How intense is the sunlight over the course of an afternoon? What’s the sun’s PPFD (µmol/m2/s) level?
Our sun is the main light source for the vast majority of plants on earth. Even when growing indoors, in a greenhouse or on a window sill for example, the sun produces the light required by our plants. But the light output from the sun can be tricky. There are many variables and they change all the time. Understand the correlation between plants’ light requirements and the sun’s light output. Knowing how much light, or how intense light, the sun puts out is the first step to calculating your plant’s light needs, or it’s DLI (Daily Light Integral).
First of all, the sun’s intensity or it’s PPFD, will vary greatly from one place to another. It is location (longitude & latitude), season, clouds, and atmosphere dependant, among other things.
We know that during a summer day, sunlight will be more intense along the equator than in northern Canada, for example. The atmosphere and clouds also absorb or reflect a fair amount.
A glass window also reduces light intensity
With all said and done, how intense sunlight (what PPFD) reaches your plants?
Real world test in Sweden (summer)
We made PPFD measurements on a sunny summer day in Lund, Sweden from noon (12.00) to 6 pm (18.00). With an Apogee MQ-500 quantum meter, we measured the sun’s PPFD curve over six hours outdoors. The majority of the results were measured with unobstructed sunlight but some readings were affected by clouds (14.30). The curve is, however, still informative.
We see that the PPFD peak is around 1 pm (13.00) and with levels slightly above 1700 PPFD umol/m2/s. The sunlight is intense all the way through the afternoon and only really dropping below 1400 PPFD after 4 pm (16.00).
So, how can we make use of this information?
Different plants require different light intensity. Here's a list of popular plants DLI requirements. See what your plant prefers. Then learn the photoperiod, or the length of sun exposure, to position your plant in a space that perfectly fits its needs.
A tomato plant (25 DLI) would, as an example, need 500 PPFD over 14 hours or 700 PPFD over 10 hours.
A standard double layer window absorbs 25-50% sunlight depending on the sun’s angle. 1400 PPFD reaching the glass window would only result in 700-1050 PPFD reaching the actual plant.
Here's a video of another light-obstructing material test.
Having this understanding of sunlight and light in general is pretty cool. It gives new opportunities to grow different types of greens and plants. Understanding if one light source is enough or if an additional one should be used as a supplement boils down to basic PPFD and DLI calculations. Is summer over and fall comes with shorter days and less intense light? If the natural DLI produced by the sun is less than ideal, increase DLI by using a grow light or LED bulb to achieve desired levels for your situation.