Fennel Plant: How to Grow Bulb Fennel and Herb Fennel

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There are two fennel plants to grow: herb fennel, cut for its feathery leaves and seeds, and Florence fennel, grown for the swollen "bulb" at its base. Both want full sun and well-drained soil. Bulb fennel tends to bolt in heat and long days, so sow it from mid-June to early August and harvest the bulb at about tennis-ball size.

Fennel (Foeniculum vulgare) is a carrot-family plant from the Mediterranean. It is a short-lived perennial, winter hardy from zone 4a to 9b, that cooler regions often grow as an annual.

Fennel at a glance

Plant type	Short-lived perennial, often grown as an annual; Florence fennel is grown as a cool season annual
Winter hardiness	Zones 4a to 9b; Florence fennel only to zone 5a
Sun	Full sun: at least 6 hours of direct sun a day
Soil	Well drained; for Florence fennel loamy, rich in organic matter, pH 6.5 to 8
Sowing	About 1/4 inch deep, into soil of 50 to 75 F
Germination	Usually 7 to 10 days, typically within 1 to 2 weeks
Spacing	Thin to 4 to 6 inches; bulbing plants 8 to 12 inches apart
Days to harvest	Florence fennel typically 60 to 90 days, by variety
Bulb harvest	About tennis-ball size, firm and under 4 inches across
Pets	Listed as non-toxic to cats, dogs and horses (see below)

Florence fennel vs herb fennel: which fennel plant to grow

Both taste of anise, but you grow them for different parts. Herb fennel is an herb, cut for stems, leaves and seeds. Florence fennel, or finocchio, is a bulb vegetable, grown for the large, flat, thick rosette of leaf stalks at its base. Common fennel doesn't bulb, so when a plant makes mostly leaves and a small bulb, check the kind of fennel first; the other causes are summer heat, in which plants generally do not develop well, and water or nutrient stress.

Herb fennel and Florence fennel compared

Trait	Herb (common or sweet) fennel	Florence (bulb) fennel
Grown for	Stems, leaves and seeds	The "bulb" of thick leaf stalks, eaten raw or cooked
Height	Can grow 3 to 6 feet tall	Shorter, with darker foliage; fronds typically 12 to 30 inches before harvest
Life span	Short-lived perennial, zones 4a to 9b	Biennial to short-lived perennial, hardy only to zone 5a
Soil and water	Drought tolerant once established; better flavor in poor soil	More fertile soil and more even moisture
When to sow	Spring, after frost	Mid-June to early August, so it grows in cooler, shorter days, or early spring

The labels get mixed up. Florence fennel sometimes gets mislabeled as anise, which is a different plant, Pimpinella anisum, and its botanical name appears as both var. azoricum and var. dulce. When you want a bulb, buy a packet labeled Florence (bulb) fennel or finocchio, and for a spring sowing, a named slow-bolting variety.

Bronze fennel is a leaf type: 'Purpureum' and 'Rubrum' are grown as ornamentals, several bronze-leafed cultivars have the same flavor, and bronze fennel forms only a small bulb.

When to plant fennel so the bulb does not bolt

Time Florence fennel so its bulb forms in cool weather. It does well in cool weather and may bolt once it turns hot, and summer heat and long days speed up bolting, or going to flower. Whenever possible, time it to reach maturity as days shorten and temperatures cool. Herb fennel is simpler: sow it straight into the garden in spring once frost is past.

When to sow Florence fennel

Sowing	When	Why
Spring	Early spring, around your last frost date	Sow early; sowings that mature in the long, hot days of summer bolt
Summer or fall	Mid-June to early August; in some areas, September to October	The bulb forms as the days turn cooler and shorter, and fennel tolerates light frost

Where summer days are long and hot, a spring sowing can bolt, so sow only in summer there. Our frost dates tool counts Florence fennel's spring dates and a last sowing date for your ZIP code. For a continuous supply, sow 3 to 4 feet of row every 3 to 4 weeks.

How to plant fennel seeds: direct sow or start indoors?

Sow fennel where it will grow if you can. Its taproot can make it a poor transplant, direct seeding often produces better results, and disturbing the roots may cause bolting, which matters most for bulb fennel. In one trial, direct-seeded bulb fennel needed 32 fewer days to harvest than transplants.

