Making a Heat Box

By Idris Anderson

A heat box is a very useful addition to your bonsai garden. Among its uses is keeping tender trees warm in the winter, as can be seen in this photo of Kingsville Boxwoods. When left in the cold, their leaves turn orange. The heat table enables them to retain their green color year around.

Heat Box in Use

A heat box is also used to keep roots warm after repotting. A temperature between 60-70 degrees helps roots recover rapidly after they’ve been pruned for repotting. This is especially true when there is a big differential between the soil temperature and the air temperature.

Here is how the heat box looks on the inside with the cables laid in.

Construction Overview

Your trees will thank you for providing them a warm bed.

Construction Details

These directions assume your box will be 4-ft x 4-ft square, but you can build a smaller one by using a single heating cable and smaller pieces of lumber, etc.

Before beginning consider whee you will install your box. I recommend raising it off the ground using concrete blocks or a wood frame so that it is easy to work on plants being kept in the box. If you can arrange a slight tilt water will drain out of the box better. If you want the box at ground level at least raise it a bit using bricks or blocks so that water don't collect around the box.

Materials:

Heating Cables - I used two Hydrofarm Jump Start Soil 12 Foot Heating Cables w/Thermostat(available on Amazon). You may be able to find 24 Foot Heating Cable.
Plywood - use a half-sheet, 4 x 4. 1/2-inch to 3/4-inch in thickness.
Boards for side - two 1x6 boards 8-ft long. Alternatively you can use 1x4 or 2x4. Redwood recommended.
Corner Posts: four 2x2 or 2x4, 6-inch or 4-inch long, depending on height of box frame boards. Will help secure boards in the corners.
1/4 inch wire mesh, aka hardware cloth - 4-ft square
Rigid Foam Board Insulation, aka Styrofoam - 4x8 sheet, 1-inch thick. Foil lined
Heavy plastic sheet - two sheets 5-ft square. Recommend using 6mil weight plastic.
Zip ties. 6-inch ones are fine
1 1/2 to 2- inch galvanized or stainless steel screws. 12 needed
1-inch galvanized or stainless steel screws. 32 needed

Tools: saw, drill, utility knife, wood glue and wire cutters. One or two wood clamps are handy, but optional.

Construction:

Cut the 1x6 sides 4-ft long.
Cut drain holes about every six inches along one side of the 1x6 boards. Triangles are easy to cut. Make sure the drain holes are large enough for electric plugs of the heating cable to pass through.
Lightly sand so the surface is smooth.
Arrange the 1x6 sides on a flat surface such as a patio in a box shape. It's important that the box is flat so that when the plywood bottom is added later it will attach snugly to the box.
Place corner posts in the corners. Drill two holes thru each 1x6 into the corner posts. Stagger the holes so that the screws don't collide inside the post.
Apply glue to the post and screw 1x6 board to the post. A wood clamp to hold them together will make this process easier.
Allow glue to dry for a couple of hours before brushing on several coats of polyurethane. Let dry overnight.
Place the frame on a flat surface again. Make sure the drain cutouts are facing up.
Cut a piece of plastic sheeting 4 to 6-inches wider in both directions than the box. Place the sheet over the box (sides) with about 2 inches of plastic sheet extending past the box. This will help keep your plywood bottom dry and minimizes moisture and rot.
Lay the sheet of plywood on top of the box and plastic. Pre-drill holes into the box below. Then insert screws to hold the plywood to the box
Turn the box over and move it to in its permanent location.
Cut your insulation board to fit loosely inside the box allowing at least 1/4 to 1/2 inches on all sides, so water can drain off. A utility knife cuts through the foil easily; just score it and the insulation board will crack where it’s scored
Cut wire mesh to fit tightly inside the box. Place in box and use something heavy, like rocks, to hold it flat for the next steps.
Feed the electric plug and cable through the holes you cut earlier.
Lay out your heating cables on top of the wire mesh according to directions on the package. Make sure wires don't cross.
Once you are satisfied with the arrangement, use zip ties to connect the heating cables to the wire mesh, at corners and about every six inches of long runs. This will keep the cables from shifting later.
Cover wires with another layer of plastic that just fits the frame, so that water can go over edge of mesh and insulation board and drain through drain holes.
Cover with 2-3 inches of pumice, lava, sand or gravel. Remove heavy rocks as you replace with the weight of the soil.

Installation and Use

You will be making shallow wells in the filler material to place your pots. Some people recommend pumice or lava because you already have a bunch for making soil - and any roots that escape from the pots will be able to grow. Sand will also work, but is heavier and doesn't drain quite as well.
If your location is near enough to a weatherproof outlet use that to plug your heater cables into. Make sure the box closes.
If not close enough, you'll need an outdoors extension cord - and preferably one with a 3-way head. To connect the heater cable to the extension cord, use a plastic box such as Tupperware. Cut holes in the side of the box that are just large enough for the cables to enter the box. Connect the cables inside the box and then put the cover back on the box. Place this somewhere where it can stay dry - like underneath the plywood bottom.