A relative of celery, lovage is a hardy perennial herb whose leaves can be used in soups, salads, and bouquets. Unlike most herbs, lovage thrives in clay soil that is slow to drain. Lovage is a large plant, reaching up to 5 feet tall, so grow it at the back of the border where it can spread and not compete with other plants. Its small yellow-green flowers attract beneficial insects. After planting, lovage grows vigorously for about four years and then slows. Because lovage is a self-seeder, clip off flower heads before they mature unless you want more plants.