To start it indoors anyway, sow 3 seeds per cell in 1 to 2 inch cells 4 to 6 weeks before your last spring frost, thin to one, and set the plants out without disturbing the roots; the seedlings can be delicate and hard to transplant. A head start indoors gives fennel the extra time it needs to produce a mature plant and seed head.

1. Prepare the bed. Work the fertilizer a soil test calls for into the top 6 inches, and add no more than 1 inch of well-composted organic matter per 100 square feet.
2. Sow shallow. Sow about 1/4 inch deep once the soil is between 50 and 75 F.
3. Thin. Thin to the spacing below.

Fennel spacing

Fennel	Plants	Rows
Bulb fennel	Thin to 4 to 6 inches, then 8 to 12 inches apart once bulbing	Can be 12 to 18 inches apart
Herb fennel	10 to 12 inches; stake at 18 inches tall	3 feet apart

Herb fennel grown for seed can grow much taller and may need wider spacing.

Soil, water and feeding

Give fennel full sun and soil that drains well. It prefers moist, well-drained ground with plenty of organic matter and tolerates clay and sand, though it grows with less vigor in clay. For bulb fennel, aim for a pH of 6.5 to 8. In windy spots, common fennel in fertile ground may need staking.

Water. Water Florence fennel on a regular schedule and keep the soil from drying out between waterings: 1 to 2 inches per week, by soil type. Drip irrigation helps keep the soil moist. Swings between dry and wet soil may cause stems to split. A mulch of straw or lawn clippings holds down weeds and helps keep the soil cool and moist.

Feeding. Fennel needs moderate nitrogen but little phosphorus or potassium. Let a soil test set the bed fertilizer, then, when the bulbs start to form, feed 3 tablespoons of a 21-0-0 nitrogen fertilizer per 10 feet of row. To head off bolting, avoid high-nitrogen fertilizers.

Blanching. Once the bulbs swell to the size of an egg, you can mound soil around them for a paler, more tender bulb, the way leeks are blanched.

Why fennel bolts, and which varieties resist it

Bolting ends a bulb: once a plant has flowered, the bulb turns woody and poor in flavor, so clip any seed stalk that may form. What sets it off:

• Heat and long days.
• Dry soil: if the soil dries out, the plant bolts and the bulb suffers, since dry conditions promote blooming at the expense of the bulb.
• Stress and root damage: lack of water, damage to the plant or its roots, and roots disturbed in transplanting, which may cause bolting.
• Variety: choose one of the newer slow-bolting varieties.

Variety labels don't always hold up. 'Zefa Fino' and 'Orion Hybrid' are commonly grown and described as resistant to bolting, but in a two-year Washington State University trial in Mount Vernon, Washington, where summer days are long, 'Zefa Fino' and plain 'Florence' bolted and produced very few bulbs in both years, and 'Orion' had good bulb production in only 1 of the 2 years. 'Finale', 'Orazio', 'Preludio', 'Solaris' and 'Tenace' led that trial on bulb production and yield, with good bulb quality.

Florence fennel varieties

Variety	Notes
'Finale'	Open pollinated, very large flattened bulbs, good bolt resistance, 75 to 80 days; among the trial's top five
'Preludio'	Uniform, good for fall harvest, 80 days; among the trial's top five
'Orion'	Hybrid, bolt resistant, large bulbs that hold well, about 80 days; good in only 1 of 2 years in the trial
'Zefa Fino'	Described as resistant to bolting; bolted both years in the trial
'Florence'	Bolted in the spring-sown trial; with 'Zefa Fino' and 'Idillio', it made the fewest bulbs both years (24% of plants on average)
'Cantino'	Very slow to bolt, good for early planting

No label holds everywhere, so for a spring sowing pick a slow-bolting variety and check what does well in your region.

How to harvest fennel bulbs, leaves and seeds

Pick leaves whenever you need them, and the bulb before the plant flowers. Leaves of either kind can be picked at any growth stage; take no more than 1/3 of the foliage at a time. For stems, pick the tender flower stalks right before they bloom.

