Selecting a High Tunnel

It seems that the most important considerations when you are selecting a high tunnel are:

Cost:  How much do you have to spend?
Use:  How do you plan to utilize the space?
Comparing these two will get you a tunnel that works for you.  For example, if you are looking to manage the high tunnel to take advantage of our long days and the heat to grow hot climate crops like squash and corn, then you may want a bigger high tunnel with lots of space.  But you could save money on that purchase by planning to take the top off in the winter so the structure doesn’t need to be build stout enough to support snow load.

If, on the other hand, you want to be eating broccoli by May, then your management will be different.  You will need to have the tunnel covered year-round to be able to heat the soil up early enough to get those early crops going or to keep it warm late in the season to run past when it frosts outside.  In that case, you will be very intensely farming so a small tunnel may be all you can handle.  You will save money on size, but you should plan to spend more up front on the structure to get a system built strong enough to hold snow over the winter.

If time and money is not a concern, you can always build a big, stout building that can handle anything and everything.  Below are more of the specifics that will affect your high tunnel selection decision.

First off, you need to determine the cost considerations.  If you qualify for the NRCS high tunnel program, you can get a grant to help pay for the purchase of the high tunnel, but then you will have to purchase one that is commercially made/engineered.  The shipping costs can be quite expensive and the pre-determined designs might not fit your needs.  Building a high tunnel with rafters built with anything from EMT (electrical conduit) to chain link fence pipe gives you a great deal more freedom to be creative and save money without the deadlines and restrictions of the NRCS high tunnel program.  Check out the Constructing a High Tunnel page for designs and more information on building your own high tunnel. On the other hand, that NRCS high tunnel program is pretty handy if you don’t have the finances for such a big investment. 

Gothic or Quonset? This is one of the most frequently asked questions. In general, gothic-style high tunnels are more expensive, but are preferred for their snow-shedding ability. If you’re going to remove the plastic for winter, that’s not as much of an issue.  Testimony from Kodiak, however, has been that the Quonset style handles wind well.

During a community gathering in 2012 discussing collapses caused by that winter’s heavy snows, a few aspects were identified to be helpful in determining high tunnel durability. 
First and foremost, the strength of the metal used.  “Rolled metal” beams of the Farm Tek Pro Solar Star is quite structurally strong, the 14 gauge pipe of a Farm Tek ClearSpan or Premium high tunnel is not so much.  Oregon Valley high tunnels use a pipe with high gauge metal so they are stronger.  To translate all the different company descriptions, check out this handy metal gauge chart.

Another consideration is the shape/size of the structure.  If the high tunnel is wide, the manufacturer may create that shape by sending you various pieces to bolt together.  Those connection points along the rafter will then be weak points.  Try to select a design that has uses a single piece of pipe/metal to span a side or select a narrower size so that the manufacturer’s parts can be used as is. Narrower structures will be inherently stronger for snow load such as 20 foot wide models.

Truss kits, purlins, and braces, of course, add strength to a high tunnel.  They cost more, but the value of avoiding collapse is invaluable.  During the 2012 heavy snow year, no high tunnels with truss kits installed collapsed that year.  One high tunnel owner didn’t put in the truss kit, but instead connected steel cable from one side of the rafter to the other to hold the shape from bowing out when the top is loaded with snow.  Innovative and cheaper, this technique has proven itself so far.

In addition to the space you have available, you’ll also want to consider how much time and effort you want to put into your high tunnel, both during the growing season and in terms of winter maintenance. A high tunnel can be 30 feet wide and more than twice that long, but keep in mind that is 2000 square feet is a lot of work for the typical home gardener. Even commercial growers seem to prefer 20’-26’ wide structures. Keep in mind that a high tunnel is generally just one part of a growing system that includes greenhouse for starts and outdoor garden for the crops that thrive in Alaskan conditions. 

Single or double-layer?  According to the Cooperative Extension publication, “Controlling the Greenhouse Environment,” double layer polyethlene is 67% more efficient than a single layer for heat retention.  Renown greenhouse owner Al Poindexter of Anchor Point, Alaska has put double poly layers over his polycarbonate walls just because of the insulation value they provide.  This is most important for folks who are looking to extend the season into the colder months of the year.  A double layer properly inflated has also been attributed with better snow shedding.  If you take off the cover for the winter, however, a stronger fabric, single layer is definitely recommended.

Drop-down or roll-up sides?  NRCS strongly recommends using drop-down or roll-up sides in order to achieve adequate ventilation. In some cases, big doors and high gable end vents provide enough air movement, but movable sides add a lot of flexibility. This feature can also be added after initial construction. Proponents of drop-down sides say they keep critters out and maintain warm air temperature at the soil surface. Proponents of roll-up sides say they facilitate weeding the perimeter of the high tunnel with a weed whacker or flame thrower.  Others disagree with NRCS and say that opening the sides cools off the high tunnel too much and that adequate ventilation can happen from doors and vents in either end wall.  This will probably depend on the size and length of your high tunnel.

Even most commercial high tunnel kits sell end walls separately so you can build your own to save money.  The commercial zippered ends have been consistently problematic. Other options include wood frame covered with plastic or polycarbonate. Door options include: single doors, double door with removable center post to allow tractor access, sliding barn door, double-fold “closet door.” Think about what equipment you will be using for bed preparation, planting, harvesting, etc., and plan accordingly. Venting options that can be built into the end walls include: manual, solar-powered automatic, and electrically-powered automatic vents. Think about whether you will be around during the day to adjust the venting and temperature. If not, an automatic option is highly desirable.