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Are your peppers more poky than perky? Is your basil more balky than bouncy? Do you wish your seeds would speedily sprout into sturdy seedlings? Bottom heat could be the answer. The seed catalogs piling up by your easy chair tout the wonders of seedling heat mats – and they are right! Heat mats can produce remarkable results. But are they the only answer?
For some types of seed, warmer temperatures are critical to germination. I successfully used the warmth from my oven light one winter to germinate two precious seeds of Vigna caracalla (Snail or Corkscrew Vine). While carefully monitoring and adjusting the temperature by propping open the oven door just so with a folded towel, I made up my mind: I wanted a seedling heat mat!
I ordered two heat mats and a temperature controller a couple of years ago. I love them! Seeds all but leap out of the seed starting trays in half the time it previously took. My seedlings look so much stockier and healthier when it's time to plant them out. Although there are DIY alternatives to "real" seedling heat mats, I fell for the appeal of something I could unpack, plug in, and put to work right away.
Seedling heat mats are usually sized to hold one, two, or four of the 1020 size standard nursery flats. You can plug multiple mats into a typical thermostat controller (check specifications for max wattage). [1]
Why get a controller? Seedling heat mats typically raise the temperature of your flats or pots 10 to 20°F above ambient (room) temperature. My basement occasionally warms to over 70°F on a sunny day, and a 20°F rise in temperature on top of that could mean the death of my seeds. A controller to maintain the temperature at an even 78°F seemed like a good investment.
If the prices on seedling heat mats and controllers make you wince, there are some DIY alternatives. You can supply some bottom heat simply by putting your seed flat on top of the refrigerator, television, or other warm spot. A 40 watt incandescent bulb positioned just under a metal shelf makes a good heat source for a seed pots placed on the shelf.
You can also construct a light box, an enclosed space warmed by one or more incandescent bulbs. If you don't want to wire your own sockets, you can use a pot light, desk lamp, or clamp-mounted fixture. Make sure to keep wiring away from possible wet areas, and leave plenty of space around the bulb. The enclosed space can be as simple as a shelf of your existing light stand, wrapped in plastic sheeting with front flaps to allow easy access.
Monitor the temperature inside your light box to figure out what wattage bulb(s) will give you even heat of 75 to 80°F. Putting a thermometer into a pot of moist potting mix will let you monitor actual soil temperature. If you are comfortable with wiring projects, you can install a thermostatic switch to turn the lights on and off as needed.
Heat cables are often much less expensive than seedling heat mats, especially for larger numbers of seedling flats. They are designed to be buried in flats of sand or gravel, with the flats of seedlings placed on top of the material for even heat distribution. A similar DIY bottom heater option uses lengths of Christmas rope light (not LEDs), buried in clean kitty litter. The amount of heat provided is regulated by the length of rope light. [2]
What about that old electric blanket? No! Electric blankets and household heating pads are not designed for use 24/7 and are not for use in potentially wet environments. All those safety warnings are on them for good reason. Even if you have been using one under your seedlings for years without burning down the house, please consider a safer alternative.
Whether you choose to use an "official" seedling heat mat or a DIY alternative, please be safe. This isn't the time to just throw something together and see if it works. Use heavy duty grounded extension cords and GFI outlets. Get expert advice if you have any questions
How long should seedlings stay on the heat mat? In my experience, pepper and basil seedlings benefit from a couple of weeks on a seedling heat mat, but after up-potting they do just as well on an unheated shelf in my 60-70°F basement. Tomato seedlings sprout almost overnight on a heat mat but must be removed from the heat after the first sign of germination, or they will become very leggy.
Germination instructions for many seeds recommend growing on the seedlings at a cooler temperature once they have sprouted. Since most seedlings won't need to stay on the heat mat long, a single heat mat can help start enough seeds to fill dozens of flats. See Tom Clothier's germination database for temperature requirements of many types of seeds. Seeds that need a warm start (70°F more) all seem to do well for me when I set my heat mat controller to 78°F.
Start with plants that appreciate a longer head start. When they're ready to transplant into larger pots or cell packs, it'll be time to put the next round of seed starting trays on your heat mat. Flowers like Torenia (Wishbone Flower), Geraniums, and Wave Petunias are among the first on my sowing schedule, followed by peppers, then by basil and tomatoes.
Whether you buy a seedling heat mat or put together a DIY alternative, I hope you'll consider adding extra heat to your seed starting shelf this winter. The results will amaze you!
