Fennel (Foeniculum vulgare) is an aromatic crop, but “fennel” can describe plants grown for quite different purposes.
Common or leaf fennel is primarily harvested for its feathery foliage, stems and aroma. Florence fennel (Foeniculum vulgare var. azoricum) is grown for its enlarged, bulb-like stem bases.
That distinction matters when greenhouse conditions are discussed.
A lighting or climate strategy designed to maximize leafy biomass and aroma is not automatically the best strategy for producing a large Florence fennel bulb.
There is also an important scientific limitation:
Current research does not establish one universal stage-by-stage PPFD, DLI, CO₂ and VPD recipe for greenhouse fennel.
What research does provide is increasingly useful experimental evidence showing how fennel responds to supplemental light, CO₂ concentration, water availability and developmental stage.
Fennel Prefers Strong Light — But “More PAR” Is Not the Whole Story
University horticultural guidance generally describes fennel as a full-sun crop. Both the University of New Hampshire and University of Wisconsin recommend strong light or full sun, while noting that Florence fennel performs best with relatively even moisture.
In a greenhouse, however, saying “full sun” is not precise enough.
The light reaching a crop is better described with:
PPFD — Photosynthetic Photon Flux Density, measured in µmol/m²/s,
and:
DLI — Daily Light Integral, measured in mol/m²/day.
PPFD describes light intensity at a particular moment.
DLI describes how much photosynthetic light accumulates across the entire day.
For a greenhouse crop receiving variable sunlight, these answer different questions.
New 2026 Research: Supplemental Red Light Increased Fennel Growth
A 2026 study in Food Chemistry: X directly examined hydroponic leafy fennel grown in a greenhouse.
Researchers compared plants under natural greenhouse light with plants receiving additional red LED light at:
100 µmol/m²/s
for:
14 hours per day.
That supplemental treatment added approximately:
5.0 mol/m²/day of DLI
in addition to the natural greenhouse sunlight.
The result was substantial.
Supplemental red lighting increased fresh biomass by approximately 57% and dry biomass by approximately 64% compared with the natural-light control.
This is an important finding, but it must be interpreted correctly.
The study does not show that fennel needs only 100 µmol/m²/s.
The plants were already receiving natural greenhouse sunlight.
The 100 µmol/m²/s was supplemental PPFD, not total PPFD.
Likewise, approximately 5.0 mol/m²/day was supplemental DLI, not total DLI.
That distinction is essential when translating greenhouse research into practical lighting decisions.
Supplemental Light Also Changed Fennel Aroma
The 2026 study is especially useful because researchers did not look only at biomass.
They also analyzed aroma-active compounds.
Supplemental red LED increased several important aroma compounds in fennel leaves, particularly phenylpropanoids including:
(E)-anethole
and:
estragole.
The researchers also identified changes in gene expression associated with phenylpropanoid biosynthesis.
This directly challenges one of the claims in the old AquaHorti article.
The old version states, in effect:
higher light → weaker aroma
and:
high PAR + high VPD → fast growth but poor fragrance.
Current fennel-specific research does not support such a simple relationship.
Under the 2026 greenhouse experiment, additional light increased both biomass and important aroma compounds.
The real relationship depends on light quantity, spectrum, natural background radiation, plant stage and other environmental conditions.
Aroma Changes Naturally as Fennel Develops
Growth stage also matters.
A 2024 study compared the aroma chemistry of hydroponically grown fennel microgreens with mature fennel leaves.
Researchers identified 32 key aroma-active compounds in microgreens and 28 in mature fennel.
(E)-anethole was one of the dominant aroma-active compounds, while microgreens contained approximately 81.4–98.1% more monoterpenes than mature fennel, depending on the compound comparison.
The overall aroma profiles of microgreens and mature foliage were clearly different.
This is another reason not to attribute every change in fennel aroma to VPD or PAR.
The plant’s developmental stage itself changes aroma chemistry.
