When getting more serious about gardening comes the realization that starting your seedlings helps you save money and get a head start on spring and fall transplants. Starting your plants inside is a big step towards increasing production.
I like to start onions, herbs, flowers, solanaceous (tomatoes, eggplants) crops, and some leafy greens under grow lights. I also use some of the indoor grow space to continue growing microgreens throughout the year.
Knowing what exactly plants need from grow lights will help you decide which lights will best serve your needs inside. Except for onions or celery, we don’t want to keep our plants inside for more than a month. Having a greenhouse or frost-protected area is key to utilizing a small indoor space as a nursery step for lots of great spring starts. Although, if you want to fruit a tomato plant inside its possible, but you would need a wider spectrum of light that simple lights for seedlings.
How plants utilize light
In the process of photosynthesis, plants absorb energy through light and combine that energy with carbon from the atmosphere along with water to form sugars. The best form of specific artificial light that plants will utilize ranges from 400 (violet) to 700 (red) nanometers. Typically, seedlings utilize a blue-green light and a warmer red light in later stages of reproduction.
Light intensity
To avoid having leggy seedlings that have too much top growth without a base to support it, we need optimal light intensity and spacing.
When buying a grow light there should be indications of watts, PPF (photosynthetic photon flux), and lumens on the packaging. Watts is a measurement of the amount of energy needed to power a light. PPF is a measurement of the amount of light received by a 1 square foot located 1 foot away from the light. In short a measure of photons, or light falling on a surface. There’s also another measure if you get into more expensive lighting fixtures that may advertise a PPFD (D=Density). This stands for photosynthetic photon flux density falling on a particular surface. PPFD will tell us how wide of an area we can grow our seedlings without having issues with stretching. Generally speaking, the further you move away from the center of light the photon density will also decrease as you move further from the center.
Optimal PPFD for seedlings
The ideal PPFD for seedlings is around 100-300 measured in μmol/m2/Second.
You may be able to source shop LED strip bars for less than an advertised grow light but you will likely only see specifications of wattage and either lumens or lux capacity. Lumens are a measurement of the visible light emitted from a light source. Generally, the brighter the light the higher the lumens. This does not always correlate with PPFD because plants can only use PAR (Photosynthetic Active Radiation) or the photosynthetic spectrum of 400-700 nm.
To conclude we are looking at a Vegetative growth that best occurs at 440 to 470 nm in the blue end of PAR with a PPFD of 100-300, and a sweet spot of around 170 (Cultivate Tech, 2022). We already discussed growing out the majority of our plants for no more than 30 days under the lights so it may be beneficial to lean on the higher end of PPFD towards 300. You may not utilize all the light but we want to be careful of causing stretching if there is insufficient PPFD towards day 30.
Light Options
What type of light should I be looking for?
Metal halide and high-pressure sodium lights will provide the correct light for your plants but waste a lot of energy throwing all colors of the visible spectrum and not only what the plants need.
The best modern solution is LED fixtures, specifically strip tape tube bars. These LEDs are replacing the 4-foot t8s that was once the favorite shop light of many. LEDs are not prone to burnout like other fluorescent tube lights.
T-5 Fixtures
There are more modern t-5 options more suitable for growing plants than the t-8s but they will run at hotter temperatures than LEDs. This may be good if it’s late winter and your indoor space is not as warm as you need for proper growth. The t-5 2 ft option from Vivosun is a great option to get started. They also sell at 4 ft t-5 option for larger spaces. If you do buy a t-5 make the most of your shipping and get some replacement bulbs. They tend to burn out after 2 or 3 years of use. I have some t-5 fixtures that work great for anything that we would start as early as January and it helps to have that extra heat then. You can also technically use the backs of the lamps as heat mats for your starts. Only keep in mind that your light fixtures are not waterproof.
LED Strips
I love LED strips for the amount of space they cover with minimal cost for the output. I have a mixture of t-5 and led strips to start all our indoor transplants. My favorite LED strip is the 4ft LED Shop Light 6500K because it comes in a 6 pack and you can fill up a decent space with these lights. It comes already wired to a plug, has an on/off relay switch and you can even connect them in pairs using one plug if you wanted. 6500K is in a similar PAR range to the midday sun. It will work great for starting your plants or even using a wider spacing of lights for microgreens.
Full spectrum LED options are only necessary if you wish to grow the entire cycle of the plant indoors. The PPFD requirements will increase as well as the PAR spectrum depending on the crop. In my opinion, there are very few plants that make sense to grow indoors when you could just use the sun!
Making the most of our lights
Now that you know more about grow lights for seedlings, it’s time to think about how to best utilize indoor space for seed starting. My go-to is using vertical shelving systems. I’ve repurposed organizational shelves I already had and invested in Saferack NSF storage racks specifically with the 4 ft LED tubs.
If you are starting lots of plants you probably want to be thinking about using cell trays or 1020-sized flats if making soil blocks. Bootstrap farmer sells great cell tray options, my favorite being 72s, and 128s. And while you there you may as well check out their great microgreen trays! I prefer using a tray with holes in the bottom to utilize bottom watering.
Sources
Cultivate Tech (2022) Understanding spectrum, lumens, par and PPFD, Cultivate Tech.
Hoidal, A.N. (2022) What type of grow lights should you use for starting seeds?, UMN Extension.