For additional information on seed starting, see my previous article, Seed Starting 101: Setting up Light Shelves. Growing your own plants from seed can be both cost effective and fun. Nothing beats the satisfaction of cutting flowers or picking heirloom tomatoes from plants you started yourself from tiny seeds. Go, sow, grow!
[1] Thanks to Park Seeds for additional information about the seedling heat mat and thermostatic controller that they offer. They gave me good advice and assistance when I purchased mine. You can check them out in The Garden Watchdog.
[2] We've had some great discussions about heat mats and DIY alternatives on the DG Propagation Forum (subscribers only). I'd especially like to thank Heathrjoy for her contributions, including posting the link for the rope light soil heater.
About Jill M. Nicolaus
Better known as "Critter" on DG, Jill gardens in Frederick, MD. This week, I've been cutting lavender, putting up pluots (plum-apricot fruits), and pulling weeds (always, LOL). I love being outside in the cooler morning -- coffee cup in one hand, watering hose in the other -- watching the birds and butterflies among the flowers.(Images in my articles are from my photos, unless otherwise credited.)
Posted by Tink2U (from Rockford, IL) on April 21, 2008 at 6:27 PM:
Jill:
Great article! I appreciate when alternatives are given to get to a goal...healthy seedlings in this case. But what I really wanted to comment on is education. Everyone should keep learning....boards like this, garden shows, new books on newer methods of gardening, what ever peaks your interest!
In my case, I have been gardening since I could walk (let's just say a very long time.) I have tried lots of things, some sucessful, some not so much. But the secret is to keep trying! Case in point. I have a bottom heat unit (your DIY version---rope lights between 2 plastic tubs...top tub filled with cell packs or with seed starter) and it makes a a great place to practice your clump transplanting, or to start individual plants. I transplanted tomatoes this year and placed them within 2" of my lights, adjusting them as they grew. But for some reason this year they got leggy. Now I know why. I had tomato and pepper seeds both and left the tomato seedlings waiting on the pepper seeds to sprout to transplant. Since they had overhead light as well, I didn't think it would harm anything, but thanks to your article on heating mats I think I have the answer!! Bottom heat after they sprouted! So you see, you should always keep reading, you never know if someone else has the answer you are searching for, and where you might find it!
Thanks for your articles, I really enjoy your style!
Bonnie aka:seeks roots
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Posted by critterologist (from Frederick, MD) on April 21, 2008 at 6:40 PM:
I'm glad you found a useful tip! I don't think any of us every stop learning from each other here at DG... :-)
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Subject: Cycad Palm
Posted by Franki_2008 (from Drayton Valley
(Canada)) on January 28, 2008 at 12:10 PM:
I was blessed with one of these wonderful plants (Cycad Palm) at Christmas, but unfortunately, I've run into a major problem. It was bright green but now it is a sandy brown. I was told to water it once a week through the core of the bulb but it has just up an "croaked on me". Is there any hope to revive this wonderful plant?
Any info would be greatly appreciated.
Thanks
This message was edited Jan 28, 2008 9:11 AM
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Posted by critterologist (from Frederick, MD) on January 28, 2008 at 2:34 PM:
Franki, I'm sorry, but I don't know anything at all about Cycads. "Sandy brown" doesn't sound good, and I'd check to make sure the soil and roots don't seem waterlogged -- repot immediately if so, or just take it out of the pot and put it on a paper plate while figuring out the next step.
Although the "Palms and Cycads" discussion group is for subscibers only, you might try posting a "Cycad needs emergency help" thread in the Beginners Houseplants forum, [HYPERLINK@davesgarden.com]
Also, Plambob wrote a detailed article about cycads last year. See [HYPERLINK@davesgarden.com] You might be able to find answers in his article.
...
Posted by Franki_2008 (from Drayton Valley
(Canada)) on February 2, 2008 at 5:08 PM:
Hi Critterologist,
Thank you so much for taking the time to respond to my query on Cycad Palms. I found the link you sent to me very informative and very helpful on a few of my other questions. Thanks Again!
Franki
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Posted by critterologist (from Frederick, MD) on February 3, 2008 at 2:25 AM:
You're welcome!
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Subject: Multiple heat mats
Posted by woofie (from Chewelah, WA) on January 24, 2008 at 11:10 AM:
Have you had any luck connecting different size heat mats to the same controller? I have several small mats and one large mat and even tho the combined wattage is well within the spec, it just doesn't work right. Fortunately the small ones seem to provide just the right amount of heat without the controller, but it's one of those little things that just bug me. I love 'em anyway! Great article and timely. (Now where did I put those torenia seeds?)
