Learn how to grow and care for fennel in the herb garden. Plus some companion planting tips and solving common problems with this stately culinary herb.
Perennial fennel is an upright, branching herb that is prized for its anise-flavored foliage and seeds. The feathery dark bronze leaves provide a nice contrast in the herb garden. Growing 3 to 5 feet tall and 2 to 3 feet wide, Fennel grows best in the back of the border.
Fennel’s delicate anise flavor and its digestion-soothing properties make it a popular ingredient in many recipes as well as in herb tea. Its delicate foliage adds visual beauty to your yard. (‘Purpureum’ fennel is particularly popular as an ornamental.) Fennel is also a host plant for black swallowtail butterflies and other beneficial insects.
This article is about growing perennial fennel or herb fennel, whose variants include common Fennel, bronze fennel, copper fennel, and more. These tall fennels are grown for their foliage and seeds.
The shorter Florentine Fennel also called bulbing or annual Fennel is grown for its thick stem bases. The growing process for bulbing fennel will be covered in a separate article.
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Growing Fennel Plants in the Garden
Fennel will grow as a hardy perennial in zones 4-9–that is, in most of the Lower 48 states. It prefers full sunlight. Fennel should be set in well-drained soil. It will grow well in both acidic and alkaline soils.
Fennel can be grown in many types of soil. In leaner soil—that is, with less rich organic matter—fennel will have a more robust flavor, according to the Wisconsin Horticulture Society. Your fennel plants won’t grow as tall, so won’t require staking in this environment.
When grown in richer soil with frequent feedings, your plants will grow taller and have a milder flavor. It may benefit the discerning chef to try both conditions depending on the flavor profile needed for their favorite dishes.
Give your perennial fennel plants plenty of space. They can grow from 3-6 feet tall. Plant in rows 3-4 feet apart and thin the plants to every 10-12 inches within the rows.
Popular Varieties of Fennel
Sweet or common fennel, the standard herb fennel variety, has dark green leaves. This Fennel is grown as a perennial and left in the ground to self-seed.
Bronze fennels are quite similar to common Fennel in their growth habits and their culinary uses. But they are often grown as ornamentals because of their rich bronze or red-colored foliage. There are several cultivars of bronze fennel, including
- Giant Bronze
Their young foliage bears the colors they are named, but older leaves tend to revert to green.
Florence Fennel is the bulbing variety and grown as an annual & thought of as a vegetable.
Propagation: Starting New Plants
Fennel is generally propagated by seed. Like parsley and dill, it has a long taproot that doesn’t like to be disturbed. Plant seeds after the last hard frost in the spring and cover them with ¼” of soil.
Seedlings can be started indoors and set outside after the danger of frost has passed. Keep seedlings moist, provide them with plenty of light, and set them out when they are 3-4 inches tall.
Once your first plantings reach maturity, they’ll probably self-seed freely.
How to Take Care of Perennial Fennel
When your fennel plants are young, they’ll need frequent watering to keep their new roots from drying out. Later in the year, when the root system is well established, fennel is fairly drought-tolerant. Do make sure to choose a well-drained site, as soggy soil can promote root rot.
Fennel usually flowers and set seed in its second year. If you want to control the seeding in your garden, cut the blooms as they mature and start to fade.
Occasionally you may get a few flowers late in the season. This happens more often in drier conditions when the plant bolts in the hot sun.
Companion Planting with Fennel
Fennel can help your other garden plants in several ways. It attracts beneficial insects, including butterflies, bees, syrphid flies, lacewings, and small wasps.
Fennel should be kept away from dill. Dill and fennel can cross-pollinate, which will prevent either plant from self-seeding properly. It should also be planted well away from tomatoes, beans, and cilantro because it can inhibit their growth.
In fact, fennel has also been said to inhibit the growth of many other garden plants, so you may want to grow it somewhere outside your main herb and vegetable gardens.
But don’t let that deter you from growing it, especially if you value the seeds. And it is an excellent foundation plant in a butterfly garden combined with the taller butterfly weed and coneflowers or gaillardia in front. In ornamental gardens, bronze fennel combines well with Black-eyed Susans and goldenrod.
Common Problems With Fennel
Fennel is not bothered by many pests or problems. But there are a few you may encounter.
Many kinds of insects find fennel attractive. This can be a very good thing for your other garden plants, but not always for the Fennel itself.
Aphids and slugs may chew on your fennel plants.
Aphids can be easily remedied with organic insecticidal soap. Spray your plants either in the early morning or late afternoon as the solution needs to be wet to work correctly.
Organic insecticidal soap is safer to use than many other commercial applications on edible plants and leaves no harsh residue. They are also not harmful to animals and birds.
To stop the slugs from reaching your plants, you can make a slug trap with yeast, sugar, and water. Beer also works to attract slugs.
Pour the liquid into a plant saucer or other shallow container. Set the container into the ground, so the dirt comes right up to the lip of the container. Slugs will be attracted to the scent, crawl in, and drown.
Caterpillars Love Fennel
You may also find caterpillars feeding on your fennel plants. Well-established fennel plants can support a few caterpillars, and the butterflies can help to pollinate your garden.
Swallowtail caterpillars particularly enjoy fennel, and they hatch into singularly beautiful butterflies. But one hungry caterpillar can devastate a row of tiny new seedlings. Handpick caterpillars if needed.
Fennel can also suffer from stem or root rot in soggy soil. Select a well-drained planting site to prevent this.
In windy areas, you may need to support your fennel plants by tying them loosely to stakes. Supports may not be necessary if you plant your fennel in soil that is not too rich and it has established a deep root system.
In mild climates, perennial fennel may self-seed aggressively and take over areas where you hoped to grow something else. Self-seeding is less of a problem where the winters are cold.
Harvesting Perennial Fennel
You can harvest fennel leaves at any time during the growing season once they are large enough to use. Don’t remove more than a third of the foliage from any plant.
For fennel seeds, cut seed-heads off when they have turned brown but before they shatter and fall.
Bring them inside and place them on screens or in a paper bag to complete the drying process. Collect the seeds when they fall from the stems. For more detailed information, see our article on How to Harvest and Preserve Fennel.
Fennel can delight your senses of sight, smell, and taste, as well as soothing your digestion and attracting beneficial insects to your yard. Use a little care in where you plant it.