Geraniums generally perform best with substantial light.
But there is no scientifically supported rule saying that they require:
15–20 mol/m²/day as seedlings
then:
20–28 mol/m²/day during vegetative growth
and finally:
28–35 mol/m²/day for flowering.
Research on geraniums gives us a much more useful picture.
Light quantity affects:
rooting
biomass
plant compactness
and:
flower development.
But the response is not linear forever.
At low DLI, additional photons can strongly improve growth and development.
At higher DLI, the benefit can begin to plateau.
And temperature can become just as important—or more important—than additional light.
Quick Answer
For common bedding or zonal geraniums, substantial light is beneficial.
Illinois Extension recommends:
full sun
for Pelargonium × hortorum.
Controlled research on Red Elite geranium found that, at approximately 18°C, flower development toward the macrobud stage slowed when DLI fell below about:
13 mol/m²/day.
Increasing DLI above roughly:
17 mol/m²/day
produced little additional improvement in developmental rate to macrobud under that experiment.
Separate propagation research found that increasing DLI from approximately:
2 to 13 mol/m²/day
substantially increased biomass accumulation in geranium cuttings.
So a useful evidence-based interpretation is:
Geranium quality can improve strongly as very low DLI increases, but more light does not keep accelerating flowering indefinitely.
First: Which “Geranium” Are We Talking About?
The word:
geranium
is commonly used for two different groups.
The colorful annual bedding plants commonly sold as geraniums are usually:
Pelargonium
especially:
Pelargonium × hortorum — zonal or bedding geranium.
True hardy geraniums belong to:
Geranium
and include cranesbills such as Geranium maculatum and Rozanne.
Their biology and landscape use are not identical.
Illinois Extension, for example, lists bedding Pelargonium × hortorum as:
full sun,
while hardy Geranium species can grow from full sun into partial shade depending on the species.
This article therefore focuses primarily on:
zonal / bedding geranium — Pelargonium × hortorum.
Use PPFD, Not “PAR,” for µmol/m²/s
When a quantum sensor shows:
300 µmol/m²/s
the quantity is:
PPFD — Photosynthetic Photon Flux Density.
PAR refers to the photosynthetically active wavelength region, conventionally approximately:
400–700 nm.
PPFD describes the photon flux density in that region.
DLI integrates PPFD over the whole day and is expressed as:
mol/m²/day.
So:
PPFD = photon flux now
and:
DLI = total daily photon exposure.
That distinction is especially important for geranium because a high noon PPFD does not tell you how many photons the plant received through the whole day.
Geraniums Are Generally High-Light Bedding Plants
For outdoor landscape geraniums, Illinois Extension recommends:
full sun
with moist, fertile, well-drained soil.
Scented Pelargonium types are also commonly recommended for full sun.
This means geranium should not generally be treated like a:
low-light houseplant.
But “full sun” still does not equal one fixed PPFD.
Outdoor photon flux changes continuously with:
time,
season,
cloud cover,
latitude,
and shade.
Full Sun Is Not 700 PPFD
The old AquaHorti article treated values such as:
700–880 µmol/m²/s
as if they defined the ideal flowering condition.
That is not the right interpretation.
Outdoor geraniums can experience PPFD:
well above 700
for part of the day.
But they may experience much lower values:
in the morning,
under clouds,
or late in the afternoon.
So:
peak PPFD ≠ daily requirement.
For whole-day light quantity, DLI is much more useful.
One of the Best Geranium DLI Studies Tested 109 Environments
A classic study by White and Warrington examined:
Pelargonium × hortorum ‘Red Elite’
under 109 combinations of:
daily light integral
and:
daily mean temperature.
DLI ranged approximately:
9–25 mol/m²/day
and mean temperatures ranged:
13.5–22.5°C.
This gives us much stronger evidence than personal garden observations.
Low DLI Slowed Development Toward Flower Bud Formation
At a mean temperature of approximately:
18°C
the time from seedling emergence to the macrobud stage did not change substantially until DLI fell below approximately:
13 mol/m²/day.