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Posted by critterologist (from Frederick, MD) on January 24, 2008 at 11:34 AM:
Hmm, that's strange. Mine are the same size. I guess that wouldn't have occurred to me. I wonder if you could use it to your advantage, though, if you know which one runs warmer (as long as it's consistent).
If, for example, the little one runs at 80 degrees when the controller is monitoring set to 72 degrees with its sensor stuck into a pot on the big mat (I'm making up these numbers), you could put seeds that need an extra-warm start on the little mat and put all the seeds marked "germinate at 70-75 degrees F" on the big mat.
But I guess we've learned something... thanks!
Tip for anybody considering purchasing multiple heat mats and a controller: Get heat mats that are the same size!
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Posted by pixie62560 (from South China, ME) on January 24, 2008 at 12:03 PM:
Great article, and yup Parks was where I bought both of mine!
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Posted by woofie (from Chewelah, WA) on January 24, 2008 at 12:47 PM:
One other thing I'd like to mention. The manufacturer of these heat mats, HydroFarm, stands behind their product 100%. The first mats I bought (the small ones) died after just one season. I contacted the mfr and they replaced all five of them with no question. Didn't even ask for a receipt. Apparently there had been a problem with a particular run of that size mat. They were very nice and I wouldn't hesitate to buy more.
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Posted by Terry (from Murfreesboro, TN) on January 25, 2008 at 9:31 AM:
Thanks Jill - this article spurred me to take the leap and purchase a 2nd mat and a thermostat yesterday!
The first mat is the ubiquitous HydroFarm 10x20 mat; the second one is coming from HarborFreight, so I don't know what brand it will be yet. I'm anxious to get them both set up and the thermostat in place - I don't know if I really need the thermostat since this room (my office) stays a fairly constant 65-70 degrees, but I suspect it'll come in handy as the days start to warm up and stronger afternoon sun will bring more warmth in here.
I've used heating cables in the past (mine are probably close to 20 years old, and I chose them over heat mats because of the cost.) A word to the wise for anyone who chooses cables: make sure the cable doesn't touch or cross itself anywhere because the heat can cause the cables to melt at the touchpoint. I recommend using regular masking or electrical tape to hold them down after you lay them out in the pattern you want.
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Subject: Very interesting
Posted by doccat5 (from Fredericksburg, VA) on January 24, 2008 at 8:47 AM:
Really enjoyed your article. Very interesting and informative, thanks for sharing.
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Posted by cathy4 (from St. Louis County, MO) on January 24, 2008 at 10:49 AM:
Great article, never thought to cover my indoor plant shelf before, and I'll be buying a heat mat.
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Posted by randbponder (from Hornick, IA) on January 24, 2008 at 3:19 PM:
Thank you for the article
Would like to use the mats. but I can't just go out and buy them right now. I have to get my fuel bill under control first.
I used rope lights last year. They seemed to work very well. as everything sprouted just fine. Some of the tomatoes were a little leggy, but I just planted them a little deeper. I thought that may have been cause I just had the regular floresenct lights. I think I should have gone with the higher priced floresent tubes, daylite or the grow lites. I will plan on changing them out later. For me as long as the rope lights work, I will probably just use them. I will try to make a tray of sorts though to hold some kitty litter to cover the ropes. Thanks again.
Russ
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Posted by critterologist (from Frederick, MD) on January 24, 2008 at 3:55 PM:
Regular florescent lights work just fine for me with tomato seedlings. Be sure the lights are just barely above the tops of the tomatoes, and also take the tomatoes *off* the heat at the very first sign of germination -- both those things will help prevent leggy seedlings. I bury my tomato seedlings pretty deeply when I plant them out, anyway (if they're especially tall, I plant them at an angle in a trench so the stem is covered without having to dig a 12 inch deep hole).
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Posted by Indy (from Alexandria, IN) on January 24, 2008 at 3:55 PM:
Very nice article. I use another option for seed starting. I have a small bathroom with its own thermostat. So for the short time it takes to pop the seeds up at 88°F, I have them in there with the door shut until they start coming up.
Oh, and perhaps most important, I can start a LARGE number of flats at one time for the same outlay.
This message was edited Jan 24, 2008 3:00 PM
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Posted by McGlory (from Southeast, NE) on January 24, 2008 at 4:35 PM:
Thank you for taking some of the mystery out of seed starting. I've always avoided it because of lack of knowledge about such things as lights and heat mats. You provided good information clearly and consisely. Some of us need that, so thank you again!
