Seed starting: Make a functional, affordable heat box

Special guest post by Matthew

Functional, Affordable Seed starting heat source . . . I’ve used this one with great success and reliability for four sets of plants now.  We set our thermostat pretty low in the winter, so the heat box helps keep the seeds cozy (i.e., at an ideal temperature for sprouting).

The basics:

  1. Build a 5 sided wooden box to hold the heat, sized to fit your light source (or your tray size)
    1. Four 1”x4″ s (or whatever wood you want) for the sides
    2. Very thin plywood (or planks) for the top
    3. I left the bottom open, just set it on #3
    4. Nails or screws to join
    5. I made two boxes, each 10.5″ x 32″ x 3.5″
  2. A string of nightlight (4 watt) bulbs (or other incandescent lights) as a variable heat source (screw them in or unscrew some of them to get the soil temperature you want)
    1. Optional: Tack the wire of the nightlight string to the sides of the box to keep the bulbs out of direct contact with the box or radiant barrier –be careful to avoid damaging the wire’s insulation
    2. You may want to add a timer; I only use heat during the 16 hours of light I give my plants
    3. We have a string of lights that have a little metal cage around each bulb, so they sit directly on the reflective material
  3. A sheet of reflective radiant barrier insulation (see photo above) for below the box
    1. I used a scrap left over at the hardware store from someone’s home insulation project

Set your seed starting trays directly on top of the heat box.  When starting with this, put a thermometer into the soil of your seed starting tray and adjust how many bulbs are on in your box until you get the right temperature for your seeds.


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