Building a hot box or cold frame is a great way to extend the gardening season, especially in colder climates. Hot boxes, also called hot beds, are typically heated with electrical heating coils; cold frames are not heated, other than with a blanket. Cold frames are usually movable structures, while hot boxes are usually stationary. The best placement for a hot box is along the southern wall of the house, where the plants will get the most benefit from sunlight in early spring or late fall, and near electrical outlets and a water source. Construction is generally inexpensive, especially if salvaged material is used.
1 Lay the window sash or plexiglass sheet on the ground in a sunny location and near an electrical outlet and water source. Trace around the window with the powdered chalk to create an outline. Remove the window and set it out of the way.
2 Dig a hole within the chalk outline that is a little over 2 feet deep, making sure that the sides are straight. This will be the bed of the hot box.
3 Lay the bricks or wooden planks along the inside edges of the hot box, building them up at the back of the box to about 12 inches above the ground at the front to about 9 inches above the ground. This allows the window to rest at a downward angle. Make sure to leave a small gap near the electrical outlet for the plug. Only frame the sides; leave the bottom as soil.
4 Pile the excavated soil around the hot box to the top of the bricks or planks. This will help insulate the box.
5 Pour 3-4 inches of gravel into the bottom of the box for drainage, and level it out. Lay the burlap or weed block cloth over the top of the gravel, and add 2 inches of sand.
6 Loop the outdoor soil heating cable over the sand, staying 3 inches away from the sides of the box. Thread the plug through the hole near the outlet.
7 Spread the screen over the heating coils. Cover the top of the window screen with about 4 inches of soil so that the thermostat bulb on the heating cable is about 1 inch below the soil's surface.
8 Place the window or plexiglass sheet over the box. If you used bricks, you can let it sit unattached on top. If you used wood planks, you can, if you wish, attach the window or plexiglass sheet with wood screws, but if you do so, be sure to add hinges for easy access to the box.
Items you will need
- Window sash or sheet of plexiglass
- Measuring tape
- Powdered chalk
- Bricks or wood planking
- Burlap sack or weed block cloth
- Outdoor soil heating cable
- Metal window screen
- Wood screws (optional)
- You can use your hot box for starting warm-weather plants early, hardening off plants, forcing bulbs and any other way that you would use a greenhouse.
- If the ground doesn't have good drainage, add a thick layer of course gravel to the the bottom of the hole.
- Check with neighbors and construction or demolition sites for used building material.
- Wood should be treated with a preservative that is not toxic to plants, or painted with white latex-based paint for extra sun reflection.
- An energy-efficient alternative to heating coils is manure; however it makes it much more difficult to maintain desired temperatures. To heat the hot box with manure, dig the box an extra 2 feet deep and fill the extra 2 feet with manure where the coils would otherwise be.
- Make absolutely sure that all electrical equipment used is designed for outdoor use. This includes the heating coils and all extension cords. Check both at least once a year, and replace them if they show wires or fraying of any kind. Make sure everything is unplugged while checking them.
- If using wood, be sure that it hasn't been treated with creosote or pentachlorophenol. These are toxic to plants and in a closed area will kill them.
- Hot boxes are great for cold weather, but not for warm weather. Unplug and vent your hot box when the outdoor temperature rises above 45 degrees Fahrenheit, or it could fry the plants. Once unplugged or manure is removed, the hot box will act as a cold frame. If there is still frost at night, cover the box to protect the plants.
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