High tunnels are generally considered to add two to four weeks to a growing season; however, there has been little research done in coastal Alaska to ground-truth or better understand this assumption largely developed in the lower 48 states. Climate in the Homer area is dominated by cool and wet summers, with dramatic microclimatic differences in temperature and precipitation at varying elevations. Air and soil temperature and relative humidity data were collected hourly inside and outside of 10 high tunnels around Homer, Alaska. Based on our data, we do not see support for the idea that high tunnels alone in this area add two to four weeks of growing season. Season extension activities should likely be concentrated in the fall; and, regardless, some additional heat source or other temperature control methods (low tunnels, row cover, etc) likely must be employed to protect crops from cold temperatures. Moisture control in the fall is also a challenge that must be addressed. Our data do suggest; however, that high tunnels alone provide a great amount of season ‘enhancement’ – increasing the growing degree days an average of 2,000 over field conditions. In our coastal, sub-arctic climate this is a huge advantage and collective experiences have anecdotally confirmed faster growth rates and increased yields of certain crops. Farmers at higher elevations will likely see greater positive impacts from double-poly tunnels; however, added heat retention is likely greater and when using additional heat double-poly tunnels are likely advantageous regardless of the elevation.
Since 2010, the United States Department of Agriculture has funded the construction of over 200 high tunnels in the Homer District through the Natural Resources Conservation Service (NRCS). The Homer District covers the southern Kenai Peninsula and Kodiak Island, all within the southcentral region of Alaska. This is one of the highest concentrations of NRCS-funded high tunnels in the nation. The southern Kenai Peninsula is home to over 15,000 people in nearly a dozen small communities. We are at the literal “end of the road,” and growers and families are increasingly interested in questions of food security and the development of local sustainable agricultural opportunities. Many feel that the NRCS High Tunnel Program is providing local producers with an opportunity to extend the short growing season in this maritime sub-Arctic region.
High tunnels are generally considered to add two to four weeks to a growing season; however, there has been little research done in coastal Alaska to ground-truth or better understand this assumption largely developed in the lower 48 states. Climate in the Homer area is dominated by cool and wet summers, with dramatic microclimatic differences in temperature and precipitation at varying elevations.
The NRCS is continuing to fund the high tunnel program and their numbers are consistently increasing in the Homer area. We believe there is considerable benefit to taking advantage of this dense concentration of high tunnels to collect regionally-specific and publically-available information to help engage and inform producers and growers at all scales. Through this project, we employed temperature and humidity data loggers and collected data in 10 high tunnels to understand the effects of high tunnels on growing conditions at different elevations (from sea level to 1,500 feet) and with single- or double-poly glazing in coastal southcentral Alaska. We also developed a website specifically for high tunnel owners around the state of Alaska to share and disseminate information.
We had three primary objectives going into this project:
Objective 1. Establish a baseline of data for air temperature, soil temperature, and relative humidity effects of high tunnels at representative elevations in the Lower Kenai Peninsula of Alaska.
Objective 2. Begin to understand the effects of single- versus double-layer polyethylene high tunnel covers on soil temperature, air temperature, and relative humidity in this region.
Objective 3. Educate and engage the agricultural community of established and new local producers in order to maximize the potential of high tunnels in this region using local data.
The lower Kenai Peninsula is located in southcentral Alaska along Cook Inlet and Kachemak Bay and is generally considered to cover the area from the community of Ninilchik south. There are approximately 14,000 year-round residents in this rural, coastal region. Homer boasts a large and growing Farmers Market with around twenty produce and flower growers selling their products between May and October. The nearby community of Anchor Point began a Farmers Market twice per week in the past several years, with an increasing number of producers in both communities selling locally to individuals, restaurants, and other businesses.
The lower Kenai Peninsula experiences a moderate, coastal sub-arctic climate with cool, wet summers and generally snowy winters that are moderately cold. The 2013 and 2014 winters were remarkably mild with low snow, especially compared to the high snow year of 2012. Climate change predictions indicate warming summers with more wintertime precipitation in the form of rain rather than snow. USDA Hardiness Maps generally show Homer in zone 5a/6b. Local growers know better, however, and generally rely on plants hardy up to zone 4 outside.
