Herb to Know: Giant Fennel

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Genus: Ferula communis ‘Gigantea’

• Hardy to Zone 7

One small herb plant in the spring, one giant in the herb border! These are the words that best describe the giant fennel (Ferula communis ‘Gigantea’), which is quietly making its debut in the Pacific Northwest. Although related to the medicinal species of ferula, this plant should be considered toxic and purely ornamental. Although not a true fennel (Foeniculum vulgare) as the common name may suggest, it nonetheless looks like one, on steroids!

In very early spring, this inedible ferula (ferula is Latin for fennel) emerges from a thick, woody rootstock to form a substantial, green, feathery mass — similar to fennel — but these finely divided, medium- to dark-green leaves continue to unfurl until they are 18 inches wide or more. It is quite resilient to cold temperatures, as any new shoots that get frostbite by late frosts are just replaced by newer shoots.

This sun-loving plant initially forms an airy, fernlike clump 3 feet tall and as wide. It is quite decorative at this point, providing an attractive foliage foil to other plants and fine textural contrast. At this stage, it continues to build up strength until early summer, when a great thick flower stalk suddenly erupts from the woody root base. This emerging flower stem has an appealing and attractive purplish tint or cast to the basal stem — akin to the color of asparagus spears — as it continues to push upward typically to 6 to 10 feet or even higher to 12 to 15 feet tall! (It is said that due to the large amount of pith contained within the stalk, Prometheus first used it as a vessel to transport the stolen fire from the gods). From this towering herbaceous behemoth of an herb, hundreds of tiny bright yellow flowers form on numerous 3-inch-wide rounded umbels. This continues for several weeks and produces a stunning treelike floral canopy. This is followed by an equal amount of developing large, brownish-gray, hanging fleshy seeds. These ripen in late July or August and are the best means of successful propagation. Sometimes the plant will die after seeding but removal of the seed stalk has prompted mine to last for years. However, if grown in a dry climate or a summer drought period, this plant often will go summer dormant. An entire leaf will suddenly turn completely yellow and then the entire plant may die back to the ground only to wait until fall rains when it will re-emerge.

This plant enjoys a fairly moist, deep, fertile soil as it has a large tap root system which resents transplanting unless at an early seedling stage.

So, choose a site wisely to enjoy this splendid and most unusual Paul Bunyan of herbs.