The Woodfired Bottom-heated Greenhouse Bench
The Woodfired Bottom-heated Greenhouse Bench

Cultivating Questions Concerning the Bio-Extensive Market Garden

The Woodfired Bottom-heated Greenhouse Bench

by Anne and Eric Nordell of Trout Run, PA
illustrations by Ed Ochsner of Hohenwald, TN

Hi Anne and Eric,

This is Sam and Rich Wickham writing to you. We have been living and working on farms in the Finger Lakes for the past few seasons and, by way of our Amish neighbors and all the talented draft powered farmers in the area, we have been inspired to pursue farming with horses.

We have been passively gleaning knowledge from you through the SFJ, the world wide web, and your wonderful DVD and booklet which we got for Christmas this year. Your in depth, thoughtful coverage has been, far and away, the most valuable resource to us in our fledgling farming career. At first, we were just excited to find another couple who wanted to keep their farm a two person operation and how they went about that. However, it quickly became apparent that that knowledge was the very least we would stand to get out of poring over your old SFJ articles.

Anyway, we think we have exhausted our google search capacity in trying to find out more about your farm so we figured we had better sit down and write a letter to you both. We are quite curious about your wood stove greenhouse seedling set up. It has been difficult for us to determine the possible dimensions and arrangement of the system. Any insight into this would be much appreciated. Also, are you only heating up the stove at night or is it continuous throughout the day?

We are not prone to pestering but we are so intrigued by this idea that we just can’t help ourselves in this instance. We have been wanting to drop a line your way anyway to let you know that your work has elevated our understanding of farming (and appreciation of the craft) by leaps and bounds. Thank you for your generosity!

Take Care,
Sam and Rich
Caseville, MI

Before launching into the construction details of the bottom heated bench, we should explain why we chose this unusual method of heating the greenhouse in the first place. Looking for a low tech, low cost heat system which did not require electricity, we were intrigued with the nineteenth century greenhouse design in Peter Henderson’s Gardening for Profit. [see below – ed.] The flue from the wood stove of this renowned market gardener’s greenhouse ran underneath the length of the central bench and back again before joining the chimney built directly on top of the firebox. The heat from the stove created enough draft in the stack to pull smoke and heat through the long horizontal flue, warming the 20′ by 60′ greenhouse and providing bottom heat to most of the plants.

As much as this innovative heating system appealed to our minimalist outlook, we were concerned that this setup would be overkill for the 10′ by 16′ structure we intended to build in time for the 1988 greenhouse season. It seemed to us that a clean burning fire would make the small greenhouse too hot for the health of the plants and that moderating the greenhouse temperature with a smoldering fire would quickly lead to creosote buildup in the horizontal flue, a sure recipe for a chimney fire. We decided a safer, less polluting alternative would require thermal mass directly under the plants to capture the intense heat from a clean burning fire and release the warmth slowly for twelve or more hours.

The Woodfired Bottom-heated Greenhouse Bench

It took several incarnations to come up with a satisfactory design for the bottom heated greenhouse bench. In the final version we used two 55 gallon drums welded end-to-end for the firebox and a salvaged piece of 12” stainless steel chimney for the horizontal flue. (Metal culvert pipe or gasline casing might work just as well.) We learned the hard way that a large firebox and flue are necessary to dissipate the intense heat into the surrounding air chamber and to minimize heat stress on these components.

To shield the wooden greenhouse structure from the heat, we built the end walls of the bench with 4′ by 4′ pieces of sheet metal, cutting a large enough hole in one end for the door of the barrel stove and a smaller hole at the other end for the horizontal flue to exit and join the chimney outside of the greenhouse. A farmer that copied our design but neglected this safety feature had to call the fire department when the heat from the stove caught the greenhouse on fire.

The Woodfired Bottom-heated Greenhouse Bench

For the same reason, we built the side walls of the bench with concrete blocks. We placed salvaged grating from a sidewalk heat vent across the top of the block walls to support an 8″ layer of field stone and gravel, doubling the depth of this thermal mass over the barrel stove where the most heat is radiated. The weight of the stone on the grate keeps the side walls in place so mortar is not needed when laying the block.

The stove and flue rest on a bed of gravel sloped so that the pipe rises 12″ over the 16′ length of the bench. This modest slope along with the warmth retained by the thermal mass surrounding the flue produces a strong and dependable draft. When heat waves replace the smoke coming out of the chimney, we close the damper installed in the horizontal flue just outside of the greenhouse to hold as much heat inside the bench as possible. Managed this way, there is no creosote or ash buildup in the flue or chimney.

Since our goal is a brief, clean burning fire, we load the barrel stove with a couple of armloads of very dry wood. Scrap wood, tree trimmings, small pieces of seasoned hard wood all work well. For a number of years we used hemlock slabwood from a local sawmill. A half cord of slabwood was more than sufficient for the typical greenhouse heating season running from mid-March to mid-May.

