Traditional gardening wisdom suggests starting seeds indoors about six weeks before frost and providing seedling trays with bottom heat to warm the soil. Rather than take up valuable indoor space and spend money on pricey seed germination mats, you can make a hot bed that uses the heat generated from fresh manure as it decomposes. Soil and seeds are planted on top of the manure layer so the heat radiates up into the soil, extending the growing season by several months. In mild climates, you can often grow vegetables and other plants throughout winter.
Pile fresh manure about 4 to 5 feet high in a location that receives full sun. Spray the pile with water to keep it damp, but not wet. Use horse or cow manure mixed with bedding, such as straw or a similar carbon-rich organic material. Mix in more straw if the manure contains less than one-third straw.
Turn the pile after about four days when the pile begins to generate heat; the manure on the outside of the pile should be turned to the inside. Leave the pile for another four days until it begins to generate heat. This begins the heating process and rids the fresh manure of excess ammonia that can kill plants.
Stack 8-by-8-by-16-inch cinder blocks about four blocks high to build a 2-foot tall raised bed bin for the manure hot bed. Build the bed to the desired length and width, depending on the number of seedling flats or plants you wish to grow. Dig out an 8-inch wide, 8-inch deep trench so you can set the in the ground to create a foundation. Stagger the joints on the remaining three courses of block. Drive 3-foot long pieces of steel reinforcing bar into the corners to stabilize the raised bed. Use additional pieces of rebar along the sides for additional stability, if desired.
Fill the raised block bed halfway deep with manure and straw mixture. The raised bed has three courses of block visible above ground, so add straw halfway up the second course of blocks. Add the manure about 6 inches at a time, spray with a mist of water until damp, and pack the manure with a hand tamper.
Cover the manure with permeable landscaping fabric to add a layer of separation between the manure and plants that still allows heat to travel up to the plants.
Place seedling trays directly on top of the landscaping fabric. Alternatively, you can spread a 6-inch layer of equal parts garden soil and finished compost over the landscaping fabric and plant directly into the garden soil. This leaves a 6-inch clearance to the top of the blocks that protects young plants from wind.
Cover the raised bed with frost cloth, clear plastic, or a similar material in freezing temperatures to prevent frost damage. Push stakes into the soil to keep the protective covering elevated above the plants because plants can suffer freeze damage if they come in contact with the cold cover.
Transplant the seedlings to a permanent spot in the garden when the danger of frost has passed. Manure hot beds can generate heat for about two to three months.