Step 4: Flatten the succulent leaves onto the dirt. A: To understand why the succulent leaves have a black stem, you must first take the first step. You can read more about this topic at Why is my String of Pearls Shriveling. Stem cuttings should not exceed 2 inches long if you plan to propagate using them. This method can be done with the use of timers to make sure you do not overwater them. Sometimes you can use these pots before transplanting the seedlings, especially when using seeds for propagation.