Harvest the bulb at about tennis-ball size, while it is still firm and under 4 inches across, before the plant bolts. Quality peaks at about 3 inches across and may drop above 4 inches, though some guides wait for a plump bulb 4 to 6 inches wide. Florence fennel typically takes 60 to 90 days, by variety, so start checking early, and sooner if the weather turns hot.

How to cut. Cut the bulb free just above the taproot with sharp pruners, or at the soil line, then cut the leaf stalks 1 to 2 inches above the bulb. You can also dig the bulb and cut off the root. If you leave the root in the ground, shoots may resprout from it.

Our pick for this job is the Felco 2, a bypass pruner rated by its maker to 0.98 inch (card below); for other pruners, see our pruning shears buying guide.

More in our shop: harvest tools.

Storing bulbs. Trim the tops and keep the bulbs for several weeks in a cool place or bagged in the refrigerator.

Seeds. Let a few plants flower. Cut the seed heads once they turn brown but before they shatter, then dry them in a paper bag hung in a cool, well-ventilated place, so the seeds drop as they dry, or on screens or trays. Our herb drying guide covers seed heads in more detail.

Leaves for later. The foliage can be air dried, but the leaves do not keep their flavor well that way; fennel freezes well.

The sap can irritate the skin of sensitive people, especially in sunlight.

Swallowtail caterpillars, aphids and other fennel problems

Fennel has few pest problems and generally resists most common diseases, though stem and root rot can strike in very wet or poorly drained soil.

Swallowtail caterpillars. Anise and black swallowtail caterpillars feed on fennel, especially the bronze types. Where you want the crop, handpick the larvae; they can be moved to host plants outside the garden. Damage is negligible on mature plants, though young seedlings can potentially be overwhelmed.

Aphids and other insects. Aphids attack fennel, and slugs can cause trouble too. Thrips and cabbage moth larvae may cause leaf damage that leads to smaller bulbs; control them with insecticidal soap or another appropriate insecticide, applied by the label. You usually don't need to control them until you see a problem. Our aphids guide starts with water and your hands.

Helpful insects. The flowers draw many beneficial insects, and cilantro, dill or fennel planted near lettuce and greens draws many kinds of insects that eat aphids (see our companion planting chart).

Does fennel spread? Self-seeding and invasive fennel

It can. Where winters are mild, fennel reseeds readily, naturalizes and may turn invasive; in cooler regions it behaves better and rarely escapes. Every type can self-seed until it turns invasive, and fennel has escaped gardens and naturalized.

• California: Foeniculum vulgare is rated a Moderate invasive plant, though it is not yet clear whether culinary varieties are invasive. Common and bronze fennel are not recommended for the Bay Area, and even bulbing fennel self-sows if allowed to flower.
• Washington: the state's weed control board classifies Foeniculum vulgare as noxious but not Florence fennel; remove the flowering stalks once the bulb is harvested.

Cut off spent flowers and stems before seed sets; deadheading prevents unwanted reseeding.

Neighbors and rotation. Keep dill away from fennel: the two can cross, and seed saved from either can come up with an odd flavor. Where space allows, keep fennel out of beds that grew fennel or its close relatives, dill, carrots and parsley, in the last 3 to 4 years.

Is fennel toxic to dogs and cats?

The ASPCA's poison control database lists fennel (Foeniculum vulgare, also called Florence fennel or finocchio) as non-toxic to cats, dogs and horses, with the note "Low if consumed in small quantities in food; avoid concentrated oil." It also cautions that eating plant material of any kind may cause vomiting and stomach upset in dogs and cats. If a pet may be ill or may have swallowed something poisonous, call a veterinarian or its poison hotline, open 24 hours a day, at (888) 426-4435.

Fennel seeds and supplies

What you need: bulb fennel seed (for spring, a slow-bolting variety such as 'Finale') or herb fennel for leaves and seed; 1 to 2 inch cells to start bulb fennel indoors, such as the Bootstrap Farmer tray below, made to sit in a 1020 flat; straw mulch and a drip line; and sharp pruners. Fennel can grow in a large container, though it typically grows better in the ground, where its deep taproot has room. There is no bulb fennel seed card: the only Amazon packets we found of the varieties that led the trial above come from a reseller that names no seed company, and 'Florence' bolted in that trial.