Below that level:
flower development became increasingly slower.
This does not mean:
every geranium requires exactly 13 DLI.
It means that for:
Red Elite
under the tested temperature and production system,
lower daily photon supply increasingly restricted development.
More Than About 17 DLI Showed Diminishing Returns for Development Rate
The same study found that increasing DLI progressively accelerated development toward macrobud up to a point.
But above approximately:
17 mol/m²/day
there was virtually no further increase in the developmental rate to macrobud.
This is a very useful result.
It directly contradicts the simplistic assumption:
28–35 DLI must be better for flowering than 18–20 DLI.
For flowering timing, additional photons can reach a point of diminishing return.
Higher DLI Can Still Affect Other Plant Traits
A plateau in:
flower-development speed
does not mean additional light has no biological effect.
Higher photon supply can still influence:
biomass,
branching,
plant compactness,
carbohydrate status,
and flower number.
So different production objectives can have different useful DLI values.
The important point is:
there is no reason to assign 28–35 DLI as a universal flowering requirement.
Temperature Became Especially Important Later in Flower Development
White and Warrington divided geranium development into phases.
For development from macrobud to anthesis, temperature was more influential than DLI when light was already within approximately:
14–25 mol/m²/day.
This is critical.
Once daily light is no longer strongly limiting, increasing DLI may do less than correcting:
temperature.
So when a geranium has visible buds but flowering is slow:
do not automatically add more light.
Check temperature too.
More Light Cannot Fully Compensate for the Wrong Temperature
Light and temperature interact.
The same DLI can produce different development rates under different:
average daily temperatures.
Likewise, increasing temperature can accelerate flowering but may also reduce:
plant dry mass
and:
flower or bud number
if development becomes too fast.
So the goal is not:
maximum temperature + maximum light.
It is:
an appropriate combination for the production objective.
Geranium Cuttings Also Respond Strongly to DLI
Light matters well before flowering.
Research on geranium cuttings found substantial increases in growth as propagation DLI increased from approximately:
2 to 13 mol/m²/day.
A later review summarized that root, stem and leaf biomass of:
Pelargonium × hortorum ‘Fantasia Dark Red’
increased by as much as approximately:
122%
across an increase of roughly:
11–12 mol/m²/day in propagation DLI.
This shows that very low DLI can strongly restrict the production of a high-quality rooted cutting.
Propagation DLI Is Not the Same as Finished-Plant DLI
This distinction matters.
A newly rooting cutting has:
limited roots,
limited leaf area,
and a very different water balance
from a mature flowering geranium.
Therefore, a useful DLI during propagation should not automatically become the flowering-stage target.
Research reviews commonly identify roughly:
8–10 mol/m²/day
as a useful rooting-phase reference for many commercial vegetative cuttings.
That is a production reference, not a universal biological minimum.
Commercial Young-Plant Production Often Targets Around 10–12 DLI
Commercial greenhouse literature commonly identifies approximately:
10–12 mol/m²/day
as a useful target for producing high-quality bedding-plant plugs when natural greenhouse light is inadequate.
Geranium is among the bedding crops studied in this context.
But again:
10–12 DLI is not “the geranium number.”
It is a practical commercial young-plant reference.
Seedlings Do Not Have a Proven 300–450 PPFD Requirement
The old article assigned seedlings:
300–450 µmol/m²/s
and:
15–20 mol/m²/day.
Those numbers should be removed.
Commercial young-plant research demonstrates that useful geranium production can occur under considerably more nuanced light programs.
What matters is:
daily photon supply,
photoperiod,
temperature,
spectrum,
and crop stage.
There is no biological switch saying:
299 PPFD = inadequate
and:
300 PPFD = correct.
Vegetative Geraniums Do Not Require 500–700 PPFD
The old article then increased the supposed requirement to:
500–700 µmol/m²/s
and:
20–28 DLI
during vegetative growth.
