Florence fennel

Quick facts

Category

roots bulbs

Difficulty

intermediate

Days to harvest

80 to 110 days

Harvest type

single harvest then replant

Spacing

30 cm between plants

Environment

Temperature

12–24°C

pH

6 to 7

EC (hydroponic)

1.5 to 2.4 mS/cm

Daily light

14 to 20 mol/m²/day

Climate and zones

USDA zones

5 to 10 (winter low around -29°C or warmer)

Frost tolerance

tolerates light frost

Season

cool (spring and fall crops)

Viable growing environments:

• outdoor year-round (in zone)
• outdoor in growing season (annual)
• unheated greenhouse / hoop house
• heated greenhouse
• indoor (heated home)
• indoor hydroponics under grow lights

USDA zone bounds reflect outdoor year-round survival. Anywhere outside the bounded zone range, this crop still grows as an annual in the warm months (outdoor_seasonal), under cover (greenhouse), or indoors under lights.

Growing systems

Florence fennel works in:

• media bed (ebb and flow)
• wicking bed
• soil bed

Growing media

The substrate the roots sit in. Choice depends on the system (clay pebbles don't fit NFT channels; rockwool isn't used in media beds) and the crop (florence fennel works in the media listed below).

Bacterial surface area matters for aquaponics: clay pebbles, lava rock, and pumice double as biofilter substrate. Low-surface media (rockwool, perlite, pea gravel) work in hydroponics but need a separate biofilter in aquaponics.

Nutrient demand by stage

NPK ratios are relative weights at each growth stage; the nutrient mix calculator scales them to absolute grams or ml. EC targets shift through the plant's life: seedlings need a much lighter solution than fruiting adults.

Companion-growing notes

• Heavy uptake of potassium. Co-grown crops with the same demand will end up deficient even at "correct" EC. Plan around this in shared reservoirs.
• Releases compounds through the roots that can mildly inhibit other crops in the same reservoir or bed. The effect is usually subtle but worth knowing if neighbors look stunted.

Aquaponics suitability

Compatible with typical aquaponics nutrient profiles. Fish waste provides enough nitrogen for healthy growth; supplemental potassium, calcium, and iron may still be needed depending on fish stocking density.

Care notes

A cool-season hydroponic crop requiring careful temperature management to prevent bolting before the bulb develops. Media bed or large container systems (the bulb forms at or above the media surface). EC 1.5-2.5 mS/cm. pH 6.0-7.0. Temperature: 12–22°C (critical: above 24°C, the plant bolts before forming a usable bulb, producing only a thin, fibrous base). Moderate light (DLI 14-20 mol/m2/day). From seed to bulb harvest: 70-90 days. Transplant seedlings carefully; fennel is sensitive to root disturbance and transplant shock can trigger bolting. Hill media around the developing bulb to blanch it white. Harvest when the bulb is 8–12 cm diameter and firm. Overmaturity causes fibrous texture and splitting as the flower stalk pushes through. The feathery fronds are harvestable as a herb throughout growth. Bolt-resistant varieties ('Orion', 'Perfection') perform better in systems with imperfect temperature control. A rewarding specialty crop for Mediterranean cooking enthusiasts. For commercial hydroponic growers, fresh fennel bulbs command $4-8/kg and occupy a specialty niche at markets and restaurants.

Further reading

• Best crops for hydroponic beginners
• The Kratky method: grow lettuce with zero electricity