What you'll need for fennel

Picks on Amazon

• Eden Brothers Fennel Seeds, Smokey Bronze Check price on Amazon
• Bootstrap Farmer 32 Cell Seed Tray Check price on Amazon
• Felco 2 Pruning Shear Check price on Amazon

Sources we used

• "University of Wisconsin-Madison Extension: Fennel, Foeniculum vulgare" (revised July 2026) and "Black Swallowtail, Papilio polyxenes"
• "Utah State University Extension: How to Grow Fennel in Your Garden" (published May 2020)
• "Illinois Extension, Herb Gardening: Fennel"
• "NC State Extension Gardener Plant Toolbox: Foeniculum vulgare"
• "K-State Research and Extension, Harvey County: Fennel"
• "UC Master Gardeners of Santa Clara County: Fennel"
• "Identifying Bulb Fennel Cultivars Suitable for Production in the Northwest United States" (Miles and others, Washington State University; HortTechnology, August 2019) and "Washington State Magazine: Fennel"
• "Johnny's Selected Seeds: Fennel, Key Growing Information"
• "University of Missouri Extension, G6470: Growing Herbs at Home" (reviewed May 2022)
• "University of Maryland Extension: Fennel" (updated February 20, 2023)
• "Clemson Cooperative Extension, HGIC 1311: Herbs"
• "UNH Cooperative Extension: The Herb Garden" (Master Gardener manual, chapter 16)
• "Penn State Extension: Herbs Make Good Plant Partners and Companions" (updated April 22, 2026)
• "Mississippi State University Extension, Southern Gardening: Make areas friendly for swallowtail butterflies" (May 23, 2022)
• "California Invasive Plant Council: Foeniculum vulgare"
• "ASPCA Animal Poison Control: Fennel", with the general notes on its plant list page
• Felco's specification page for the Felco 2 pruner

Related guides

• Herb garden guide, which lists fennel among the herbs to sow straight into the garden.
• How to grow cilantro, another herb that bolts, with its own section on why.
• How to dry herbs, including seed heads.
• Companion planting chart, where each pairing's evidence is graded.
• How to grow lettuce, a crop to plant near fennel to draw insects that eat aphids.
• How to grow carrots, a carrot-family crop to rotate away from fennel.

Frequently asked questions

Is fennel a perennial or an annual?

Both, depending on the climate. Fennel is a short-lived perennial, winter hardy from zone 4a to 9b, that gardeners in cooler regions often grow as an annual. Florence fennel is hardy only to zone 5a and is grown as a cool season annual for its bulb.

When should you plant fennel?

Sow herb fennel in spring after frost. Time Florence fennel so its bulb forms in cool weather: sow it from mid-June to early August for a fall bulb, or in early spring, around your last frost date.

Why is my fennel bolting?

Heat, long days and dry soil. Summer heat and long days speed up bolting, and water stress or damage to the plant or its roots can set it off. Keep the soil evenly moist, sow so the bulb forms in cool weather, and choose a slow-bolting variety.

When is a fennel bulb ready to harvest?

Harvest the bulb at about tennis-ball size, while it is still firm and under 4 inches across, before the plant bolts. Quality peaks at about 3 inches across. Florence fennel typically takes 60 to 90 days to mature, by variety.

Does fennel grow back after you cut the bulb?

It may. After you cut the bulb at its base, new shoots may sprout from the root, and they can be eaten while small.

Is fennel invasive?

It can be where winters are mild: fennel reseeds readily there and may turn invasive, while in cooler regions it rarely escapes. In California, Foeniculum vulgare is rated a Moderate invasive plant, though it is not yet clear whether culinary varieties are invasive. Remove spent flowers before seed forms.

Can you grow fennel in a pot?

Fennel can grow in a large container, though it typically grows better in the ground, where its deep taproot has room.

Is fennel toxic to dogs or cats?

The ASPCA lists fennel as non-toxic to cats, dogs and horses, with a note to avoid concentrated oil. It cautions that eating plant material of any kind may cause vomiting and stomach upset in dogs and cats; if a pet may be ill, call a veterinarian or its poison hotline, open 24 hours a day.