How to Build a Warming Box for Vegetables

Locate a level area that has a clear southern horizon on your property to construct a vegetable warming box. It is important to have a unobstructed southern exposure to maximize the amount of sunlight that enters the warming box.

Determine the size of the warming box that you want to build. Most warming boxes are at least 3 feet deep and 6 feet long to generate enough heat for vegetables to grow in cold conditions.

Excavate the soil in the area where you want to build the warming box. Dig a clean, level hole 18 inches deep.

Cover the base of the hole with a 4-inch layer of gravel to improve soil drainage and prevent the saturation of the growing medium.

Cut four pieces of treated timber to fit the dimensions of the hole. Use timber that is at least 1 inch wide and 12 inches tall for the base of the growing box.

Construct the walls of the warming box by attaching the timber together with galvanized screws on each corner.

Create a lid for the cold box by constructing a frame out of 1- by 4-inch treated wood that has the same dimensions as the base. Use L-brackets and galvanized wood screws to attach each corner of the frame.

Stretch a sheet of clear polyethylene material over the wooden frame and attach to the underside using a staple gun. Attach staples every 2 inches to create a draft-free seal on the lid.

Attach two small door hinges on the back of the wooden frame and secure the lid to base so that the lid opens from the front, making it easy to access the vegetable plants.

Add 2- to 3-inch shims under the back side of the growing box walls to give the growing box a slight angle toward the front. This allows more sunlight to enter the warming box and water to easily drain off of the cover.

Add 6 inches of manure and straw mix over the gravel drainage. In addition to the greenhouse effect of the warming box, fresh manure retains heat well and the bacteria that break down the manure also generate heat, both of which help insulate the warming box and allows vegetables to grow healthily.

Add gardening soil and plant and care for vegetables as needed.