Starting Seeds Under Lights

I have been using fluorescent lights for growing seedlings and plants for at least 35 years. First of all, it’s fun. Starting successive replacement plants in advance of final harvest of those producing creates more generations than the two or three I might otherwise sneak in during one growing season. Lights also allow me to do something “garden” from about the first of January to Thanksgiving, reserving December for pouring over new catalogs and placing seed orders while the snow is falling and I am eating way too many cookies.

With two four-foot fluorescent fixtures, each of which takes two fluorescent tubes (making that a total of four tubes) you can easily start about 100 plants for your garden. That’s a whole lot of plants. The least expensive light hoods are actually longer than 48 inches. So when selecting a place to accommodate them keep their total length in mind. I have an incredibly simple unit of plastic utility shelves in my kitchen. The shelves are slotted across the top and make it easy to hang and adjust the lights. The ends are open to accommodate the length of the light hoods and through which air can flow. The shelf supports are plastic tubes that can easily be cut to length with a plastic pipe cutter in a matter of minutes. These units are 18 inches by 3 feet. So I set two up as an L-shape to accommodate the length of the light hoods. By putting them in the kitchen the extra shelf space also gave me a place to stash some kitchen overflow and gives me the freedom to add another bank of lights on a second shelf. In other words, if you shop around you can find inexpensive modular shelving. You won’t have to hire a carpenter or get into a complicated building project. If counter space permits you can also hang lights from the cupboards above the counters.

 There is much to-do about full spectrum lights. Certainly if you are supplementing the light in your home they are beneficial to you and a bit more attractive. I have used them but truthfully cheap four-foot fluorescent lights grow seedlings every bit as well and I would dare say better than full spectrum lights. I just can’t justify the cost either.

 Okay, we have our shelves and our lights are hanging. I would add one more feature. Purchase one plastic-ceiling tile that is virtually clear and has been cast to have a raised, honeycomb surface. Cut this panel (Carefully!!!) in half so that you have two strips 1’ X 4’. Lean or attach one at the back of your shelf and have one for the front to stand in front of your seed flats once they are set under the lights. Be sure that the honeycomb faces your plants. This simple step enormously increases the available light to your seedlings. And in my case, prevents my cats from eating my plants.

 You must put your lights on a timer (plugged into the outlet) because you want to leave them on for 16 hours a day. I have a surge bar plugged into my timer and I plug the individual light hoods into the surge bar. Most surge bars or multiple outlet units have about six outlets. Consequently, quite a few fluorescent lights can be connected to just one timer. I can also switch off the lights entirely with just one switch and my timer continues to run regardless. This configuration is quite handy when the timer is plugged into a difficult to reach outlet behind your shelves, as mine is.

So what do we start our seeds in? Hands down my favorite system is from Park Seed ( They call it their Bio Dome System. It uses Bio Sponges that are basically spun peat moss as far as I can tell. The sponges, available in three sizes, slide into hard foam blocks and replacement sponges can be purchased. The blocks, regardless of the sponge size, fit into a tray that comes with a dome.

 Now, as much as I love these sponges (and I only use the Whopper 3” X 1¾” diameter) there are genuine drawbacks to this system. I don’t like the domes at all; they make my seedlings weak, fleshy, and water dependent. And the height of the dome puts the grow lights too far away. Park’s does not sell separately the flats in which the blocks sit nor do the blocks fit efficiently in less expensive standard greenhouse flats. Consequently, the system becomes way too pricey for my taste. But through some experimentation I found a great way to use the large sponges with any shape flat available. Activia Yogurt Containers (individual serving size). Cut the bottom out of the recycled yogurt cups, set them on a one inch layer of perlite or aquarium gravel spread into the bottom of the flat, and slide the sponges into the yogurt cups pressing them slightly into the perlite.

 From this experiment I discovered than even when I used the commercial foam blocks designed for the sponges adding that layer of perlite or in the tray easily tripled the performance of the seedlings. The perlite created a hydroponic layer that was constantly moist, the perfect failsafe for both over and under watering. And do make this reservoir nutrient rich by always watering with a ¼ dilution of water-soluble organic plant food. I mix up several gallons at a time and just water the trays as needed. Be aware that over watering kills seedlings. Be sure that the water level does not remain above the surface of perlite layer. Once the sponges absorb what they need the water level should drop below the surface of the perlite. Note that you must wait to begin feeding seedlings until they have developed two sets of true leaves. While you are waiting for your seeds to germinate the sponges must remain damp to the touch. Check them often.

 I did a search for a possible alternative to the bio-dome system that should be able to accommodate the large sponges. International Greenhouse Company ( carries the following products to consider but they do not provide a perfect alternative:

 #1020 Trays no holes (10 for $12.00)

#TLC Plug Flats/Hobby Pack 50 cell (10 for $13.00)

These are large quantities and would be a great group purchase among friends if you think these alternatives would serve your purpose. I can’t vouch for these products only because I have already invested in Park’s system. But I think I would in the future if I needed to increase the number of specimens I grew or I had to replace the system I have.

