PAR & DLI Requirements for Geraniums at Different Growth Stages
When I first planted geraniums in my garden and containers, I treated light the same way I treated most flowering annuals: put them in the brightest spot available and assume that was enough. At first, that seemed reasonable. Yet over a couple of seasons I noticed a pattern: some geraniums produced abundant blooms and sturdy growth, while others in nearby “bright” locations looked leggy, faded in color, or had fewer flowers. That inconsistency made me question what “bright” really meant from a plant’s perspective.
I began using a PAR meter to measure usable light intensity in different parts of my garden and to estimate Daily Light Integral (DLI), which is the total usable light plants receive over the course of a day. What I learned from tracking both PAR and DLI helped me understand how light actually affects geranium growth at different stages.
This article shares real measurements and how geraniums responded under different lighting conditions, so you can place and manage them more effectively.
Why Usable Light Matters for Geraniums
Geraniums, like many flowering plants, depend on usable light to support leaf development, stem strength, and flower production. Usable light refers to the portion of the light spectrum that plants can use for photosynthesis.
- PAR (Photosynthetically Active Radiation) measures usable light intensity at a given moment in micromoles per square meter per second (µmol/m²/s).
- DLI (Daily Light Integral) sums all usable light a plant receives over the entire day in moles per square meter per day (mol/m²/day).
Knowing both values gives a clearer picture of light conditions than observing brightness with the naked eye. Spots that look bright at one moment might have low total usable light because they receive short windows of direct sun or are shaded part of the day.
How I Measured Light Around My Geraniums
I took PAR meter readings at canopy height — the height where the geranium leaves and flowers were — at several times: early morning, mid-morning, midday, early afternoon, and late afternoon. Then I used those data points to estimate daily usable light totals.
Here’s an example of midday usable light measurements from a sunny bed where geraniums were thriving:
| Time | PAR (µmol/m²/s) |
|---|---|
| 08:30 | 220 |
| 10:30 | 540 |
| 12:00 | 730 |
| 14:00 | 670 |
| 16:00 | 400 |
From that pattern I estimated a DLI in the range of about 25–30 mol/m²/day on a clear late spring day. In this range, the geraniums in this location produced vibrant leaves, strong stems, and abundant flowers.
Light Needs at the Seedling Stage
In the earliest stage — when geranium seedlings are developing their first true leaves — they do not yet need the highest light levels of maturity, but they do need steady usable light to develop compact structure and avoid legginess.
In my observations:
- Midday PAR around 300–450 µmol/m²/s helped seedlings form thick stems and broad leaves.
- Daily totals around 15–20 mol/m²/day correlated with steady early growth.
- In spots where midday PAR stayed below about 250 µmol/m²/s and daily totals were under 12 mol/m²/day, seedlings tended to stretch and delayed the transition to robust vegetative growth.
Balancing usable light at this stage prevents weak, elongated seedlings and helps establish a solid framework for later flowering.
Usable Light During Vegetative Growth
Once geraniums begin producing more leaves and stems, their light needs increase. In this vegetative stage I found that:
- Midday PAR values of about 500–700 µmol/m²/s in sunnier areas supported broad, healthy foliage.
- Daily totals between 20 and 28 mol/m²/day under steady sun helped build energy reserves for later flowering.
- In areas that rarely exceeded about 400 µmol/m²/s and had lower daily totals, foliage expanded more slowly and stems grew taller and thinner.
Here’s a midday PAR profile I recorded during strong vegetative development:
| Time | PAR (µmol/m²/s) |
|---|---|
| 08:00 | 200 |
| 10:00 | 500 |
| 12:00 | 680 |
| 14:00 | 640 |
| 16:00 | 350 |
Patterns like this indicated consistent usable light throughout much of the day, which translated to lush foliage and compact growth.
Light Requirements for Flowering and Bloom Production
Flowering is the stage where geraniums really show their ornamental value, and it is also when usable light matters most.
In my garden, I saw that:
- Midday PAR frequently above about 700 µmol/m²/s supported strong flower production.
- Daily totals in the 28–35 mol/m²/day range correlated with abundant blossoms and sustained bloom cycles.
- In spots where midday peaks were lower and daily totals stayed closer to 18–22 mol/m²/day, plants still bloomed, but flower counts were lower and blooms were less prolific.
Here’s an example of midday PAR in a location that consistently produced abundant flowers:
| Time | PAR (µmol/m²/s) |
|---|---|
| 07:45 | 250 |
| 10:00 | 700 |
| 12:00 | 880 |
| 14:00 | 820 |
| 16:00 | 500 |
With usable light distributed well across morning and afternoon, these plants not only produced more flowers but maintained vibrant color and consistent form.
Seasonal and Weather Considerations
Usable light patterns shift with seasonal changes and weather. In early spring, days are longer and usable light accumulates more evenly, making it easier to hit target ranges for growth and bloom. In midsummer, intense midday light can spike, but the combination of strong midday peaks with high temperatures sometimes led to slight leaf fading or heat stress in my afternoon-sun plots, even when total daily light was high.
On cloudy days, midday PAR peaks were lower, but usable light often extended across more hours. In those cases, daily totals sometimes remained adequate for steady growth even though peak values looked modest. Tracking usable light across several days helped me understand patterns that individual midday readings did not capture.
How I Apply These Insights
From repeated measurement and plant observation, I developed practical usable light ranges that supported petunias through different stages:
Seedling stage
- Midday PAR around 300–450 µmol/m²/s
- Daily totals around 15–20 mol/m²/day
Vegetative growth
- Midday PAR around 500–700 µmol/m²/s
- Daily totals around 20–28 mol/m²/day
Flowering and bloom production
- Midday PAR often above 700 µmol/m²/s
- Daily totals around 28–35 mol/m²/day
These ranges reflect what I measured in my garden and how petunias responded under real conditions. They helped me place plants where they could develop strong foliage and abundant flowers rather than struggle in spots that looked bright but didn’t deliver enough usable light over the whole day.
Final Reflection
Growing petunias taught me that light needs are not just about brightness at a glance. Instead of relying on subjective labels like “sunny spot,” measuring usable light intensity and total usable light over the day gave me a much clearer understanding of how plants actually experience their environment.
Using a PAR meter to measure usable light at different times of day and estimating daily totals helped me place geraniums where they could thrive at each stage of growth — from compact seedlings to prolific flowering annuals. This approach allowed me to make informed decisions about placement, shading, and timing of plantings based on what the plants truly experienced, not just what looked bright to my eye.
If you want petunias that grow strong and bloom abundantly from spring through fall, thinking in terms of usable light intensity and daily light accumulation gives you a practical, data-informed way to understand their needs.
View on AmazonAmazon is a trademark of Amazon.com, Inc. or its affiliates.