Hot beds provide enough bottom heat to transplants enhancing germination, stimulating root growth and keeping transplants above freezing on cold nights. I have been using both systems for three years and thought I would share some pluses and minuses as well as step-by-step instructions on how to build them.
I use two different kinds of hot beds. One uses an insulated electric cable running through sand to transmit heat and keep transplants warm. The other is a larger box filled with composting horse manure and bedding. The composting process provides free heat with no need for a source of electricity.
Steps to Make a Heated Hot Box
1. Build a frame. I built my frame using 2 x 12 inch boards. The box is 44 x 84 inches. Instead of making a bottom I set it on pallets on cinder block legs. Use what you have on hand but make sure the table is level otherwise your transplants will be at an angle, the nemesis of even watering and you will have to use more sand than you need. Size your box to fit the flats you plan to use. A slightly deeper frame will better protect tall transplants.
2. Insulate the bottom of your frame. An inch of foam insulation will help trap heat. Drill or pierce holes in the foam to enhance drainage.
3. Line the bottom with landscape fabric or other woven material. You want something that will hold the sand but allow the water to drain.
4. Fasten heating cables in a grid. The heating cables need to line the bottom of the box with no more than three inches between the cables. The manufacturer suggests cutting a piece of hardware cloth and attaching the cable to it with twist ties or string. I used window-screen this time as a less expensive alternative. However, it was a little more difficult to force the twist ties through the small holes of the screen. Make sure the cables do not cross. Crossing will fry the cables and you will have to replace them. It is recommend that built in thermometer is near the center of the bed.
5. Cover with one inch of sand. The sand if kept damp will transmit the heat provided by the cables evenly across the bottom of the hot bed.
6. Water the sand well, cover plants at night to trap heat.
Pros and Cons to a Hot Box with Heating Cables
I like this system because it uses very little electricity. I figure it uses less than $20 worth of electric for the whole transplant production season. It is pretty reliable. I have measured temperatures of 23 oF in the unheated greenhouse where the hot box is located and the temperature inside the hot-box when covered stayed well above freezing. Temperature fluctuations can still be hard on delicate seedlings and they never look as lush and large as seedlings produced in a heated greenhouse. But, I feel they are hardened off well and do not suffer as much from transplant shock. My biggest concern with this set-up is that you cannot sanitize it. Your transplant roots will grow into the sand and if there is a disease problem it can persist on the decaying root material in the sand. It is important to get your greenhouse up to high temperatures during the middle of the summer after your transplants are all outside to kill any remaining disease organisms. Freezing over winter does not do the job.
Steps to Make a Hot Box Heated by Composting Manure
I learned about this method from Teena Bailey at Red Cat Farm who in turn learned it from Steve Moore, the greenhouse guru. I have since seen similar designs used on a number of farms. The key is to get the right mix of manure and bedding or other carbon type material so that your 'compost' is producing plenty of heat.
1. Build a box to contain the composting manure and bedding. Often this box is made out of cinder blocks. They are sturdy, sometimes available on the farm from old projects and don't take up too much space in the greenhouse. I have also seen the box made from bales of straw. These will need to be replaced every year. I made my box three blocks high with an additional set of blocks dug into the ground and leveled to create a foundation. Pounding rebar stakes into the corners will help make it more sturdy.
2. Fill the box with manure and bedding, water and turn well. I use horse manure with bedding because it is available from my neighbor and seems to compost well. If your manure/bedding mixture does not heat up well the first time even with plenty of water and turning you may have too much bedding.
3. Level and cover with landscape fabric.
4. Allow ammonia to dissipate. You will want to wait a few days before putting in your transplants. The ammonia released from the fresh manure could be toxic to small plants.
5. Cover the box to trap heat at night. I have used row cover, or old greenhouse plastic to stretch over the bed at night. At Red Cat Farm, Teena Bailey uses rigid plastic roofing. See farm profile Red Cat Farm.I think this is a better option since if frost develops on top of the plastic it will burn seedlings that touch the plastic. A rigid surface would not sag and touch the plants.
Pros and Cons to a Hot Box Heated with Composting Manure
Unlike more sophisticated systems you will need to keep an eye on the temperature. Overtime the compost will start to cool and you will have to remove your plants, turn the compost, and water to get the compost process back in full swing. I have also worked with growers who have had trouble with insects such as pill bugs. They are attracted to all the decomposing plant material in the compost and may wander up and munch the roots of your seedlings as well. I have also heard of rats and mice being attracted to the heat. In addition to making a warm home in your pile they might eat germinating seedlings - especially sunflowers.
Prepared by Tianna Dupont, former Penn State Extension educator