Foxglove is the source of digoxin, a medicine used to treat congestive heart failure and heart rhythm problems. Foxglove is a biennial that sows itself freely in most parts of the United States if the seed pods are allowed to ripen. It can be grown in moist, well drained soil in USDA Hardiness Zones 4 through 8. You can see foxglove in the National Herb Garden, where it is scattered throughout the Antique and Heritage Rose Garden and the Theme Gardens.