Getting Florence fennel started at the right time is the single most important factor for success. Because fennel is sensitive to both frost and heat, your sowing window must fall during reliably cool—but not cold—weather. After growing fennel for many years in Sonoma Valley’s mild Mediterranean climate, I’ve learned that aligning sowing dates with your last spring frost is the surest way to avoid bolting, stunting, or poor bulb formation.

This region-by-region guide gives you practical sowing windows based on typical frost patterns across the U.S., with notes and insights from long-term gardening experience.

Understanding the Timing: Why Last Frost Matters

Florence fennel grows best when daytime temperatures stay between 55–75°F.

Too early: frost damages seedlings or slows growth.
Too late: heat triggers bolting before bulbs form.

Using your local last frost date helps you hit the narrow but ideal early-season window for spring sowing.

My Insight:
I’ve found that spring fennel must grow quickly before heat arrives. Planting even two weeks late often leads to bolting in May or June, especially in warm regions.

General Sowing Formula

Use this simple rule to find your sowing window:

Direct Sow Outdoors:

1–2 weeks after your last expected frost

Indoor Sowing for Transplanting:

Start seeds 3–4 weeks before your last frost, then transplant immediately after.

Region-by-Region Sowing Guide

Northeast (Zones 4–6)

Last frost: Late April to mid-May
Direct sow: Early–late May
Indoor sow: Mid–April

Notes:

Spring heat can arrive suddenly; sow as early as frost protection allows.
Fall is often more reliable—sow again in mid–July to early August.

Upper Midwest & Great Lakes (Zones 4–5)

Last frost: Early–mid May
Direct sow: Mid–May
Indoor sow: Late April

Notes:

Short spring cool periods make timing tight—favor fall crops.
Protect seedlings with row covers during unpredictable cold snaps.

Central Midwest (Zones 5–6)

Last frost: Late April to early May
Direct sow: Late April–mid May
Indoor sow: Early–mid April

Notes:

Excellent spring sowing region if heat doesn’t come early.
For summer heat, fall plantings (August) produce better bulbs.

Pacific Northwest (Zones 7–8)

Last frost: Late March–mid April
Direct sow: Early–late April
Indoor sow: March

Notes:

Cool, mild springs offer one of the best fennel climates.
Avoid sowing too late—the June warm-up causes bolting.

Northern Rockies & Intermountain West (Zones 4–6)

Last frost: Late May–early June
Direct sow: Early–mid June
Indoor sow: May

Notes:

Short spring; use heat-resistant varieties if possible.
Fall sowings can be difficult due to early frosts.

Southwest High Desert (Zones 6–7)

Last frost: Late April–early May
Direct sow: Late April–mid May
Indoor sow: Early April

Notes:

Rapid spring warm-up makes timing critical.
Fall is the preferred growing season—sow mid-August to September.

California & Coastal West (Zones 8–10)

Last frost: January–March
Direct sow: February–April
Indoor sow: January–February

My Insight:

In Sonoma Valley, I usually sow spring fennel in late February to early March, but fall crops (August–September) produce the best bulbs of the year.

Southeast (Zones 7–9)

Last frost: February–mid March
Direct sow: March
Indoor sow: February

Notes:

Spring heat and humidity make spring fennel challenging.
Fall plantings are far more reliable—sow August–September.

Southern Gulf Coast (Zones 9–10)

Last frost: Late January–early February
Direct sow: February
Indoor sow: January

Notes:

Spring window is extremely narrow.
Fall and winter crops are the most successful.

Fall Sowing Snapshot (All Regions)

No matter where you live, fall fennel follows this formula:

Fall Direct Sow:
6–10 weeks before your first expected frost

This gives the plant enough cool, stable weather to form a full bulb before the first freeze.

Final Tip

If you’re unsure whether to sow spring or fall fennel, choose fall. After decades of growing it, I’ve seen that fall conditions—cool nights, mild days, and steady moisture—consistently produce better bulbs in nearly every region.