The sprouts, which look like mini cabbages, form along with the 2- to 3-foot stems under umbrella-like foliage. They take up to 100 days to mature. To start your Brussels sprouts plants from seeds (indoors or out), plant them 1/2 inch deep. When seedlings are between 5-and 7 inches tall, they should be thinned to about 2 feet apart.