Again, this was based on invented observation rather than controlled evidence.
A geranium may certainly experience those values outdoors.
But that does not make them:
minimum vegetative requirements.
Under greenhouse production, strong growth can occur at lower average PPFD when the photoperiod produces an adequate DLI.
Flowering Does Not Require PPFD Above 700
The old article stated that flowering geraniums needed:
midday PAR frequently above 700 µmol/m²/s
with:
28–35 DLI
for strong flower production.
This is the largest correction needed.
The Red Elite study found strong diminishing returns in developmental rate above approximately:
17 DLI.
Therefore:
28–35 DLI cannot be presented as the universal requirement for geranium flowering.
Does That Mean 17 DLI Is the Optimum?
No.
The 1988 research focused heavily on:
developmental rate,
temperature interaction,
and specific flowering stages.
Another cultivar or commercial objective may respond differently.
A grower seeking:
more biomass
or:
more flowers
may choose a different photon supply than a grower seeking:
fastest economically efficient flowering.
So the study should be interpreted as:
DLI response begins to show diminishing returns for flower-development rate in the mid-to-high teens under the tested conditions.
Not:
Every geranium should receive exactly 17 DLI.
Very Low DLI Can Delay Flowering Strongly
The same research also modeled a very low-light region where flowering would fail to proceed normally.
The authors reported a lower threshold around:
3.3 mol/m²/day
for flowering in Red Elite under their model.
That should not become a practical target.
Quite the opposite.
It illustrates that:
survival and successful commercial flowering are very different standards.
A plant receiving only enough light to barely flower may be far from high-quality production.
A Better Way to Think About Geranium DLI
Instead of rigid stage tables, the evidence suggests a response curve.
| Production situation | Evidence-based interpretation |
|---|---|
| Very low DLI | Growth and flowering can be strongly limited |
| Rooting/cuttings | Increasing DLI toward ~8–13 can greatly improve biomass and quality |
| Young plants | ~10–12 DLI is a common commercial reference |
Finished Red Elite | Development slowed below ~13 DLI at ~18°C |
| Mid-to-high teens DLI | Flower-development response approached a plateau in the classic study |
| Higher outdoor DLI | Can occur naturally; not automatically harmful or necessary |
| Slow flowering with adequate DLI | Check temperature, cultivar and plant condition |
These are:
research reference points
not:
one universal recipe.
PPFD Depends on Photoperiod
Suppose artificial lighting runs for:
16 hours.
Then:
150 µmol/m²/s × 16 h = 8.64 DLI
200 × 16 h = 11.52 DLI
250 × 16 h = 14.4 DLI
300 × 16 h = 17.28 DLI
This demonstrates why:
“Geraniums need 500 PPFD”
is incomplete.
The same daily photon supply can be created with different combinations of:
PPFD
and:
photoperiod.
But Same DLI Does Not Mean Every Lighting Program Is Identical
For example:
300 PPFD × 8 h
and:
150 PPFD × 16 h
both provide:
8.64 DLI.
But plants experience different:
instantaneous intensities
and:
photoperiods.
Spectrum and temperature may differ too.
So DLI is a powerful metric.
It is not a complete description of the growing environment.
Outdoor Geraniums Can Receive Much Higher DLI
Geraniums grown in full sun outdoors may experience DLIs far above the ranges commonly used in winter greenhouse production.
That does not mean a greenhouse grower needs to reproduce:
summer outdoor sunlight
with electric fixtures.
Outdoor sunlight:
rises gradually,
peaks,
falls,
and changes with clouds.
An indoor fixture operating at constant PPFD for 16 hours is a different environment.
Do not copy:
peak outdoor PPFD
directly into:
indoor lighting settings.
High Summer Light and Heat Can Interact
If geranium leaves fade or growth slows during very hot summer conditions:
do not automatically diagnose:
excess PPFD.
At the same time the plant may experience:
high leaf temperature,
high root-zone temperature,
rapid water loss,
and high VPD.