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Posted by Dea (from Frederick, MD) on January 24, 2008 at 5:22 PM:
Great info and thanks :)
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Posted by rvnsbrk (from Leesburg, VA) on January 24, 2008 at 6:29 PM:
Thanks a bunch. I have lots of seed to try again this year and am hoping for results like yours. I have 3 large seedling carts with lights that I use mostly for cuttings. This year I am going to try using them for their intended purpose!
Juanita
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Posted by tucsonjill (from Tucson, AZ) on January 24, 2008 at 7:10 PM:
Thanks for the info, critter! I had been thinking of a heat mat, and wondering about DIY alternatives, so the timing on this was perfect for me. Also, I really liked the germination link--that one's getting bookmarked for future reference!
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Posted by Tokoro (from Sacramento, CA) on January 26, 2008 at 2:36 PM:
Re the rope lights: intriguing idea, since I already own some -- but I'm not clear on the mechanics of how you handle them.
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Posted by randbponder (from Hornick, IA) on January 26, 2008 at 3:07 PM:
Tokoro;
I have a chrome plant stand each shelf is made of a heavy gauge chromed wire like material Last year I used nylon wire ties to hold the rope lights in place at each end of each shelf. I laid them back and forth from one end of the shelf to the other.. Then set the flats on top of the rope lights. This year I think I will make a long box to fit the shelves and use the wiring staples to hold the ropes to the inside of the box. Having the rope run back and forth, as I did without the boxes. Then do as suggested by filling the boxes with either sand or kitty litter.
If I put aluminum foil down with shiny side up, before I secure the rope to the bottom, it should reflect more heat upward through the kitty litter to each of the flats.
I have 2 power strips attached to one end of the plant rack which all the lights and rope lights plug into of course the power strips each have a switch that I can turn all the lights off with one switch and the same would be true with the rope lights. I was going to send a picture but there don't seem to be a way under the article section. I will try find a pic and send a Dmail. Russ
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Posted by critterologist (from Frederick, MD) on January 26, 2008 at 3:16 PM:
Russ, thanks for the additional information! It sounds like the kitty litter helps to even out the heat, just like when people install heating cables in trays of sand.
Also, the information at that link seemed to suggest that 3 ft. of rope light would provide enough heat for a 1020 flat of plants... so 12 ft. of rope light ought to be enough for a 2x4' shelf. It sounds like some rope lights can be shortened safely, but not all -- be sure to read the instructions and precautions that come with them before cutting any of them apart. :-)
This message was edited Jan 26, 2008 2:16 PM
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Posted by randbponder (from Hornick, IA) on January 26, 2008 at 3:34 PM:
Critter; They are the 12' ropes. and my shelves are 2' x 4'. So hopefully by making a 2' x4' box for each shelf and using the kitty litter over the ropes it should work out. Crossing my fingers LOL
I know a lot of the heat went elsewhere. So I am hoping that this don't direct too much to the flats. But I would think I might be able to control that, by the amount of kitty litter in each box. ( ????) Trial and error.
The temp run around 65 with the ropes exposed. I will experiment some and get back to you on that.
Russ
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Posted by critterologist (from Frederick, MD) on January 26, 2008 at 5:00 PM:
I think using the kitty litter will help keep the heat where it belongs, under the flats. I'd stick a little thermometer into one of the pots to keep track of soil temp (you should be able to pick up a cheap one in an aquarium store, or by the guppies at WalMart). Keep me posted -- I'm curious!
For some types of seed, warmer temperatures are critical to germination. I successfully used the warmth from my oven light one winter to germinate two precious seeds of 
seedling heat mat!
Seedling heat mats are usually sized to hold one, two, or four of the 1020 size standard nursery flats. You can plug multiple mats into a typical 
(room) temperature. My basement occasionally warms to over 70°F on a sunny day, and a 20°F rise in temperature on top of that could mean the death of my seeds. A controller to maintain the temperature at an even 78°F seemed like a good investment.
from possible wet areas, and leave plenty of space around the bulb. The enclosed space can be as simple as a shelf of your existing light stand, wrapped in plastic sheeting with front flaps to allow easy access.
provided is regulated by the length of rope light. [2]
ve any questions
Seeds that need a warm start (70°F more) all seem to do well for me when I set my heat mat controller to 78°F.
basil and tomatoes. 
