How to Build The Cheapest Heated Seed Bed

To get a huge jump on the spring planting for vegetables or flowers, it's a great idea to plant and germinate your seeds absolutely as early as possible. If you know in February that only 10% of your squash is going to pop up, you've still got time to start another crop if you want to.

In addition, you can have peppers hanging on the plant 6 weeks ahead of your neighbor, because your plants popped out of the dirt 6 weeks earlier then hers. All you have to do is warm the seeds in the soil, and keep them warm.

Commercial heat mats are insanely expense per square foot. They use a tremendous amount of electricity, and if it stops working for any reason at all, there is NOTHING you can do to repair it.

This home built germination bed will hold literally hundreds of seedlings for about the same cost as a single commercial heat mat, and uses a fraction of the electricity.

(2) 2x8x10 boards
(1) Sheet 5/8" plywood
(2) 16 foot rope lights any color
(1) Standard electrical timer
Mason or playground sand
Concrete blocks as needed
Cooking thermometer

Instructions

- 1
  Begin by cutting both 10 foot 2x8's down to 7 feet. This will give to 2 pieces 7' long, and 2 pieces 3' long. Any other dimensions can be used, but this seemed to be a great compromise between cost and size. I've built the seed bed from 2'x2' up to 4'x16'... It's all the same procedure.
- 2
  Stand one 7 foot 2x8 on edge and screw the 3 foot piece into the end of it, making sure that it's square. Screw the other boards all together, to make a rectangle 7'3" x 3'.
- 3
  
  Cut your piece of plywood to the footprint size you're building to. (ie: 7'3" x 3') and screw it to top of the frame you just built. In the photo, you'll see that I covered the box in plastic. I was worried that moisture would rot the wood away. Since then, I have stopped doing this on all of my seed beds, since they get hot enough to constantly evaporate any moisture in them.
- 4
  
  Flip it over and set the box in its final location on top of the concrete blocks or frame as appropriate. If your using concrete blocks, I recommend you put one at each corner and two in the middle to distribute the weight evenly. Remember, it's going to be very difficult to move once it's complete because of its weight. Make sure your base is strong enough, and make sure it's where you want it to be.
- 5
  
  Add and spread the masons sand until it covers the entire bottom of the box to a depth of about 4".
- 6
  
  Attach the 3 strands of rope lights so they make a single continuous strand. Lay out the rope lights back and forth in the sand. I highly recommend you leave a foot or so exposed at the beginning of the strand, and 4-6" exposed at the end. Just drape them over the back edge of the box for now. The rope lights are going to want to twist and shift around, since they have been rolled up in their packaging, but a couple well placed bricks will hold them in place until you pack the sand around them. By the way, the color of the ropes is absolutely irrelevant. They are ALL white lights, with a colored plastic wrap. Buy whatever is on sale. I have some that are white, some purple, some orange, and some that started life as Christmas Candy Cane lights. They all work well.
- 7
  
  Fill the remaining 2 inches of sand into the box covering the rope lights completely again, leaving the beginning and end of the rope light exposed. The reason for leaving the beginning of the rope exposed, is so that you'll know when you have power to them and they are turned on. With it covered in sand, you won't be able to tell otherwise. The reason for leaving the end of the rope exposed is so that you'll have an indication if the lights stop working somewhere in the middle.
- 8
  
  Plug the rope light into the timer, turn it on and be patient. The rope lights put off a very small amount of heat (They also consume a very small amount of electricity!!), but the large thermal mass of the sand stores that heat very, very effectively. All of the little tiny air gaps between grains of sand insulate the lights and the more sand, the higher the insulation. It will take a number of days for the mass of the sand to get up to maximum temperature, but the positive side is that it also takes a couple days to lose that heat if the environment cools. The end result is a very stable temp for the seeds to draw warmth from as they germinate and set roots.
- 9
  
  Place your seed trays on the surface of the sand, but I really discourage you from embedding them in the sand. They trays may absorb too much heat and all your seeds will cook in the soil.
  
  Monitor the temp with the cooking thermometer embedded an inch or so into the surface of the sand. If it starts running too high, cut the time on the timer by a few hours a day and continue to monitor.
  
  32 feet of rope lights will heat a 7x3 foot box with 6" of sand an amazing amount above ambient temp. The area just below the surface will be in the neighborhood of 40 degrees higher, if the surrounding space is calm and no breezes are trying to cool the sand.
  
  With that kind of temp change, a seed box in a shed will easily germinate tomato, pepper or eggplant seeds even if it's still 40 degrees outside!
  
  As you can just make out in the photo, these pepper seeds are sprouting in the middle of December!
  
  Good Luck!!

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