Wednesday, December 29, 2010

In Closing

This year marked the passing of several people instrumental to the Market.

Virgina Johansen was a long term past member of the Market Commission. She had the unusual background, at least for this century, of growing on a farm, but then moving to the city. This dual experience provided her with the insight of what the Ann Arbor citizen wanted the Market to be, and what was also advantageous for the farmer.

Peter Pollack will also be missed. Peter served on the Market Commission until a few months before his death in December 2010.

Tuesday, December 21, 2010

Hoophouse II - A Qualitative Investigation

Let's think about how a hoophouse works. This is the first step toward ideas to improve the design.

First, consider why there are the seasons. The reason is that the axis of the earth's rotation is inclined to the plane of the orbit around the sun. In our summer the northern hemisphere is more directly exposed to the sun; that is, the surface is more nearly perpendicular to a line between the center of the earth and the center of the sun. As a result each unit of surface area receives more solar radiation in summer than it does in winter when it is inclined to this line. In addition, the surface is exposed to the solar radiation a greater length of time each day. For both of these reasons the hemisphere that is in summer receives the greatest amount of solar energy per day.

There is an additional effect that contributes to the timing of the seasons. In the northern hemisphere the maximum solar energy is received around the summer solstice, about June 20. But the hottest days occur in July and August. In these months we receive less energy per day than at the end of June, but we still receive more energy than is being lost each day, consequently the temperature of the surface of the earth continues to rise. A similar effect occurs in winter. The winter solstice is around December 20, but the coldest days are typically in January or February. In this case the surface of the earth is receiving less energy than it is losing, stored heat continues to be lost, and the surface continues to cool. This lag in the maximum and minimum temperatures is a result of the earth storing thermal energy. This storage is often called the “thermal mass” of the system

What does this have to do with a hoophouse? An unheated hoophouse modifies the season to make winter in the hoophouse milder than winter outside. But it cannot increase the solar energy received, in fact it reduces it slightly. Therefore the effect must be a result of reducing the heat lost. The hoophouse reduces the heat loss and allows the stored thermal energy to maintain a higher internal temperature.

It may seem surprising that a few thousandths of an inch of plastic can provide sufficient insulation to change the climate in the hoophouse, but it can because the effect is accumulative over a long period. This is exactly the same reason that the slight increase in greenhouse gasses in the atmosphere can lead to global warming.

How can you use this information to improve the design of your hoophouse? The micro climate in the hoop house is a result of the solar energy received, the stored thermal energy, and the rate of heat loss. You need to decide which of these variables you can modify, the effect you may achieve, and the cost.

The hoophouse designer has very little control over the solar energy received, other than the transparency of the covering. Theoretically, one could collect solar energy from a larger area and transport it to the hoophouse, but in practice this would be more a method of heating than increasing the solar energy received at the hoophouse. The solar energy received is a variable the hoophouse designer cannot control for a given location.

In a similar manner the hoophouse designer has little control over the stored energy, or thermal mass of the system. It may be possible to have an effect with something like barrels of water in a very small hoophouse, but in any moderate sized system the thermal mass of the earth inside the hoophouse will be many times larger than any energy storage system that is practical to add. If you chose to grow hydroponically the nutrient solution will had some thermal mass, and you will probably need to heat it to prevent freezing. But again this is more an approach to heating the hoophouse or greenhouse than increasing thermal mass.

This leaves the insulation variable. If 6 mill polyethylene can create a more favorable micro climate, clearly better insulation would have a greater effect. The problem is that the hoophouse covering must be transparent. Transparent materials that are also good insulators are expensive. The way to avoid this dilemma is to look for areas and times the hoophouse does not need to be transparent.

During the night the hoophouse does not receive solar energy and the plants can be covered with an insulating blanket to retain more heat. The hoophouse side walls and end walls do not receive very much solar energy and they can be insulated with very little reduction in solar energy received. Night time insulation and insulation of the areas that receive little solar energy can be very cost effective, yet are rarely done. Of all the hoophouses I have seen, only one used side and end wall insulation and night time row covers.

If anyone is interested in this analysis continuing in a more qualitative manner, leave a comment or email.

Tuesday, December 14, 2010


An item of great interest at the Market this season has been hoop houses. It seems many vendors either have one or are considering one.

What is a hoop house?

The best definition is that it is an unheated greenhouse. Most hoop houses are built with steel or plastic arches, or 'hoops', and that is the basis for the name. But whatever materials are used the primary distinction between a greenhouse and a hoop house is that the hoop house is not heated.