You no doubt can see now that with a few lights, some trays, an ample supply of bio-sponges and something to hold them in, rapid successive planting is easy to accomplish. Once one generation is planted in its final destination the next generation can be started immediately under lights. Where only containers or a very small garden exists you can keep plants coming for an extraordinary length of time.

Assuming you have acquired all of these components and your seeds, don’t forget to buy plant markers. Don’t be tempted to buy cheap, wooden markers. They get moldy, they rot, and then you can no longer read them. Use plastic and a permanent marker. However, I don’t like to buy these and have found that I can cut strips from plastic food containers and they work every bit as well. It is also a way to recycle at least a small amount of plastic much in the same way the yogurt cups were recycled.

 Last step. Your seeds are now sown into the sponges and labeled. Now the lights have to be adjusted to just 2 inches above the sponge surface and later remain 2 inches above the foliage. And yes, moving the lights can be a real pain. So what I do is put something like a flat box, bricks or 2X4’s, whatever you can salvage around the house, under the flats and raise them to the right distance in relationship to the lights rather than constantly have to raise or lower the lights themselves. As your seedlings grow simply lower the flats by removing whatever you used to elevate them.

 This system is so efficient and easy no matter how busy your life is you can do it. Once every two weeks you spend maybe a half-hour sowing a few seeds and sliding them back under the lights. Every couple days you check and see if the sponges and perlite are damp. If not just pour a little of that premixed plant food into the perlite. Of course when your seedlings are stout and growing like crazy you must check more frequently. But the perlite layer buys you time and often insurance should you forget to check.

 So spring is here and you have a fabulous crop just waiting to go to containers or gardens. Check your local last frost date and watch the nightly weather report carefully. Its time to harden off your first generation of plants. Plants grown indoors and under lights, the ideal environment, must be toughened up. Have a spot outside, like a porch or a deck, someplace a little protected, and move your flats outdoors. Rig a small piece of shade cloth over them. Then for about a week or so withdraw them off the shade cloth by giving them more and more time exposed to real sunlight and air. This is the one time you really must stay on top of the watering because your plants are big and healthy with lots of roots. They will need plenty of nutrient-rich water. This process can be started before your last frost date on warm, sunny days provided you bring them in at night. Once hardened and the all danger of freezing has passed bio-sponges can be planted directly into your containers or gardens. One note, I raise a lot of perennials so I often advance them into gallon containers before planting them into the ground. But I live in the desert and this may not be necessary for you to do if you have good soil and abundant spring rainfall.

 As a footnote, please sterilize your blocks, trays and perlite every now and then in a mild solution of bleach water. And if possible, allow them to dry in the sun.

 This is a great place in this blog to make a case for container growing. Perhaps you are not physically able to plant in the ground or you lack the space, energy or time to do so. I use big containers because I can sit on a bucket and work my container gardens sitting. You can prepare your containers while your seedlings are growing. When they are ready just plant the sponges directly into the containers and in minutes you are done. And best of all, in just a few weeks you will have a gorgeous, productive garden.

 I am deeply cognizant that some of you might not be able to afford a system like this, so let’s discuss some alternatives. Literally anything can be used as a container provided that it will drain excess water. Seeds will germinate in trays of damp sand, perlite, vermiculite or peat moss. All of these mediums can be problematic but well worth a try. Seeds need warmth and sunlight to germinate. Often windowsills do not provide adequate light. If your seedlings are long, leggy, and leaning toward the window you will know that the light isn’t adequate and they should be moved from window to window with the path of the sun everyday. In other words, chances are money will still have to be spent, it’s vastly more work, and usually the results aren’t satisfying. And I figure I still have to turn on lights in the winter and at night. Why not use efficient fluorescent lights rather than carbon-fat incandescent ones? By the time you buy something to germinate your seeds in, you might as well buy bio-sponges and get creative with what to put them in. Once the equipment is purchased it will last for years and years if it is given some care. Fluorescent light tubes last for about five years. They don’t burn out as much as they simply fade away. But chances are your light hoods will not need to be replaced for decades. So think about it and put that coupon money in a jar. A growing system is a superb and reasonable investment.

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Responses to “Starting Seeds Under Lights”

  1. Angela Cheetham Wilkinson March 1st, 2014 - 5:06 pm

    A great wealth of detailed information and useful tips, including bits for the cash-strapped and encouragement not to buy where recycling can do the job.
    With over 35 years of gardening under lights, including all the interesting growing happening in your kitchen, this proves a very worthwhile subject from a trusted, experienced writer.

    • Thank you, Angela. Years ago I was asked to do this blog for those who had never gardened but wanted to. Gardening is easy; it isn’t nearly as daunting as some of the newer techniques suggest.