Illinois Extension specifically recommends avoiding overwatering during hot weather, illustrating how cultural management changes with environmental conditions.
Light must be interpreted with temperature and water.
Geraniums Can Become Lanky Indoors
When geraniums are overwintered indoors, insufficient light can produce:
long,
weak,
lanky growth.
Illinois Extension recommends a:
bright, sunny window
or supplemental lighting for overwintered Pelargonium and notes that plants become lanky when temperatures are warm and light is inadequate.
This is a useful diagnostic clue.
But legginess still does not tell you an exact PPFD.
Temperature Can Also Cause Stretching
Warm conditions combined with low light are particularly likely to produce poor plant form.
Therefore:
long internodes ≠ proof of low DLI alone.
Also evaluate:
temperature,
plant spacing,
spectrum,
fertility,
and cultivar.
The same principle applies to commercial greenhouse production.
Spectrum Can Change Geranium Morphology
Geranium seedlings have also been included in LED-spectrum experiments.
Research comparing red/blue LED mixtures with HPS lighting found differences in characteristics such as:
stem diameter
and plant quality
even under comparable lighting programs.
So PPFD is important, but:
PPFD alone does not describe spectrum.
Two fixtures can deliver the same PPFD and still produce somewhat different morphology.
Supplemental Lighting Is Most Valuable When Natural DLI Is Low
Northern greenhouses can receive only:
1–5 mol/m²/day
during dark winter periods.
Research on bedding-plant production therefore focuses heavily on supplemental lighting because low natural DLI reduces:
growth,
quality,
and uniformity.
When natural DLI is already high:
the benefit of additional electric photons may become much smaller.
So supplemental-light decisions should be based on:
the light deficit
rather than simply operating lights at full output every day.
Measure at Canopy Height
If measuring geranium PPFD:
place the sensor approximately where the active leaves are.
Do not measure:
at the fixture
or:
well above the crop
and assume the same number reaches the plant.
For benches or multiple containers, measure several positions:
center,
edges,
corners,
and between fixtures.
Light uniformity matters.
DLI Is Better Than One Noon Reading Outdoors
Imagine two containers.
One receives:
very strong noon sun
but afternoon shade.
The other receives:
moderate sunlight for most of the day.
The first may have the higher peak PPFD.
The second may accumulate a similar or higher DLI.
So if the question is:
Which location gives my geranium more light over the whole day?
measure:
DLI.
Do Not Estimate DLI From Five Spot Measurements
The old article measured a few supposed points such as:
220 → 540 → 730 → 670 → 400
and declared the day:
25–30 DLI.
That section should disappear.
A few selected readings do not reliably reconstruct a natural-light DLI without a defined integration method.
Clouds and moving shade can make the actual curve substantially different.
For outdoor DLI:
log PPFD through time.
A Better Practical Framework
For common zonal geraniums:
Outdoor garden or container: provide substantial light; full sun is a standard starting point.
Propagation: avoid very low DLI; cuttings respond strongly when daily photon supply rises from very low levels.
Young plants: around 10–12 DLI is a useful commercial greenhouse reference, not a universal minimum.
Finished plants: light in the mid-teens DLI can strongly support development; classic Red Elite research found diminishing returns in flowering development above approximately 17 DLI.
Slow flowering despite adequate light: examine temperature.
Weak, lanky indoor plants: investigate both light and temperature.
Hot-weather foliage stress: evaluate water and temperature together with PPFD.
Frequently Asked Questions
Do geraniums need full sun?
Common bedding geraniums (Pelargonium × hortorum) are generally recommended for full sun.
Is “geranium” the same as Pelargonium?
Not botanically.
Most annual bedding plants commonly called geraniums are:
Pelargonium.
True hardy geraniums belong to the genus:
Geranium.
What PPFD do geraniums need?
There is no single universal PPFD.
The appropriate PPFD depends on:
photoperiod,
DLI,
temperature,
cultivar,
and crop stage.
