How to Build a Hot Box Planter

Seedlings get an early start on growth in a hot box planter.

Hot box planters are advantageous for germinating seeds and growing seedlings while it is still cold and frosty outside. The hot box protects the new seedlings by keeping the interior warm. It is called a hot box because of radiated heat coming from the bottom that is usually produced with the use of manure or peat moss. A hot box is essentially a cold frame with heating ability within and is similar to a mini greenhouse.

Things You'll Need

• Wood window frame
• Plywood
• Saw
• Wood screws
• Drill with screwdriver attachment
• Hinges
• Gravel
• Peat moss

Instructions

1. Locate an old, wooden window frame with the glass still intact. These can be found at construction salvage yards, stored in old barns or at yard sales. If you have trouble locating one, simply put the word out to friends and family and see what you can find.

2. Cut four pieces of plywood to create a box with the same dimensions as the window frame. Each piece should be cut 12 to 14 inches wide.

3. Screw the four pieces of plywood together, using three to four screws at each corner vertically over the 12 to 14 inches, to form a box that is the size of the window frame.

4. Join the window frame to one side of the plywood box with hinges. The number of hinges required will depend on the size of the window. Plan to space the hinges every 6 to 8 inches. Screw one side of each hinge onto the frame and then the other side of the hinge to one top-side of the box.

5. Place a 1-inch layer of gravel, to help with water drainage, in the location where you will place your hot box. The location should receive six to eight hours of sunlight each day. You can’t control the weather -- rain and clouds are bound to make appearances -- but you can ensure that plants in the hot box won't be shaded for the majority of the day, every day.

6. Place a layer of peat moss over the gravel, 1/2 to 1 inch thick. This will keep the inside of the hot box warm.

7. Place the hot box over the peat moss and gravel. Put your seedling pots inside the hotbox and close the hinged window frame. Water your seedlings daily. Lift the window for short periods of time when the weather is warmer, three or four times a week. Seedlings need air ventilation.

References

• Ed Hume Seeds: Growing Vegetables in a Cold Frame or Hot Bed
• The Old Farmer’s Almanac: Cold Frames Gardening
• Cornell Cooperative Extension; Cold Frames and Hot Beds; Eric de Long; October 2001