What Does Research Tell Us About CO₂?
Fennel has unusually useful crop-specific CO₂ research.
A 2022 study published in Frontiers in Plant Science compared hydroponic fennel grown at:
400 ppm CO₂
and:
800 ppm CO₂.
Plants were grown for approximately five weeks in plastic-film-covered greenhouse chambers with a 12-hour light / 12-hour dark photoperiod. The reported mean environment was approximately:
27°C
and:
80% relative humidity.
The plants responded strongly to the higher CO₂ treatment.
Plant length increased from approximately:
49.81 cm at 400 ppm
to:
57.64 cm at 800 ppm.
Leaf fresh weight increased from approximately:
12.97 g
to:
43.86 g,
while stem fresh weight increased from approximately:
11.24 g
to:
77.09 g.
These are unusually large responses.
But again:
800 ppm should not automatically be called the universal optimum CO₂ concentration for fennel.
It was one of only two CO₂ treatments tested in that particular experiment.
The study shows that fennel can respond strongly to increased CO₂ under those conditions.
It does not establish whether 600, 800, 1,000 or another concentration provides the best economic or physiological result in every greenhouse.
Higher CO₂ Changed Quality as Well as Biomass
The CO₂ experiment also demonstrates why crop quality cannot be summarized with one growth measurement.
At 800 ppm, total flavonoid content increased.
However, several other antioxidant measurements, including total phenolic content, DPPH scavenging activity, FRAP and reducing power, decreased when equal quantities of plant extract were compared.
That creates a useful lesson:
A treatment that increases biomass does not necessarily increase every quality parameter.
The greenhouse objective therefore matters.
A grower selling fresh leafy fennel may prioritize:
biomass, visual quality and aroma.
A producer growing fennel specifically for phytochemical extraction may evaluate the crop differently.
CO₂ Should Be Interpreted Together With Light
CO₂ and light are not independent inputs.
Light supplies the energy for photosynthesis.
CO₂ supplies the carbon.
If a greenhouse crop has insufficient light, simply increasing CO₂ cannot remove the light limitation.
Likewise, during bright conditions, a dense actively photosynthesizing canopy may consume CO₂ rapidly, particularly when greenhouse ventilation is limited.
This means a useful greenhouse measurement strategy is not simply:
“Set fennel CO₂ to 800 ppm.”
A better question is:
Does CO₂ around the canopy fall during periods of strong photosynthesis?
If it does, CO₂ enrichment may be worth evaluating.
If CO₂ remains adequate but DLI is low, additional light may be the more important variable.
What About VPD?
The scientific evidence is much weaker here.
There is currently no convincing fennel-specific experiment demonstrating one optimum VPD range for germination, vegetative growth, harvest or regrowth.
This means precise claims such as:
0.4–0.7 kPa for seedlings
0.8–1.2 kPa for mature growth
or:
1.0–1.3 kPa before harvest
should not be presented as scientifically established fennel requirements.
The 2022 CO₂ experiment provides a useful example.
Its reported mean greenhouse conditions were approximately:
27°C and 80% RH.
If leaf temperature were close to air temperature, that corresponds to an air VPD of roughly:
0.7 kPa.
But the researchers were not testing VPD.
Therefore 0.7 kPa is a description of the successful experimental environment, not proof that 0.7 kPa is the ideal fennel VPD.
What VPD Is Actually Useful For
VPD remains valuable because it describes atmospheric water demand better than relative humidity alone.
If greenhouse temperature rises while moisture content stays similar, VPD increases.
That generally increases the evaporative demand on foliage.
But fennel’s response will also depend on:
root-zone moisture, leaf temperature, plant size, airflow and radiation.
So instead of using VPD as an “aroma setting,” use it to answer:
How strong is atmospheric water demand right now, and can the root system supply enough water to match it?
That is a scientifically defensible use of VPD.
