The winter months can always be a challenge for floriculture producers. But have you found this winter to be unpredictably dark? Has this affected your crop?
Read on to see if this year is unusual, or part of a trend, and how this may influence your production decisions (and energy costs) in years to come.
Was This Year Really Darker?
The image below created by climatologist Brian Brettschneider shows which areas of north America were above, near or below normal solar radiation. Each “tercile” or category, represents which third of the 1991-2020 data this winter falls within.
As you can see, areas shown in grey, including southern Ontario, received less solar energy than average this season.
Figure 1. This map was created by Brian Brettschnieder using data from the European Centre for Medium-Range Weather Forecasts (ECMWF).
Understanding Light Measurements
To determine how lower than average light levels will affect your crop, we need to first talk about the importance of Daily Light Integral (DLI), or the total amount of photosynthetic active radiation (PAR) delivered to plants over a 24-hour photoperiod.
Other units for measuring light such as foot-candles or lux are limiting because they only provide a single instantaneous reading. Natural light levels change throughout the day and a single measurement doesn’t accurately represent the total amount of light plants get.
In contrast, DLI readings represent the number of light photons that accumulate in a square meter throughout the day (moles per m2 per day) and leads to more accurate recommendations to growers in terms of plant production needs or supplemental lighting use.
How can lower light levels affect floriculture production?
Low light levels can result in production lags by impacting:
- Seedling and cutting growth
- Flower number and timing
- Branching and stem thickness
- Colouration
When stacked with other environmental factors like high humidity, low light can also lead to physiological disorders such as oedema.
Each plant species has an optimal light intensity that maximizes photosynthesis and plant growth, therefore optimum DLI levels are heavily dependent on the type of crop grown. The table below summarizes the recommended minimum DLI for commonly grown ornamental crops in Ontario.
| The Recommended Minimum DLI for Various Ornamental Crops | ||
| >4 mol/m2/day | >8 mol/m2/day | >12 mol/m2/day |
| Forced bulbs | Begonia | Cut Flowers |
| Phalaenopsis orchids | Cyclamen | Vegetables |
| Ferns | Kalanchoe | Chrysanthemum (potted) |
A full comprehensive list of light requirements categorized from minimum acceptable quality to high quality was created by Jim E. Faust of Clemson University and can be found in the Purdue Extension article Measuring Daily Light Integral in a Greenhouse.
Measuring the Daily Light Integral in Your Operation
To measure DLI, there is the option of using quantum or PAR sensors that measure instantaneous light intensity at defined intervals in µmol/m2/s and can then be converted to DLI.
If you are currently collecting light data in foot-candles, the following table created by Ariana P. Torres and Roberto G. Lopez at Purdue University shows how you can convert those values to PAR and then to DLI for sunlight and high-pressure sodium (HPS) lamps.
| Step | Method | Example |
| Determine the average number of foot-candles per hour | Take the hourly foot-candle averages for the day, add them, and then divide this sum by 24 | Your 24, hourly foot-candle readings: 0 + 0 + 0 + 0 + 0 + 5 + 12 + 21 + 40 + 43 + 159 + 399 + 302 + 461 + 610 + 819 + 567 + 434 + 327 + 264 + 126 + 15 + 4 + 0 = 4,408 foot-candles 4,408 foot-candles ÷ 24 hours = 184 foot-candles per hour |
| Convert foot-candles per hour to PAR | Multiply the calculated foot-candles per hour by the following factors: Natural sunlight has 0.20 foot-candles per µmol/m2/s HPS lamps have 0.13 foot-candles per µmol/m2/s Note: the conversion factor is dependent on the light source! | PAR for natural sunlight: 184 foot-candles per hour x 0.20 foot-candles per µmol/m2/s = 36.8 µmol/m2/s PAR for HPS lamps: 184 foot-candles per hour x 0.13 foot-candles per µmol/m2/s = 23.9 µmol/m2/s |
| Convert PAR to DLI | Use the following equation: PAR (µmol/m2/s) x 0.0864 0.0864 factor is the total number of seconds in a day divided by 1,000,000 | DLI calculation for natural sunlight: 36.8 per µmol/m2/s x 0.0864 = 3.2 mol/m2/day DLI calculation for HPS lamps: 23.9 per µmol/m2/s x 0.0864 = 2.1 mol/m2/day |
Dark Winters – An Argument for Supplemental Lighting?
Management decisions surrounding the installation and use of supplemental lighting are farm-specific and comes down to the production needs of your operation.
If you are considering installing supplemental lighting, one way to reduce operating costs and maximize the effectiveness of lighting is to only run the fixtures when natural light levels are below a pre-determined threshold. For example, by connecting an outdoor light sensor to an environmental control computer, fixtures can be programmed to turn on for a certain period when the outdoor light intensity drops below the programmed threshold (i.e., on cloudy winter days) and turn off again when the sun comes out.
A case-study by Dr. Chevonne Dayboll (OMAFRA), David Llewellyn and Youbin Zheng (University of Guelph) looked at different supplemental lighting programs in potted chrysanthemum to determine which method works best with the best economic return.
The results of the experiment found that while threshold and fixed period (i.e., running lights for a solid, predetermined amount of time) supplemental lighting treatments produced plants that were similar in height and root mass, the threshold treatment allowed for less energy consumption that translated to lower costs.
| Treatment | 14-hour | Threshold | Natural Light |
| Total Light Integral | 264.4 | 222.2 | 196.3 |
| % Supplemental Light | 26% | 12% | 0% |
| Energy Use (kWh/m2) | 9.8 (0.35 kWh/m2/day) | 3.5 (0.125 kWh/m2/day) | – |
| Cost (@$0.10/kWh) m2 | $0.98 | $0.35 | – |
Full experimental details and further results can be found on this slide deck by Dr. Chevonne Dayboll presented at the 2019 Canadian Greenhouse Conference.
Although this winter seems to be a once in a century blip, accurately measuring and tracking trends in DLI can help you to decide in the long term if supplemental lighting is right for you, especially if you’re going to take on a wider variety of crops in the future.