact. Later, as the plant is nearing full size, you can shear off the entire top, and new growth will emerge from the center. Especially with basil, it's good to keep cutting it back to prevent it from going to seed.