All parts of the plant are useful. The root, harvested in early Autumn is used as a vegetable. Lovage root contains furanocoumarins - excessive use can lead to photosensitivity. The leaves which can be harvested any time are added to salads, The Stems are like celery and best when eaten young, The stems are also quite tasty when they're candied in much the same fashion as Angelica - See Candied Angelica.
The flower produces an aromatic oil, the seeds as well as the stems are sweet and used in candy making as well as flavored vinegar and oils.
In addition to bearing a taste similar to celery, it also looks something like it, although it is actually more closely related to carrots and parsley. It is a perennial that grows up to 6 feet tall with lush green foliage and a spread of around 3 feet around.
Lovage produces clusters of yellow umbrella like flowers which produce 1/2 inch long seeds. Flowers appear in early summer and last till summer, occasionally very early Autumn. Both leaves and stems can be easily dried for winter storage.
How to Grow Lovage
Full Sun is best, partial shade is tolerable. In very hot regions, partial shade is preferable. Soil should be well-drained sandy, loamy fertile. A soil pH of 6.1 to 6.8 is optimal. They are hardy to USDA plant hardiness zone 4.
Lovage can be started by direct seeding right into the garden bed so long as the soil temperature is around 60 F, but a higher success rate will be achieved by starting it indoors about 6 weeks before the last frost date in your region.