Home Made Bottom Heat for Seed Starting (or pet bed)

February 17th, 2009 by david laferney Leave a reply »
These tomatoe plants were grown under shop lights in only 4 weeks from planting the seeds!

I always had problems starting seeds in our plant room, but these tomato plants were grown under shop lights in only 4 weeks from planting the seeds!

These seedlings were planted only one week ago.  I used to wait weeks for germination that was spotty at best in my cool plant grow room.

These seedlings were planted only one week ago. I used to wait weeks for germination that was spotty at best in my cool plant grow room.

The answer was simple…
Recycling rope lights turn out to be a great way to make bottom heat for seed starting under lights.

Recycled rope lights turn out to be a great way to make bottom heat for seed starting under lights.

I built our “plant room” about 2 years ago – just a small well insulated room with a lot of windows and shop lights – and since then I’ve tried starting my own seeds with varying degrees of success.  The problem that I’ve had is that over night temperatures in the room routinely fall into the 50s which is fine for maintaining tender plants over the Winter, but makes seed germination spotty at best.  I knew that what I needed was bottom heat.

The thing is that retail bottom heat is expensive – I saw one “kit” at a local garden center that was big enough for 2 flats and was $79 – wow!  You can buy a lot of tomato plants for eighty bucks!  A low cost alternative had to be possible for a dedicated scrounger like myself.

Whatever I decided upon had to be:

  1. Safe – neither an electrical shock nor a fire hazard!
  2. Cheap
  3. Simple
  4. Big  enough to start all of our early Spring seeds.

Before proceeding – You the reader must agree that you will not hold the author or anyone associated with doorgarden.com responsible for your use of this information.  What you see being done in this article may not be safe (and probably isn’t), and could cause injury, death, destruction, mayhem, fire, dammage to your home, and prolong the economic downturn by preventing you from spending money and thereby stimulating the economy.  It might not even work.  In any event thou shalt not hold me responsible.  If you don’t agree with any of that then turn back now – don’t even look at the pictures.

After quite alot of thought I decided to use rope lights for the heat source – here’s what I did…

Rope lights going in over insulation board

Rope lights going in over insulation board

I already had a section of shelf in the plant room that was about 2 feet wide and 8 feet long that I wanted to use.  I put a 1/2 inch layer of styrofoam insulation board over the 1/2 inch OSB (chipboard) shelf and then loosely fastened the rope lights over that. This took 3 20′ lengths of rope light.  Note – after using this for a few days I would say that 2 strings of lights would be plenty for this size bench – I might rework mine to make that adjustment.

These rope lights are rated for exterior use and are double insulated.  They are safe for wet locations and direct contact with flammable materials like wood because they only get slightly warm.  Neither a shock nor fire hazard.  They were used at one time for outside lighting but hadn’t been used in a couple of years.  However it is probably always a good idea to use GFI protected outlets or plug bars anywhere that electricity and water come in close proximity.

The rope lights are fastened down using screws and wire ties that have holes for this purpose.

The rope lights are fastened down using screws and wire ties that have holes for this purpose.

zip ties with the holes for screws like this are extremely handy - look for them in the electrical wiring section of the home improvement store.

zip ties with the holes for screws like this are extremely handy - look for them in the electrical wiring section of the home improvement store.

Then I put in strips of 1 inch insulation board to act as spacers.  Looks kind of festive doesn’t it?

Then I topped that all off with a layer of 1/2 inch drywall…

Drywall going on top (I didnt fasten it in place at all)

Drywall going on top (I didn't fasten it in place at all)

Drywall is relatively fire resistant, and has good thermal properties so that the heat will be nice and even without hot and cold spots, but this kind isn’t at all water proof so I topped it all off with a left over sheet of vinyl flooring…

I already had this nice big scrap of vinyl floor, otherwise I could have used a scrap of vinyl or rubber roofing, pool liner, shower curtain, or plain old plastic sheet.

I already had this nice big scrap of vinyl floor, otherwise I could have used a scrap of vinyl or rubber roofing, pool liner, shower curtain, or plain old plastic sheet.

Here’s an end view showing the layers (except the vinyl floor)…

Most of the heat from the rope lights is directed by the insulation to go up through the seed starter.

Most of the heat from the rope lights is directed by the insulation to go up through the seed starter.

From the top you have – 1/2″  Drywall, rope lights and spacers made of 1″ EPS board, 1/2″ styrofoam, and 1/2″ wooden osb (chip board) on the bottom.  It’s all sitting on top of recycled 55 gallon drums full of water which act as thermal mass to moderated the temperature in the room.  You could probably use wood for all of the parts and it would be safe and work just fine, but I didn’t feel 100% comfortable doing that since I’m using the rope lights for something that they weren’t really designed for.  I did not close up the holes in the end.

Heres the system with fresh planted flats.

Here's the system with fresh planted flats.

The surface of the finished bench is just slightly warm to the touch, and the soil in the flats feels just like it would on a sunny day in May – should be just about right.  When I have actual results I’ll post some pictures.

In the planning stages I thought that an 8′ by 2′ starting area would be really roomy, but after my lovely wife commandeered 5 of the 8 slots for her flower starts I realized that it isn’t too big at all.  Barely adequate in fact.

  • The bottom heat stays on 24 hours a day.
  • The lights are on a timer which turns them on for about 17 hours a day.
  • When using florescent lights for plant starting the lights need to be very close to the soil / tops of the plants to keep them from getting leggy – even then they are not nearly as bright as the sun.
  • The air temperature in the plant room stays above 55° F.
  • Once the plants are a couple inches tall (probably about a month) I intend to move them out to the garden, greenhouse, or cold frame depending on the plant.

Note that I planned and built this project with an eye toward utilizing materials that I already had on hand – didn’t buy a single thing.  You should taylor your own projects similarly to use the materials, tools, skills, and spaces that you have available to you.  Some alternative ideas that I considered:

  • Light bulbs mounted under a shelf that would be like a shallow open bottomed box. – This could probably work all right but would be more likely to be a shock or fire hazard.
  • Recycling a water bed heater. – This would probably work, but I didn’t have a water bed heater.
  • Cement board tile underlayment instead of drywall.  – Good idea, but would have required the outlay of cash. I already had drywall scraps.
  • Bedding the rope lights in sand so that it would be completely surrounded by fire proof thermal mass. – This would probably work great, and I almost did it like that.
  • Bedding the rope lights in drywall joint compound or mortar for the same reasons of fire safety and thermal mass. – This would also probably work great, but if the rope lights died it would make it a huge job to replace them.  Also it would be a big messy job to put it together.
  • Using a small space heater under an open bottomed shelf. – Everything would have to be made out of fire proof material for this to be safe.
  • A lower level of shop lights to bottom heat an upper level of seedlings. – This would be a likely shock hazzard because of excess water running down onto the lights, and you wouldn’t be able to use the bottom heat 24 hours a day.
  • If / when electric heat was not an option heat could be supplied by a layer of hot compost under a cold frame – This is called a hot bed.

After running this for a few days it has occured to me that something like this would make a really nice heated dog or cat bed.  Just a thought.

Update: I planted a variety of seeds one week ago tomorrow, and almost everything has already come up including some stevia seeds that are supposed to be hard / slow to germinate.  This works.

Update 2: A couple of things that I would do different and will probably change – My bottom heat covers an area about 8′ long by 2 feet wide which is great, but it would be getter if I could independantly turn it on and off in 4 foot sections, for when I’m not using all of the area. By the way, after about a month of use I’m astounded by how well this works!


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