The area surrounding Homer is topographically diverse, with a steep bluff rising behind the bench on which town is located. Within several miles the elevation can change from sea level to nearly 1,500 feet. The summers are generally cooler down on the bench with a maritime breeze moderating any warm temperatures. Higher elevations see generally longer winters, however, with snow coming earlier and staying later than at lower elevations. The town of Anchor Point is located 15 miles north of Homer and is generally colder in the winter.
High Tunnel Selection
In order to meet our first two objectives, we outreached to area growers to find volunteers with high tunnels that were all “full size” (2,100 square feet per the original NRCS high tunnel program requirements) and who kept their high tunnel covers on year-round. We looked for a total of 12 tunnels, with a goal of three tunnels each to represent the following categories: single-poly, low elevation; single-poly, high elevation; double-poly, low elevation; and double-poly, high elevation. We had a difficult time finding producers with full-sized high tunnels who also left their covers on year-round. As a result, we ended up with a total of 10 high tunnels. We did have an eleventh participant; however, just several months after we installed the data loggers their cover blew off in a wind storm. It took them much of the season to get the cover back on and the data loggers were never reinstalled.
We used HOBO data loggers from Onset for this project. HOBO U23 Pro v2 loggers were used for recording inside and outside air temperature and relative humidity. HOBO Pendant Temperature/Light loggers were used for recording inside and outside soil temperatures. Loggers were set to record data every hour. Air temperature loggers were suspended inside small one gallon white buckets, hung upside down with holes drilled around the top for ventilation. These acted as cost-effective solar shields.
Data loggers were deployed at each site using consistent protocols. A data sheet was developed and completed at each initial site visit. Inside air loggers were hung mid-way down the northern or western wall of the tunnel, depending on its orientation. Working with the structural limitations, we aimed to hang each logger inside a solar shield at least one foot from the edge and three to four feet above the soil. The outside air logger was hung at least 10 feet away from the high tunnel and as close as possible if nothing was available for hanging within 25 feet. Both inside and outside soil temperature loggers were buried between 5-10 centimeters in the soil as close to the air loggers as possible. They were attached to blue pin flags for recovery. Data loggers were deployed in early-summer 2013 and recovered in late-fall/early-winter 2014 prior to freeze-up.
All temperature and RH data were uploaded using the HOBOware Pro software (v. 3.7.1). Data were then exported to Excel spreadsheets and trimmed for temperatures taken prior to or after the data loggers were set-up. All data summaries and analyses were done within Microsoft Excel 2010. Raw data from each tunnel is available as part of this report, and on the high tunnels website (described below, http://www.akhightunnels.org/resources/temp-rh-study.html).
Growing Degree Days (GDD)
Growing degree days (GDD) were calculated for each location in the study where data were available, both inside and outside of the high tunnels. We calculated GDD for spinach, using a maximum temperature threshold of 80F and a lower threshold of 32F. The base temperature was set at 32F. Therefore, the equation used to calculate GDD was:
GDD = ((Tmax + Tmin)/2) – Tbase, where Tmax = 80F if the maximum temperature was above 80F and Tmin = 32F if the minimum temperature was below 32F.
Daily Minimum/Maximum Temperatures and Differences
In order to better visualize the long time series of data, we looked at daily minimum and maximum temperatures inside and outside of each high tunnel. From these data, we calculated the difference in maximum and minimum temperature between inside and outside at each site by subtracting the outside temperature from the inside temperature. Thus, if the difference is positive, the high tunnel was warmer than outside. If negative, the high tunnel site was cooler than the corresponding outside site.
In order to compare the finer resolution of collected data, we chose 48-hours, from September 17-18, 2014, to compare high tunnel and outside temperatures and relative humidity (RH). We also graphed soil temperatures both inside and outside of each high tunnel over the 24-hour period of September 18, 2014.
