Plant lights - The good, the bad and the bright

Plants need the right light to grow

If you plan on starting seeds indoors you are going to need lights to accompany your efforts as plants need light for photosynthesis.  The question often asked (see the previous post's comments) is - what's the "right" light for starting plants indoors?  Since I spent a fair amount of time researching this very topic years back I figured a concise post about what I do might offer some mild guidance and insight (or not).

First off - the reason why home seed starters struggle with this question is the fact that the light you "see" is slightly different than the light plants "use".  On a technical level there are really 4 peak light wavelengths that plants use for photosynthesis.  The goal of seed starters - hit as many of those peaks as you can so your plants don't grow up leggy, tall and unhealthy.

Back when I spent a good deal of time looking into this I referenced articles published by Cornell and Purdue's agriculture departments.  The classic question being - incandescent or fluorescent.  And if fluorescent - which bulbs?  The easy starting point was NOT incandescent.  For starters they get HOT - and having enough of them to light up your plants becomes dangerous for the plants as well as the planter.  Also - they just don't produce the right light for the plants.  So rather than spend a long time discussing them - let's focus on the fluorescent options.

Fluorescent lights are inexpensive (big box stores offer a 4' light for under $10 + bulbs).  Inexpensive is good.  It's not the fixture that generates the rabid discussions amongst growers but rather "which bulb"?  And it's here in the conversation that you hear people use terms like "degrees Kelvin", "full spectrum", "cool light", "warm light"...  And while there's a TON of science behind all that, here's the basic takeaway:  Natural sunlight is the "full spectrum" of light - it's what plants have evolved to use - the closer we can get to that - the happier the plants are.  The color of the spectrum is measured in "degrees Kelvin" with light that appears blueish being higher in degrees Kelvin (around 6500K) and being referred to as "cool light" (because of the blue color - not degrees).  The other end of the spectrum is "warm light", it's reddish and is around 3500K. (here's a great explanation).  Ok - so what does all this mean - boiled down?

Well - when you pick up your $9 shop light and need bulbs there are 3 primary choices you will see:

1. very expensive "full spectrum" bulbs that are sold specifically to support seedling development and growing plants with no natural sunlight


2. cheap "cool white" bulbs - they look very blue when lit and are typically recommended for a garage or a kitchen on the package


3. cheap "warm" bulbs - these look orangeish and the package will tell you these are great for your basement or family room

So which one?  Well - the research conducted by some of the worlds best ag schools found that a lighting fixture with 2 of the "full spectrum" bulbs that cost up to 4X as much work no better than a the same fixture loaded with one "cool" bulb and one "warm" bulb.  The idea of combining the two types goes back to the 4 peaks in photosynthesis (two of them are in the blue regain and two are in the red region).  Ohh - and the single most important factor in good plant development - not the color of the bulbs but their age.  these studies showed after 2 years - you really should replace the bulbs.

At then end of the day (you timer should turn off the lights)...   At the end of the day - the choice is yours.  You can certainly opt for the expense of the full spectrum lighting but I have been using one cool and one warm light in my lamps for years and very rarely have to deal with spindly plants.  Those times  I do get leggy growth - I look at my notes and see that I must not have replaced older bulbs.  When I do get strange growth I have another trick I use that keeps my guys growing squat and robustly... more on that later...

(Thanks Shirley - for you comment and article inspiration)

Comments

[Shirley Zundell]

Andy, thank you for the thorough response. How interesting that the warm/cool bulbs work just as well as the full spectrum. I found the different Kelvin measurements confusing so your article helped to clarify this. I'll be on my way to buy the cheap shop lights, and as you suggest I'll make a note so that I can replace them after two years. Again, thank you for sharing this information and everything on your website. It's truly informative and fun to follow.