What to Plant With Fennel (and What Not to): Fennel Companion Plants

Few garden plants have a worse reputation among companion planters than fennel. Almost every list puts it on the "keep away" side, often with dramatic claims that it poisons everything nearby. The real picture is more modest, and more useful: there are good practical reasons to give fennel its own spot, one listed incompatible pair, and one listed benefit. This guide covers what extension sources actually say, the difference between bulb and herb fennel, when to sow so bulbs do not bolt, and how to keep fennel from seeding itself all over the yard.

What not to plant with fennel

Tomatoes are the only crop our checker flags, because Virginia Extension's tomato row is the only extension companion table we use that lists fennel. Its table does not give a reason. Beyond that pair, there are three practical reasons fennel is usually grown apart:

• Size and shade. Illinois Extension describes herb (common) fennel as a tall perennial reaching about 3 to 5 feet. A clump of it can shade a whole row of shorter vegetables.
• Self-seeding. Iowa State Extension notes fennel self-seeds readily if flowers are left on the plant, and UW–Madison Extension warns it can become invasive in mild climates. A fennel plant left to seed in a mixed bed tends to come back everywhere.
• Possible allelopathy. In Ask Extension answers, extension experts say there is not enough research to show fennel truly suppresses other plants in a home garden, though some studies have found that fennel extracts can inhibit seed germination, and plants in the nightshade family (tomatoes, peppers, eggplant, potatoes) are the ones most often described as sensitive. Their practical advice is to keep fennel a couple of feet away from other crops, or grow it in its own bed or a container.

Good companion plants for fennel

Lettuce and other greens

Penn State Extension's herb companion guidance suggests planting fennel, dill and cilantro near lettuce and other greens, because their small flowers attract beneficial insects that prey on aphids. To get that benefit without crowding the lettuce, put fennel at the end of the row or in a pot beside the bed rather than in the middle of it. Our lettuce guide lists more lettuce companions. Cilantro does the same job in a much smaller plant; see what to plant with cilantro.

A home for pollinators and swallowtails

UW–Madison Extension notes fennel's flat flower clusters attract many beneficial insects, and fennel is also a host plant for black swallowtail caterpillars, along with dill and parsley. If you see striped caterpillars on it, consider planting a little extra to share. Our dill guide covers those caterpillars and dill's own companions.

Florence fennel or herb fennel?

Illinois Extension describes two kinds. Herb fennel (common or sweet fennel) is grown for its feathery leaves, stems and seeds; UW–Madison Extension describes it as a short-lived perennial that is often grown as an annual in cooler climates. Florence fennel (finocchio, or bulb fennel) is grown for the swollen, bulb-like base of its leaf stalks and is treated as an annual. Bronze fennel is an ornamental form of herb fennel with coppery leaves. All of them have the same companion issues, but herb fennel's height and seed production make it the one to place most carefully.

When to plant fennel

Fennel has a taproot and does not transplant well, so Illinois and Iowa State Extension both recommend sowing it directly where it will grow. Herb fennel can be sown in spring after the danger of frost. For Florence fennel, timing matters most: Illinois Extension recommends sowing from mid-June to July, so the bulbs size up in the cool weather of late summer and fall. Spring-sown bulb fennel often bolts (flowers) before it forms a good bulb as days lengthen and heat arrives. Utah State Extension adds that seeds germinate best in soil between about 50 and 75°F and that you can sow every three to four weeks for a longer harvest. Use our planting calendar to find your frost dates, then count forward for summer sowing.

Sowing and spacing

Utah State Extension's directions: sow seeds about ¼ inch deep, 4 to 6 inches apart in rows about 18 inches apart, and thin seedlings to 8 to 12 inches. Germination takes about 7 to 10 days. Fennel likes full sun and well-drained soil; Utah State gives a soil pH of about 6.5 to 8 and says it needs only moderate fertility. Florence fennel matures in roughly 60 to 90 days from seeding.

Water, blanching and bolting

Florence fennel needs steady moisture. Iowa State Extension suggests about an inch of water a week, and both Illinois and Utah State Extension note that drought stress and heat push bulb fennel to bolt. A mulch helps keep the soil evenly moist. When the bulbs start to swell, you can mound soil around them to blanch them, which keeps them pale and tender. Utah State Extension lists aphids, thrips and cabbage moth larvae as occasional insect pests, and UW–Madison notes fennel has few problems other than root rot in very wet soils.

Harvest and stopping self-seeding

Harvest Florence fennel bulbs when they reach about tennis-ball size or a little larger and are still firm (Illinois Extension); Utah State suggests picking before they are much over 4 inches across, since they get stringy with age. Lift or cut the bulb at the base; UW–Madison notes the root may send up new shoots that can be eaten while small. Leaves of either kind can be snipped any time for the kitchen. For seed, Iowa State Extension suggests letting the flower heads mature until the seeds turn light brown, then cutting them and drying them in a paper bag. If you do not want seed, remove the flowers before they ripen, which is the main way to keep herb fennel from spreading. UW–Madison Extension also notes that the sap can irritate the skin of some people, especially in sunlight, so gloves are a good idea when cutting large plants.

Where to put fennel

• Its own container: a deep pot suits the taproot and keeps fennel apart from everything else. Florence fennel needs steady watering in a pot.
• A herb or pollinator corner: herb fennel fits a perennial herb bed or a garden edge, at least a couple of feet from vegetable rows, where its flowers can feed beneficial insects.
• The end of a greens row: a few plants at the end of a lettuce or salad-greens row, following Penn State's tip, without shading the crop.

Every pair to separate is in our keep-apart chart, and the tomato guide covers better tomato neighbors.

Frequently asked questions

What should you not plant with fennel?

Virginia Extension lists fennel as incompatible with tomatoes. Many gardeners also keep it away from most vegetables, because it grows tall, self-seeds and may inhibit some seeds from germinating, though extension experts say that evidence is limited.

What grows well with fennel?

Penn State Extension suggests fennel, dill and cilantro near lettuce and other greens, because their flowers attract beneficial insects that eat aphids. Otherwise fennel does best in its own bed, container or garden edge.

Is fennel allelopathic?

Possibly, but it is not well proven. Ask Extension experts say some studies show fennel extracts can inhibit seed germination, while there is not enough research to show a strong effect in home gardens. Keeping fennel a couple of feet from other crops is a simple precaution.

When should I plant Florence fennel?

Illinois Extension recommends sowing bulb (Florence) fennel from mid-June to July, so it forms bulbs in cool late-summer and fall weather. Spring sowings often bolt before making a good bulb.

Will fennel spread in my garden?

It can. Fennel self-seeds readily and can become invasive in mild climates, so cut off the flower heads before the seeds ripen if you do not want new plants.