Do seedlings need 300–450 µmol/m²/s?
There is no strong basis for presenting that old AquaHorti range as a universal requirement.
Do flowering geraniums need more than 700 PPFD?
No.
Geraniums can experience >700 PPFD outdoors, but research does not establish that level as a flowering requirement.
What DLI do geraniums need?
There is no one universal value.
For Red Elite, flower development at about 18°C slowed as DLI fell below roughly:
13 mol/m²/day
and showed little additional acceleration above about:
17 mol/m²/day.
Is 10–12 DLI enough?
It is a widely used commercial reference for high-quality bedding-plant young-plant production.
Finished flowering plants may benefit from greater DLI depending on:
cultivar,
temperature,
and quality goal.
Is 28–35 DLI necessary for flowering?
No evidence supports that as a universal geranium requirement.
Does more DLI make geraniums flower faster?
At low-to-moderate DLI, increasing daily light can accelerate development.
But the Red Elite study found diminishing returns above approximately:
17 DLI.
Does temperature affect geranium flowering?
Yes.
In the classic DLI × temperature study, temperature strongly affected flower development, especially from macrobud to anthesis once DLI was already adequate.
What DLI is useful for rooting geranium cuttings?
Research on geranium cuttings shows strong improvement as propagation DLI increases from very low levels toward roughly:
8–13 mol/m²/day.
Treat this as a commercial research reference rather than a universal biological target.
Why is my geranium lanky indoors?
Insufficient light is one common reason, especially when indoor temperatures are warm. Illinois Extension specifically warns of lanky growth under warm, low-light overwintering conditions.
Should I measure PPFD or DLI?
Use:
PPFD for instantaneous photon flux.
Use:
DLI for total daily photon exposure.
For comparing outdoor locations or seasonal greenhouse light:
DLI is usually more informative.
The Main Takeaway
The old AquaHorti stage table should be removed:
Seedling: 300–450 PPFD / 15–20 DLI
Vegetative: 500–700 / 20–28 DLI
Flowering: >700 / 28–35 DLI.
Direct geranium research gives a much better answer.
For Pelargonium × hortorum 'Red Elite':
low DLI slowed development;
at approximately 18°C, the effect became more important below roughly:
13 mol/m²/day;
and increasing DLI above about:
17 mol/m²/day
provided little further acceleration toward macrobud.
Propagation research also shows that geranium cuttings respond strongly when DLI increases from very low levels toward the low-to-mid teens.
So the correct model is:
avoid light limitation → use DLI to quantify daily photon supply → recognize diminishing returns → interpret DLI together with temperature and production stage.
That is much stronger than pretending each growth stage has a fixed PPFD requirement.
Measuring Geranium Light
For geraniums grown outdoors or in greenhouses where sunlight changes through the day, AquaHorti AH-PARDLI can record PPFD and daily DLI at canopy height.
AH-PARDLI → /ah-pardli
Related guides:
PAR vs PPFD vs DLI → /understanding-par-and-dli-essential-light-metrics-for-plant-growth/
Why Track PAR Changes Throughout the Day → /why-track-par-changes-throughout-the-day/
Why Measure DLI Under Sunlight → /why-measuring-dli-under-sunlight-isnt-as-simple-as-it-seems/
References
White, J.W. & Warrington, I.J. — Temperature and Light Integral Effects on Growth and Flowering of Hybrid Geraniums. Journal of the American Society for Horticultural Science, 1988.
Currey & Lopez — research on propagation DLI and growth of Pelargonium × hortorum cuttings.
Faust & Logan — Daily Light Integral: A Research Review and High-resolution Maps of the United States. HortScience, 2018.
Craver, Boldt & Lopez — Comparison of Supplemental Lighting Provided by HPS Lamps or LEDs for Propagation and Finishing of Bedding Plants. HortScience, 2019.
University of Illinois Extension — Geranium, Pelargonium × hortorum.
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