To take a closer look at the potential for spring season extension in local high tunnels, we compared inside and outside minimum temperatures at each location from March 1 – May 31, 2014. On each graph we included two reference lines – a solid line indicating a “hard” frost (27F) and a dashed line indicating a “light” frost (31F).
Website Development – www.akhightunnels.org
Kyra Wagner spearheaded the development of the Alaska High Tunnels website and database. We worked with a local website developer to create the structure for the website, and Kyra and Rachel worked on developing the content. Pages on the website include: Getting Started (Selecting a high tunnel, Site selection, Constructing a high tunnel), Soil Preparation, Management (Irrigation, Ventilation, Winter Management, Pests & Disease), Database, Resources (Temp-RH Study), and Contact. High tunnel owners can sign up for a login to the database and enter their site information, environmental data (including temperatures and RH), and variety successes and feedback for vegetables grown in their high tunnel.
High Tunnel Data Results
Ten high tunnels participated in our year-round data collection project in 2013-14. There were several locations where data were lost due to lost or moved data loggers, hardware malfunction, or in one instance an outside air temperature/RH logger that fell to the ground and froze without our knowledge. The high tunnel data report includes detailed information for all sites and what data were collected at each.
Seven of the 10 high tunnels in this study were the same size – 30’ x 72’. Three additional tunnels were 35’ x 60’, 20’ x 92’, and 26’ x 96’. There were no apparent or substantial differences in GDD, air and soil temperatures, or relative humidity attributable to the high tunnel dimensions.
At nearly all high tunnels there was around 2,000 additional growing degree days (GDD) inside versus outside. The difference between inside and outside GDD was greater with double-poly tunnels at higher elevations than those at lower elevations. The higher GDD inside the high tunnels suggests that even with the same season length, the growing season can be accelerated and enhanced inside a high tunnel in this region through potential faster crop development and higher yields.
In the 48-hour close-ups, we saw the daily pattern of cooler late-night and early-morning temperatures, with the temperature inside the high tunnels rapidly rising with the warmth of the sun. The subsequent fall in temperatures were equally dramatic as the sun set; however, double-poly tunnels seemed to lose their heat at a slower rate than the single-poly tunnels. The double-poly tunnels stayed slightly above outside temperatures overnight, whereas the single-poly tunnels tended to reach ambient temperatures within a few hours after sunset. In the 48-hour close-ups and the daily minimum/maximum graphs, we can see that the temperatures increase quickly on sunny days, with temperatures reaching over 100F in some of the tunnels included in this study. The high tunnel participants who had fans, end wall vents, and large doors seemed to have fewer temperature spikes than those that relied on more passive ventilation through roll up/down sides or smaller end-wall doors.
Relative humidity (RH) were tracked with ambient RH and were closely correlated with temperatures. In the fall, the outside air is often saturated (at 100% RH). This can create very challenging high tunnel management situations, as the only way to reduce inside RH is to heat the air. This is not always an option, due to both cost and infrastructure. This may be an area for more study, given that the Alaskan spring conditions are less ideal for high tunnel season extension than those in the fall, with the exception of the high moisture in the air and adequate ventilation concerns.
The soil temperatures recorded through this study were surprising in the degree to which they fluctuated during the course of a day. This was likely due to their shallow depth, and we would recommend burying soil temperature loggers deeper to avoid potential direct air temperature interactions.
Website – www.akhightunnels.org
The high tunnel website developed for this project is a fantastic new resource for growers around Alaska. It compiles and documents local information gathered by the Kenai Soil and Water Conservation District, Sustainable Homer, and local growers and makes it available statewide. It allows a platform for links, resources, and updates on high tunnel growing in Alaska. In a web-based culture, having this permanent resource online is a great benefit to the growing community.