What can you do with a hoop house?

You can extend the season. If you cover your plants to protect them from frost in the fall, a hoop house will do this much better. In the spring a hoop house will act like a cold frame and allow plants to be started earlier. A reasonable estimate is to expect to extend the season from about two weeks to a month in both fall and spring.

You may be able to 'over winter' plants that are not normally be hardy in this area. The hoop house captures passive solar heating and provides some insulation. It also provides protection from wind and rapid temperature changes. The climate zone in the hoop house is probably increased by 1/2 to 1 USDA zone. Ann Arbor used to considered zone 5, with climate warming many charts now consider it to be zone 6. A well designed hoop house should be able to keep most zone 6 and some zone 7 plants alive over winter. This will depend on the plants, the design of the hoop house and the severity of the winter.

You can harvest cold hardy plants later in the season. My wife occasionally sends me out to our herb garden to cut fresh herbs in the winter. Plants like oregano, thyme and sage are winter hardy. But it is troublesome to find them under a blanket of snow. A hoop house makes this much easier.

Can you grow plants in the winter?

Not really, most plants require about 65 degrees F for photosynthesis. This will not occur in an unheated hoop house in a Michigan winter. To actually 'grow' plants, that is to have the plants increase their vegative mass during the winter the building must be heated.

What vendors at the Market have hoophouses?

The ones that I know have hoophouses are Shannon Brines (Brines Farm), Dwight Carpenter (Carpenter Organic Produce) and John Hochstetler (Farmer John of Our Family Farm) Much of the information of this post is a result of conversations with Shannon Brines and Dwight Carpenter and visits to their hoop houses.

Brines Farm

Shannon Brines (Brines Farm) was one of the first vendors at the Farmers Market to construct a hoop house. They are now growing plants in three separate hoophouses. Shannon plants cold tolerant greens in the fall and uses the advantage of the hoophouse to continue to grow them in an extended season. The greens survive all winter with the hoophouse protection and Shannon harvests them and sells them all winter at the Market.

Brines Farm uses all of the potential hoophouse advantages to produce and sell excellent greens all winter. Much of their produce goes to the members of their CSA program customers. The produce has such a good reputation that they often cannot meet the demand from other customers.

Here is link to the Brines Farm website:

Here is a photo of one of their hoophouses.

Carpenter Organic Produce

Carpenter Organic Produce also has three hoophouse. In addition, Dwight also has one large heated greenhouse constructed in the manner. Some photos from Carpenter's green house are on the May 8, 2010 post of this blog. Some photos of his excellent produce are in the previous post on this blog.

John Hochstetler (Farmer John of Our Family Farm)

I have not visited Our Family Farm and do not have any information about their hoophouse. The produce they have been bringing to Market have been a hardy salad mix of spinach and members of the cabbage family. These greens have been very good, particulary when used in an oriental stir fry dish.

Here is a link to their website  with a photo of their hoophouse : The hoophouse is a larger version of plastic pipe construction.

I discussed hoophouses with John on Dec 18, and he mentioned that at least one of his hoophouses was heated. To me, this would make it a green house, but there was an interesting aspect of his method of heating. His lettuce is grown hydroponically and he heats the hydroponic solution. For the small producer that wants to grow plants that do well in a hydroponic system this is an ideal approach. If you are concerned about organic production, it is possible to grow organically, even certified organic, with the correct hydroponic nutrients.

In a few days I will add a post discussing how a hoophouse works and how they might be improved.

Saturday, December 11, 2010

The Market in Winter

It is obviously winter, but the Market is sill very active.  Many of the stalls are selling holiday greens, but there are still many selling produce from green houses or hoophouses.  Of course, the meat vendors are there and are happy to be able to display their products without refrigeration.

Just a few days ago we had a very nice dinner from local food sources.  It was not a planned effort to eat local, it was just a typical dinner from items I had purchased at the market.

The Main course was lamb chops from Hannawald Lamb.  The side dishes were; potatoes from Donahee Farms, sliced and browned with a bit of bacon from Earnst Farm; tomatoes from Carpenter Organic Produce; and coleslaw made from a cabbage sold by a market vendor that did not display his farm name.  Only the mayonnaise in the coleslaw was from a supermarket and probably not local.

End of season tomatoes from the greenhouse of Carpenter Organic Produce