There Is No Evidence That VPD Directly Controls Fennel Aroma
The old article makes several very specific claims:
high VPD causes weak aroma,
low or stable VPD produces stronger fragrance,
and VPD determines whether fennel aroma is “vibrant or flat.”
Current fennel research does not establish these relationships.
In fact, aroma chemistry is influenced by plant development and light spectrum, and the 2026 greenhouse experiment showed that supplemental red light increased important aroma compounds.
So statements linking one VPD range directly to aroma quality should be removed unless future experiments demonstrate that relationship.
Leaf Fennel and Florence Fennel Need Different Interpretations
This distinction is particularly important for growers.
Leaf fennel is primarily valued for foliage and aromatic quality.
Florence fennel is valued for its enlarged stem bases, often called the bulb.
University of Wisconsin Extension notes that Florence fennel benefits from more fertile soil and more even moisture than common fennel. Dry conditions can encourage flowering at the expense of bulb development.
That means a mild water stress that a mature common fennel plant survives easily may still be undesirable when the production target is a large Florence fennel bulb.
When interpreting PAR, VPD or irrigation data, first define:
What part of the fennel crop are you trying to maximize?
Bolting Is Another Reason to Watch Water and Temperature
Fennel naturally progresses toward flowering.
For Florence fennel, premature flowering is undesirable because resources are redirected away from bulb development.
University of Wisconsin Extension notes that dry conditions can promote bolting, while maintaining even moisture supports bulb development.
This again shows why attributing everything to VPD alone is risky.
A high VPD may coincide with:
high temperature,
rapid substrate drying,
and increased plant water demand.
The crop experiences the entire environment, not one VPD number.
Why DLI Is More Useful Than One Midday PAR Reading
Imagine two greenhouse days.
At noon, both measure:
400 µmol/m²/s at canopy level.
But one day remains sunny for eight hours.
The other becomes heavily overcast after noon.
The midday PPFD measurement tells you almost nothing about that difference.
DLI does.
For fennel, monitoring DLI can help compare:
sunny versus cloudy days, different greenhouse locations, seasons, and before-versus-after supplemental lighting.
This is particularly relevant after the 2026 fennel study demonstrated a strong response to supplemental daily photons.
Supplemental DLI Should Always Be Separated From Total DLI
This is worth emphasizing because it is a common mistake in greenhouse articles.
The 2026 study supplied:
100 µmol/m²/s × 14 hours
which equals approximately:
5.0 mol/m²/day.
But those photons were added on top of natural greenhouse light.
Writing:
“Fennel grows best at 5 mol/m²/day”
would therefore be incorrect.
The study tells us:
adding approximately 5 mol/m²/day of red photons improved growth and aroma under the natural greenhouse-light conditions of that experiment.
Those are completely different statements.
Research-Based Reference Conditions
The available evidence gives several useful reference points, but they should remain tied to their original experiments.
| Research situation | Environmental treatment | What it actually shows |
|---|---|---|
| 2026 hydroponic leafy fennel | Natural greenhouse light + 100 µmol/m²/s red LED for 14 h/day | Supplemental light increased biomass and aroma compounds |
| Supplemental lighting calculation | +100 µmol/m²/s × 14 h | Approximately +5.0 mol/m²/day supplemental DLI |
| 2022 hydroponic fennel | 400 vs 800 ppm CO₂ | 800 ppm substantially increased several growth measurements |
| 2022 greenhouse environment | ~27°C, ~80% RH, 12 h photoperiod | A successful experimental climate, not an optimum VPD prescription |
| 2024 aroma comparison | Microgreen vs mature hydroponic fennel | Aroma composition changes substantially with developmental stage |
| Extension guidance | Full sun; even moisture for Florence fennel | Dry conditions can encourage flowering rather than bulb formation |
These studies answer different questions.
They should not be combined into a single “perfect fennel recipe.”
