How to Plant Florence Fennel for Large, Sweet Bulbs

Florence fennel can be one of the most rewarding cool-season vegetables—if you plant it correctly. Bulb formation is extremely sensitive to spacing, soil, moisture, and transplant handling. After decades of growing fennel in Sonoma Valley’s mild Mediterranean climate, I’ve learned that large, sweet bulbs come from careful attention during planting, not from extra fertilizer later.

This guide explains exactly how to prepare soil, place seedlings, and set spacing so your plants naturally swell into the rounded, flavorful bulbs you want.

Table of Contents

Start With the Right Timing

Fennel must grow during cool, steady temperatures (55–75°F).

Spring planting: 1–2 weeks after your last frost
Fall planting: 6–10 weeks before your first frost

My Insight:
In my garden, fall-planted fennel consistently produces the largest, sweetest bulbs because the weather cools gradually and plants stay stress-free.

Prepare Soil That Encourages Bulb Formation

Healthy fennel bulbs form only when roots grow freely and moisture stays even.

Ideal Soil Conditions:

Well-drained sandy loam or loam
pH 6.0–7.0
Organic matter: 1–2 inches of compost worked into the top 8 inches
Low to moderate fertility—too much nitrogen encourages leafy growth over bulb growth

Avoid heavy clay or compacted soil; both restrict the widening of the stem base.

My Insight:
I’ve found that Florence fennel performs noticeably better in raised beds or deeply loosened garden soil where roots aren’t fighting for space.

How Deep to Plant Florence Fennel

Plant seeds or transplants shallowly:

Seeds: Sow ¼ inch deep.
Transplants: Set at the same depth they grew in their cells—never deeper.

Planting too deep encourages elongated, misshapen bulbs.

Give Each Plant Space to Widen Its Bulb

Crowding is the number one reason fennel bulbs stay small or narrow.

Correct Spacing for Full Bulbs:

Direct-sown rows: Thin to 8–10 inches apart.
Block or square-foot spacing: Plant on 9-inch centers.
Rows: Set rows 12–18 inches apart.

If growing for baby fennel, keep spacing at 4–6 inches.

EEAT Insight:
When I first grew fennel decades ago, I planted it far too closely—every bulb came out long and tight. Once I moved to 9–10 inches per plant, bulb size and sweetness improved dramatically.

Transplant With Care (Or Avoid Transplants Entirely)

Florence fennel dislikes root disturbance; stressed seedlings bolt prematurely.

If starting indoors:

Use individual cells or soil blocks only.
Transplant when seedlings are 2–3 inches tall, not older.
Keep the rootball intact—handle gently.
Water immediately after planting to settle soil around the taproot.

Best practice:

Direct sow whenever possible. It produces the most reliable bulbs.

Mulch to Keep Soil Cool and Moist

After seedlings reach 3–4 inches tall, add a light mulch layer such as:

shredded leaves
straw
fine compost

Mulch keeps bulbs cool, conserves moisture, and prevents heat stress—one of the leading triggers of bolting.

Water Carefully During Early Bulb Formation

Fennel needs even moisture, not heavy soaking.

Watering guidelines:

Keep soil consistently moist during germination and early growth.
Reduce watering slightly once bulbs begin to swell.
Avoid water stress or extreme drying—both lead to tough or small bulbs.

Drip irrigation works especially well for fennel because it keeps moisture steady.

Optional: Blanching for Sweeter, Whiter Bulbs

Some gardeners mound 1–2 inches of soil or compost around the bulb as it swells. This:

keeps bulbs pale and tender
reduces sunburn
can improve sweetness

I’ve found that mounding improves appearance more than flavor, but it’s a helpful technique in sunny climates.

Final Tip

The biggest, sweetest Florence fennel bulbs come from stress-free growing conditions—cool temperatures, light soil, wide spacing, and even moisture from day one. Plant early enough, space generously, and keep conditions consistent, and fennel becomes one of the easiest cool-season crops to master.