Photo A Photo B Photo C Photo D Photo E Photo F Photo G Photo H In building the mat, the rope light weaves around long thin strips of wood, and both are attached to a ‘plank’ of plywood. The gaps between the wood strips serve to help with airflow. The rope light will only emit a certain amount of heat – not enough to burn the wood. Since it’s thinner than the wood strips, the rope light doesn’t come in contact with the seed trays. Since the rope light is insulated for outdoor use, it’s protected from water.

What you’ll need: 1. Plywood ‘plank’ (I used a 1-inch thick x 12-inch wide x 4-foot long piece) 2. Two 1″ x 2″ x 8-foot wood furring strips 3. Wood screws – two packs each of #6 x 1-1/2″ and #8 x 3/4″ sizes 4. Plastic cable clamps – two packs of 1/2″ size 5. Hand saw or jigsaw 6. Tape measure and/or square 7. Pencil 8. Sandpaper 9. Rope light – incandescent type (not LED). I used an 18-foot length.

Skill Level: Easy to intermediate, depending on your wood-working/cutting skills. Construction: 1. To correctly wrap the 18-foot rope light as I wanted, I ended up having to cut the 4-foot plank of plywood to approximately 44 -1/2″ long, but you can certainly leave the excess on if you don’t want to cut it. 2. Cut the two 1″ x 2″ x 8′ furring strips to get four 40″ long pieces. Sand any rough edges or surfaces (Photo A). 3. Using the 1-1/2″ wood screws, attach the 4 furring strips to the plywood plank (Photo B). 4. Unwind and straighten the rope light (Photo C). 5. Place the closed end of the rope light at the bottom right corner of the plywood plank (if you have the long side facing you). Wrap the rope light around the 5 spaces between the furring strips (the two outside edges are two of them) (Photo D). This is only for rough placement – we’ll tighten it down in the next step. 6. Using the plastic cable clamps and the 3/4″ wood screws, attach the rope light to the plywood plank (Photo E). I ended up using five clamps along each run of the board, and put an extra one at the ends of the cable to secure it better (Photo F). 7. Light ’em up baby! I mean,… um… plug it in (Photo G). Your lighted heater is ready for seed-starting duty (Photo H). So, if you’re crafty and are out shopping, why not grab some lights to warm your seeds?

How well does it work? The heat mat takes approximately 30-45 minutes to fully heat up, and using my hand, felt as warm as the commercial heat mat. Upon taking measurements with both regular and soil thermometers, the numbers were impressive. Commercial heat mat

Surface temperature

(thermometer placed on top of soil): 73-75 degrees F

Soil temperature: 80-82

Bottom temperature (gap between heat mat and seed trays): 100-102 DIY light heat mat

Surface temperature: 72-74

Soil temperature: 78-80

Bottom temperature: 105-110 Note: To control heating times, use a thermostat or timer.