A Better Greenhouse Fennel Monitoring Strategy
Rather than programming a four-stage PPFD / CO₂ / VPD table, monitor the variables that describe what the crop is actually experiencing.
Measure PPFD at canopy height, not only fixture output or outdoor sunlight.
Track DLI so changing natural-light conditions become visible.
Measure CO₂ near the active canopy during bright periods to determine whether photosynthesis is depleting available CO₂.
Monitor temperature and humidity together, using VPD as an indicator of atmospheric water demand rather than an aroma target.
Keep track of root-zone moisture, particularly when growing Florence fennel, where drying can encourage premature flowering.
And identify whether the production objective is:
leaf biomass, aroma quality, bulb development or phytochemical composition.
The correct environmental interpretation can change depending on that objective.
Sensor Placement Matters
Fennel can become tall and develop a large, open canopy.
A PAR sensor fixed close to the substrate may accurately represent seedlings but become increasingly irrelevant as the canopy rises.
The sensor should therefore follow the active canopy.
The same principle applies to temperature, humidity and CO₂.
A sensor mounted near the greenhouse roof describes the greenhouse roof environment.
It does not necessarily describe conditions experienced by fennel leaves.
For meaningful comparisons, keep sensor placement consistent relative to the crop.
What Should Growers Actually Optimize?
Current research supports several clear conclusions.
Fennel can respond strongly to supplemental light.
A 2026 leafy-fennel experiment found substantial biomass gains from supplemental red LED lighting.
Light can change aroma as well as yield.
The same study found increased aroma-active phenylpropanoids, showing that more light did not simply make fennel less aromatic.
Fennel can respond strongly to elevated CO₂.
In one hydroponic study, 800 ppm produced substantially greater biomass than 400 ppm.
But more biomass does not mean every quality metric improves.
Elevated CO₂ altered flavonoids and other antioxidant measurements in different directions.
There is not yet a scientifically established fennel VPD recipe.
VPD is best used to understand atmospheric water demand, not as a direct control for aroma.
And:
leaf fennel and Florence fennel should not automatically be managed as the same production target.
Key Takeaway
Greenhouse fennel does not need an invented series of precise PAR, CO₂ and VPD values for every growth stage.
Modern fennel-specific research provides a better approach.
A 2026 greenhouse study showed that adding 100 µmol/m²/s of red light for 14 hours per day — approximately 5.0 mol/m²/day of supplemental DLI — increased leafy fennel biomass and important aroma compounds.
A direct CO₂ experiment found that 800 ppm substantially increased fennel growth compared with 400 ppm, but also changed different phytochemical measurements in different ways.
And aroma research shows that fennel’s volatile profile changes naturally as the plant develops from microgreen to mature foliage.
For growers, the stronger strategy is therefore to:
measure canopy PPFD, track DLI, monitor CO₂ during active photosynthesis, interpret VPD together with temperature and root-zone water, and evaluate biomass and aroma separately.
That is far more useful — and scientifically defensible — than following a fixed environmental recipe.
References
Liu, J. et al. (2026). Narrow-wavelength red LED lighting enhances growth and aroma of hydroponic leafy fennel (Foeniculum vulgare Mill.). Food Chemistry: X, 38, 104178.
Jo, N.-Y., Lee, J., Byeon, J.-E., Park, H.-J., Ryoo, J.-W., & Hwang, S.-G. (2022). Elevated CO₂ concentration induces changes in plant growth, transcriptome, and antioxidant activity in fennel (Foeniculum vulgare Mill.). Frontiers in Plant Science, 13, 1067713.
Liu, J. et al. (2024). Characterization of key aroma compounds in microgreens and mature plants of hydroponic fennel (Foeniculum vulgare Mill.). Food Research International, 197, 115229.
University of Wisconsin–Madison Division of Extension. Fennel, Foeniculum vulgare.
University of New Hampshire Extension. How to Grow Green Fennel (Foeniculum vulgare).