HIGH TUNNEL SITE SELECTION by Norman Kilmer

Where do I place my high tunnel? Which way should it be facing? Can it be placed on a grade? How close to trees can I place it?  If I build more then one high tunnel, how close can I place them apart?

These are just a few of the question I get asked by our customers. The readers of the Missouri Vegetable Growers Association most likely are no different. So let’s see if we can put some light to this subjects.

Where do I place my high tunnel?  This one is very important. First of all it should be close enough to the house or office to where you can get to it in a very short time. Why? There will be times you will need to close or open it in a hurry. This could be caused by temperature change. It can also be cause by a storm moving in that has lots of wind to it. If closing your high tunnel, close the wind side first.  Then the down wind side last. In case where you have an auto open/close system, both side will close/open together.

The next thing you need to look at is your land layout close to your buildings.  Best place-to-place your high tunnel is a nice and level location. This is not always possible. If your land has a small grade to it, it will work also. If your tunnel uses any electricity or water, you want it close to the source. The least amount of digging you need to do the better.

Which way should it be facing? Very good question. Since a high tunnel is a natural ventilated structure, we need to make sure we get good airflow through it. To do this, you need to have the wind flowing thought the shortest point possible. If your prevailing wind is from the southwest, you will need to place your tunnel in a northwest southeast orientation. If the prevailing wind is from the south, place high tunnel east and west. Each way, the wind is hitting the side of the tunnel first. This will be the shortest way for the wind to get though your high tunnel. It will also give you the most natural ventilation.

Can it be placed on a grade?  Yes it can to a point. In fact I know of some Amish that build their greenhouses on a grade intentionally. Why? We all know that heated air moves to the top. Since they do not use any electricity, they let nature do the heat moving for them. By placing the stove on the lower side, the warm air moves to the high side of the building. This pushes the cool air to the lower end. In the case of where they are using hot water as the heat source, the heated pipes are run at a grade also. Hot water moves up and pushes the cool water back down to the stove to reheat it again.

How much of a grade can I use for my high tunnel? For the width of the high tunnel, not more then a 5% grade. This would be a maximum of 1 ½’ for a 30’ wide high tunnel. On the long side, no more then a 3% grade. On a 96’ long high tunnel this would be around 3’.

If your tunnel is build on a grade make sure you cut a drainage on the high side. Cut ditch as close to high tunnel as you can. Some people line this ditch with plastic or some type of heavy fabric. Do not put any small rock on plastic or fabric to fill ditch, as the rocks will fill up with soil or plant debris. Then no longer will it drain properly. End of drainage ditch should divert water away from high tunnel. If you do not make a drainage ditch, this water will run right though your high tunnel. This will cause you to have a very wet soil in your high tunnel. 

How close to trees can I place it?  Trees are wonderful plants if they are in the right spot. In the wrong spot, they can just plain down be a pain. With high tunnels, you want the trees far enough away from the tunnel so if one does blow over, it does not reach the high tunnel. Also trees create shade.

Place your high tunnel away from the trees, the height of the tallest tree then add another 20-35 feet to this measurement. If the tallest tree is 50’, keep high tunnel 70-85’ away from the tree line. This way if a tree does fall over, it should not hit the high tunnel.

If I build more then one high tunnel, how close can I place them apart?  You want a minimum of at least 10’ between high tunnels. If you plan on mowing the grass between the high tunnels with a tractor and brush hog, 20’ would be better.

You want them far enough apart so you get good wind flow through them. Too close and they will block the wind flow.

If you are looking at building more then one unit, you might want to look at a gutter connected unit. These units can be anywhere from 2 units to up to at least 5 units wide. If more then 2 units are to be used, I would recommend putting ridge vent on the units. Units this wide need good Hope this is of help to some one.