How to build a hot box for seed germination

Updated February 21, 2017

You can get a good jump on the summer growing season when you start seeds in a hot box, sometime called a "hot bed." These devices are small growing areas that typically have a heat source and a glass or plastic cover. This protects tender seedlings from late winter and early spring frosts that would kill them if they were growing in an open, unprotected area. Hot boxes needn't be elaborate or expensive if you create one yourself with used or found materials.

Search for an old window sash containing unbroken glass. A typical size is 3-by-6-feet, which will serve well as the lid for your hot box. Also search for used boards if you want to save money.

Cut the boards of your hot box to have the following dimensions if you use a 3-by-6-feet window: rear wall, 18-inch-by-6-feet; front wall, 12-inch-by 6-feet; side walls 3-feet-by-12-inch, with the front edge measuring 12 inches and tapering to 18 inches tall at the rear edge.

Assemble your hot box by placing cut 2-by-2-inch boards at each inside corner and then screwing the front, rear and side walls to them.

Attach hinges to the rear edge of your window and the back wall of the hotbox to form a lid that you can easily open and close.

Locate your hot box in an area that receives full sun for as many hours each day as possible. You can also shield it from chilling winds by setting bales of straw around it as a windbreak but do not allow them to shade your hot box.

Lay an electric heating cable with a thermostat on the soil surface where you will situate your hot box. Then cover it with sand. Add a sheet of ½-inch mesh hardware cloth on top of the sand. Then add soil to fill the box about half full: this is where you will start your seeds.

Maintain a steady temperature of around 23.3 degrees Celsius. Ventilate your hot box by opening the lid all the way or part way during the daytime when temperatures are above freezing. You can use a thermometer inside your hot box to tell you if you have opened the lid far enough to maintain a temperature in the 70s. Monitor the temperature several times during the day and then close the lid when the outside temperature begins to drop.


Hot boxes help to germinate the seeds of tomato, pepper, melon and other types of plants that need warmth. Woody plants also benefit from the heat, which assists them in developing roots from cuttings. Instead of an old window, you can use a double layer of clear polythene plastic. Redwood and cedar are good types of wood for a hot bed. If you purchase a heating cable with 10 to 12 watts per square foot, it should be adequate for most climate zones. Water your seedlings inside the hot box early in the day, but after the outside temperature has risen to above freezing.


You must replace clear polythene plastic every year. Do not use wood that has been treated with creosote.

Things You'll Need

  • Old window frame with glass
  • Bales of straw (optional)
  • Boards (2-by-2- inch or 2-by-4-inch)
  • Screws
  • Screwdriver
  • Hinges
  • Electric heating cable
  • Thermostat
  • Sand
  • Hardware cloth
  • Thermometer
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About the Author

Barbara Fahs lives on Hawaii island, where she has created Hi'iaka's Healing Herb Garden. Fahs wrote "Super Simple Guide to Creating Hawaiian Gardens" and has been a professional writer since 1984. She contributes to "Big Island Weekly," "Ke Ola" magazine and various websites. She earned her Bachelor of Arts at University of California, Santa Barbara and her Master of Arts from San Jose State University.