The database portion of this website project has been less successful. We based our project idea on a similar type of database hosted by Cornell (http://vegvariety.cce.cornell.edu). Having heard growers talk about their desires for this type of database, we trialed a ‘google spreadsheet’ for communal data gathering. Having seen this seemingly successful model at Cornell, we took a ‘build it and they will come’ approach. Unfortunately, we had challenges in the design phase with our web design contractor, and our final product, although functional, was far from user friendly. This coupled with a reality of lack of engagement for online data entry by our local growers resulted in an underutilized and arguably defunct database.
Educational & Outreach Activities
Included in this report is a final publication that provides summary overviews of the participating producer high tunnels and the data collected at each. This report is intended to give farmers, extension agents, researchers, and others quick and user-friendly access to the type of data we collected, some basic results, and information on how to access raw data for further analysis.
During the course of this project, we gave seasonal updates on the data collection efforts at three community high tunnel meetings in Homer. These were coordinated by project partner Kyra Wagner and had an average attendance of 40-50 growers. At the March 12, 2015 meeting, Rachel gave a short presentation with slides that gave an overview of the kinds of data collected and basic analysis.
On March 4, 2015, Rachel gave a 30-minute presentation at the Alaska SARE Conference, held in Fairbanks, AK. A copy of this presentation is included in this report and is available on their website.
At the conclusion of this projectt, we will be distributing our data report to area growers utilizing Facebook, the Sustainable Homer email listserv, the Homer and Kenai Soil and Water Conservation Districts, the Homer District NRCS office, the Homer Garden Club, and the Central Peninsula Garden Club. We are excited that the akhightunnels.org website will continue to be run by Sustainable Homer and be a resource and outreach platform for growers in the future.
Based on our data and the results discussed above, we do not see support for the idea that high tunnels alone in this area add two to four weeks of growing season. Season extension activities should likely be concentrated in the fall, and, regardless, some additional heat source or other temperature control methods (low tunnels, row cover, etc) likely must be employed to protect crops from cold temperatures. Moisture control in the fall is also a challenge that must be addressed.
Our data do suggest, however, that high tunnels alone provide a great amount of season ‘enhancement’ – increasing the growing degree days an average of 2,000 over field conditions. In our coastal, sub-arctic climate this is a huge advantage and collective experiences have anecdotally confirmed faster growth rates and increased yields of certain crops.
Another interesting result from our study suggests that while the double-poly high tunnels were generally warmer than single-poly, the effects were greater at higher elevations and farmers at higher elevations will likely see greater positive impacts from double-poly tunnels. However, double-poly tunnels are better at retaining heat. Therefore, if a farmer is planning to use additional heat sources, double-poly tunnels are likely advantageous regardless of the elevation.
The NRCS High Tunnel program is intended to bring season extension opportunities to growers around the country. This is particularly appealing in Alaska where growing seasons are short. Additionally, coastal Southcentral Alaska has cool summers, bringing further challenge to producers. The data from this study are helpful in quantifying actual gain of growing degree days, air, and soil temperatures inside different types of high tunnels from 10 case studies in this area. Farmers at lower elevations may take into consideration the similar GDD results in single- versus double-poly tunnels in this study when deciding on whether or not to spend the extra money on the double poly.
Additionally, this study helps to illustrate the season ‘enhancement’ potential of unheated high tunnels in this region and should raise some concerns as to the season ‘extension’ potential – especially in spring – in a high tunnel alone without additional low tunnels, or even added heat to protect against freezing temperatures similar to field conditions. We found this to be true to some degree in all high tunnels in the study, regardless of single- or double-poly covers.
Additional study could be very useful to provide farmers with recommendations on fall ventilation and moisture control in high tunnels with and without supplemental heat. Given the increased potential of season extension in this area in the fall versus the spring, this particular challenge would be a good one to better understand the costs and returns from various moisture control options.
It would also be useful to understand the costs and returns of different market crops in high tunnels in this region, given our short growing season and marginal growing climate. How could farmers best take advantage of the ‘season enhancement’ benefits of a high tunnel in southcentral Alaska?