The Woodfired Bottom-heated Greenhouse Bench

Normally, one firing around six in the evening is all that is needed to heat the bench overnight. (We fire up the stove a couple of times at the start of the greenhouse season to warm up the thermal mass.) If the thermometer dips into the twenties or lower, then we load the stove twice a day, morning and evening. We have never had to get up in the middle of the night to heat the greenhouse. Firing the stove a couple of times in a row, on the other hand, can get the stones so hot that they will fry the plants resting on top. Keep in mind that heat released from the bench keeps the greenhouse just 20-30 degrees above the outside temperature. That is fine with us because the combination of warm potting soil and cool air produces plants with the stocky topgrowth and large root system necessary for trans- planting without irrigation. Dryland planting also benefits from larger plugs than normal. For example, we use 72 cell flats for lettuce and 50’s for the brassicas.

Only one of the two greenhouse benches is heated. This arrangement is adequate for space heating because we insulated all four sides of the small structure with foam board sticking at least 18″ in to the ground. By contrast, a grower who tried a single wood heated bench in the middle of a larger, uninsulated hoop structure found the results unsatisfactory.

The only reason this small heated structure meets our needs is our business is based on successive plantings of cool season vegetables and we move the flats through three growing areas: our wood heated house with enclosed front porch for germination (beginning mid-February); the bottom heated greenhouse for early growth of the vegetables; and an adjacent unheated 12′ by 28′ hoophouse for the later stages of growth and hardening off.

The Woodfired Bottom-heated Greenhouse Bench

So far, this low cost, multi-stage system has reliably produced enough plants for our small market garden. Over the course of the 2014 growing season we transplanted 15-1/2 of our 380′ rows to lettuce set 9″ apart (grossing $10,436), 7 rows of onions @ 7-8″ ($2,748), 5-1/4 rows of broccoli spaced 20″ ($2,704), 3 rows of kale ($2,532), 3 rows of leeks ($1,200), 1 row of peppers ($685), 1 row of cabbage ($519), 4-1/4 rows of miscellaneous vegetables ($2,789), 4,360 sq. ft. of transplanted hoophouse crops ($11,188) and bedding plants for sale ($2,713). A much larger heated greenhouse would be necessary if we grew lots of warm season crops, like tomatoes, melons and peppers.

Also in 2014, our first summer intern replaced the rusted out grates in the bottom heated bench. The refurbished greenhouse attracted a lot of attention at the Columbus Day tour, probably because it was the only spot on the farm that looked somewhat warm on that raw, chilly day.

In closing we should point out that our make-do growing system requires moving every flat between three different buildings and an intimate understanding of which part of the heated bench is best suited for each crop. There is a reason why most growers eventually put up a single structure for all the stages of plug growth. Although the initial investment and annual fuel costs are significantly higher than the woodfired bottom heated greenhouse, one large hoop structure with automated heating is much more labor efficient and straightforward to manage.

Heating By Flues

from Gardening for Profit by Peter Henderson (1890)

When personal attention can be given to the fires, by heating greenhouses with flues a great saving in cost can be made; in fact, nearly half the cost of construction; for we find that the hot-water heating apparatus is usually half the cost of building greenhouses, while, if heated by flues, the cost would be no more than ten percent of the whole. A new method of constructing flues (or rather a revived method, for it originated in 1822,) has been in use for the past few years, which has such manifest advantages that many now use it who would no doubt otherwise have used hot water heating. Its peculiarity consists in running the flue back to the furnace from which it starts and into the chimney, which is built on top of the furnace. As soon as the fire is lighted in the furnace, the brick-work forming the arch gets heated, and at once starts an upward draft, driving the cold air from the chimney, which puts the smoke flue into immediate action and maintains it; hence there is never any trouble about draft, as in ordinary flues having the chimney at the most distant point from the furnace. It will be understood that the chimney into which the flue is returned is placed on top of the arch of the furnace, and not in it, as some might suppose.

By this plan we not only get rid of the violent heat given out by the furnace, but at the same time it insures a complete draft, so that the heated air from the furnace is so rapidly carried through the entire length of the flue, that it is nearly as hot when it enters the chimney as when it left the furnace. This perfect draft also does away with all danger of the escape of gas from the flues into the greenhouse, which often happens when the draft is not active. Although no system of heating by smoke flues is so satisfactory as by hot water or steam, yet there are many who do want to go to the expense of hot-water heating, and to such this revived method is one that will, to a great extent, simplify and cheapen the erection of greenhouses.

The Woodfired Bottom-heated Greenhouse Bench

The flue will always “draw” better if slightly on the ascent throughout its entire length. It should be elevated in all cases from the ground, on flags or bricks, so that its heat may be given out on all sides. The inside measure of the brick flue should not be less than eight by fourteen inches.

After the flue has been built of brick to twenty-five or thirty feet from the furnace, cement or vitrified drain pipe, eight or nine inches in diameter, should be used, as they are not only cheaper, but radiate the heat quicker than the bricks; they are also much easier constructed and cleaned. Care should be taken that no wood work is in contact with the flue at any place. It should be taken as a safe rule, that wood work should in no case be nearer the flue or furnace than eight inches. In constructing, do not be influenced by what the mechanics will tell you as few of them have any experience in such matter, and are not able to judge of the dangers resulting from wood work being in close contact with the heated bricks. There are scores of greenhouses burned every year owing to carelessness or ignorance in allowing the brick work to be too close to the wood. The cost of such a greenhouse, at present prices, heated by flue, would be about six hundred dollars, or about sixty cents per square foot covered by the greenhouse.