Extension Specialist Jonathan Ebba Answers Real Questions About Gardening With Artificial Light

Indoor lighting - Pamela Hargest

In a recent webinar from UNH Extension and UMaine Cooperative Extension, UNH Extension field specialist Jonathan Ebba delved into light and how plants use it, and how you can use artificial light to help grow seedlings and other plants indoors. 

We didn't get to all the questions viewers had for us, so we are sharing them here as a written Q&A. Enjoy!

Is it possible to figure out PAR if you know the lumens and kelvins of an LED light?

How much of that light is useful for photosynthesis will vary based on the spectrum from that lamp, and lumens/ kelvin doesn't fully account for that, but we can get a pretty good guess.  There are some good online calculators that can convert based on the type of LED (full spectrum or blue/ red).  (Here is one: https://www.waveformlighting.com/horticulture/convert-lumens-to-ppf-online-calculator)

It's important to remember that distance affects intensity... if a lamp is listed as some number of lumens, that will vary greatly based on the distance from the lamp.

I recently checked out some of the light meter apps available for download.  These use the light sensor on your phone and convert the reading to PAR based on the lamp type (like the online calculators I referenced above).  They seem pretty good!  This will help you to get around the distance question when trying to use specs from the manufacturer ... you can get a reading using your phone and one of these apps.  (Here is one: https://growlightmeter.com/)

How do you know what the light requirements are for different plants? 

Largely, 4-6 moles per square meter per day for houseplants 

6-8 for shade plants

10-12 for part shade plants

14-20 for full sun

These are for growing/ flowering/ fruiting.  For growing seedlings prior to transplant, we can target more like 8-14, even for full sun plants.

There is a handy chart at the end of this fact sheet from Purdue University.

Do common vegetable plants like tomatoes and peppers need a certain amount of darkness to fruit?

Tomatoes actually do need dark (6 hours) to fruit.  Peppers do not.  (Interestingly, wild tomatoes were found to not need a dark period. These were crossed with commercial tomatoes and some 24 hour day tomatoes are being bred for commercial uses.

For seedlings (veg & flower) that will be planted after the last frost, would you recommend a 54W fluorescent tube, or would a 32W suffice? 

A 54W will allow you more room to grow and more options for length of run time.  The 32W may be too close, or may need to run for too long to be effective.  If you can use the 54W, that's preferred.  If not, keep the lamps close to the crop and on for up to 23 hours per day.

Ultimately, the plants will tell you.  If they are stretched and pale, you need more light, and remember to get more we can go brighter, closer and/ or longer.

If only lumens are listed in a light's specifications is there a way to know if it will be sufficient for plant growth?

You can use a calculator to get an idea of how much PAR the lamp provides.  Armed with that info and knowing what Daily Light Integral (DLI) you are targeting (12 moles per square meter per day for transplants), you can use this worksheet to calculate the number of hours you need to run the lamp.

Will LED shop lights work for starting seedlings, or do I need to purchase a LED labelled as a "grow light"?

LED shop lamps will work fine.  Remember, failure with growing plants under lamps is more often about light quantity (intensity and duration) than quality (spectrum).  Keep them low enough, bright enough and running for long enough per day.  

What a grow lamp will do is provide a spectrum that has a larger portion of its photons available for photosynthesis.

Does the type of fluorescent tube matter for seed starting? Should I be using "daylight" full-spectrum tubes?

See the above answer.  Correct spectrum is nice, but intensity/ duration is more important.  Interestingly, photosynthesis doesn't prefer daylight full spectrum... it best utilizes a red/ blue combination with less in the yellow & green wavelengths.

Is there a standard lifetime for fluorescent bulbs? I have some that are 10 years old and still produce light, but I'm wondering if I need to replace them.

The life span really depends on the bulb and the lamp industry has done a bunch of research in this area.  Typically,  good T8 fluorescents can last 20,000 - 30,000 hours.  Cheap ones, turned on and off a lot can be as little as 10,000 hours.

They do decrease in intensity over time, but in a steep curve at the end of their life...typically as they start to lose intensity, they continue to lose intensity and should be replaced.

One interesting thing to note is that keeping fluorescent lamps on for long periods before turning off and back on again (like in a grow room), instead of on and off several times per day (like in a normal room in the house),  actually helps them live longer.

What should I know about artificial light for starting seeds that need light to germinate?

Seeds which need light to or germinate better with light need only 2-15 micromoles per second.  That's pretty low intensity... you can get that from a windowsill or low intensity lamp set for 12 hours per day.  As soon as those are germinated, however, they will stretch with insufficient light just like any other seedling!

(Those few light-germinating seeds need to be left uncovered, of course, and keeping an uncovered seed moist enough to germinate can be difficult in a house, but that's another conversation!)

How do you determine how close a light fixture should be to seedlings for proper growth? 

The most accurate way would be to determine how much DLI the seedlings needed, determine how long you would like to run the lights for, and calculate the required intensity.  Then use a light meter to find out at what distance from the fixture the lamp provided the needed intensity.

More often, we simply hang the lamp (if it's a household, fluorescent or LED) 4-6 inches from the tops of the plants.  If the seedlings burn, move the lamp further.  If the seedlings stretch, move the lamp closer and/ or run it for longer.

In seed starting, how important is the temperature of the soil compared to the light source? 

That's a great question!  Soil temp is often more critical for germination than it is for growing... that's why we often place seedling trays on a heat mat until they emerge, then we move them off to prevent stretch.  During that germination phase, for the most part we don't need light anyway until the plants are emerged.  This is why some commercial growers use a germination chamber with no lights but with a heat  and humidity source.  Seedling trays are kept in this chamber (hot, moist and dark) until the seedlings emerge, when they are immediately moved to a cooler, drier, brighter area.

I guess the quick answer to your question is that heat is more important before the seeds emerge, and light is more important afterward, but that is of course within limits (it doesn't matter how bright your tomato seedlings are if they are at 35 degrees F... they won't thrive!)

Do you love learning about stuff like this? 


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Landscape & Greenhouse Field Specialist
Extension Field Specialist, Landscape & Greenhouse
Office: Coop Extension, Taylor Hall, Durham, NH 03824