Have you wondered if all the hype surrounding grow lights is true and if these lights will help your seedlings grow better? Chances are, your online quest for answers left you tangled in contradictory advice. (I've been there myself while delving into this topic for Master Gardeners.) Perhaps you're now teetering between abandoning the idea altogether or impulsively snagging lights adorned with all the persuasive catchphrases, hoping for the best. Today, I hope to explain the cases where supplemental lighting is beneficial and guide you through the key specifications to consider when purchasing grow lights.
First, we need to step back and evaluate what light is
needed during the seedling phase and what light actually makes its way into
your home. There are three important
aspects of light for plants – quantity, quality and duration. Light quantity is the number of light particles
capable of performing photosynthesis.
This means the amount of usable light that actually reaches the leaves.
This is different from light perceived by the human eye. Light quality refers to the specific range of
light wavelengths plants use to photosynthesize. As illustrated in the figure, blue and red
wavelengths are what plants primarily use for photosynthesis.
Plants thrive on a consistent dose of light each day, with
the required duration contingent upon factors such as light intensity and the
developmental stage of the plant—be it seedling, vegetative growth, flowering,
or fruiting. There’s a term - Daily
Light Integral (DLI) - that brings
together both intensity and duration in order to quantify the total light
received by a plant over a 24-hour period. DLI is measured in moles of photons
per square meter in a day. Different
light sources (sun or grow lights) provide varying light quantities. To
give an example, while a tomato seedling may meet its DLI quota in just five
hours of full sun exposure, achieving the same DLI under fluorescent light
might need a staggering 48-hour period—yes, all within a single day—owing to
its lower photon output.
What DLI value do seedlings need – about 12 to 16 mol/m2/day. For reference, this is the same light levels
needed for succulents and flowering houseplants (considered high indoor light
level.)
Now let’s look at what light actually makes it through the windows into your home. Did you know that low-E glass windows can block up to 90% of light intensity? I decided to test my windows to see how much light intensity is blocked. I purchased a light meter and took foot-candle readings inside and outside the window at the same height at noon. I found the quantity of light that makes it through is 32%. You can use a light app on your phone to take your own measurements (inside foot-candle or lux divided by outside foot-candle or lux)*100%. An additional item to note is that the further away you move from your house’s window, the light availability diminishes for your plants. Fortunately, the quality of light is not affected by windows. Likewise, it's also important to note that the duration of daylight indoors doesn't match outdoor conditions due to obstructions such as walls and external objects that impede sunlight.
If you want to go further into determining what the actual DLI value is in your home at a specific location, please reference this website.
https://www.extension.purdue.edu/extmedia/HO/HO-238-W.pdf
To go into all the details to calculate DLI
values would take too long for this blog.
I will share with you the findings I had for my home 6” away from both
south and west windows – 9.3 mol/m2/day and 5.6 mol/m2/day
respectively. I definitely do not have
the 12 to 16 mol/m2/day of light to grow healthy, strong seedlings
with just the sunlight . I need
supplemental lighting. Calculating DLI
can provide a precise measure of light requirements, but there are simpler
indicators to gauge if your plants need grow lights. Remember, seedlings have
high light needs akin to succulents. If you notice your succulents or other
high-light plants stretching and appearing spindly, it's a sign they're not
receiving adequate light. Similarly, if plants that should be flowering fail to
bloom despite proper care, insufficient light might be the culprit. If you
observe any of these signs, it's worth considering adding grow lights to
provide your seedlings with the light they need to thrive.
What to look for in grow lights?
The market is inundated with so many choices, and
unfortunately there are several bad products available. All lights are not created the same, and
remember this blog is only discussing needs for seedlings. Fluorescents, LEDs, and HIDs vary in cost and
provide different specific or generic wavelengths and intensities. Prioritize manufacturers whose main focus is
producing lights, rather than those who dabble in various products. Such
specialists are more likely to offer lights tailored to plant growth needs.
Additionally, read all the details for the grow light. Good manufactures will list specifications in
plant light terms (PAR, PPF, PPFD). If purchasing LED lights, check to see if the
LED diodes/chips are actually designed for plants (like from Samsung) instead of designed for the human eye. Seek
out lights that offer a full spectrum covering wavelengths from 400 to 700
nanometers, or at least encompassing both the blue (400-499nm) and red
(600-699nm) ranges. Single gooseneck
lights or single LED tubes will only broadcast light to a small narrow
area. Keep in mind your seedling tray
dimensions and how much light area your grow lights need to cover. You may
discover that you need several rows of lights. Regarding light intensity, the
high end grow lights will give you a PPFD value which you want to be high (in
the thousands). However, smaller lights
like gooseneck types won’t give you an intensity indication and they have lower
wattage due to their compact size. For
these products you need to make sure that you are purchasing the correct
wavelength ranges and you will need to adjust the light distance from the
seedlings to get compact, strong growth.
And with that last thought, examine how easy you can adjust the grow
lights up and down. The intensity of the
light will be stronger closer to the bulbs and quickly get weaker further from
the lights. Some manufacturers will show
you values at 6”, 12” and 18” from the light source.
I hope you understand better what seedlings need and what
your house allows for natural sunlight. If you do need supplemental lighting,
consider what I mentioned above and happy shopping.
Resources
Important considerations for supplemental light
Factors to consider when purchasing lights
https://yardandgarden.extension.iastate.edu/how-to/sources-supplemental-light-indoor-plants
Growing seedlings under lights
https://extension.unh.edu/resource/growing-seedlings-under-lights-fact-sheet
