2016/2017 Spinach Trial Temperature Data Summary
Winter spinach production in unheated high tunnels typically involves a harvest in December, a long dormancy period, and then a late-winter and early-spring flush. The purpose of these trials is to evaluate the yield response when adding a little heat and 12 hours of supplemental light. At each of three locations we installed three low tunnels within the high tunnel. All three low tunnels are covered with frost blanket. One low tunnel receives no supplemental heat, one low tunnel is heated to maintain a minimum temperature of 32F, and one low tunnel is heated to maintain a minimum temperature of 45F. For this trial we are heating with forced air from an electric space heater. Half of each high tunnel also receives 12 hours of LED light.
The photo to the right shows the low tunnels at the Washburn High Tunnel. The heated low tunnels receive heat from an electric space heater with forced air blown down the length of the plastic tube. The low tunnels are covered with frost blanket to hold the heat. It is important to note this heating system is used in order to conduct the experiment and may not be the best system for expanded commercial production.
The Low Tunnel Effect
Low tunnels are a common season extension strategy used by vegetable producers, but what impact do they have, especially when used in a high tunnel? In our trials we use a 1.5 oz frost blanket draped over wooden frames as shown in the photo to the right. Whenever the air temperature in the high tunnel drops below 45F the blankets are put on. This basically means they are on all night through the winter. As shown in the first chart below, at all three locations the frost blanket low tunnels did maintain a higher minimum temperature. The Mason location had much lower minimums than the Washburn and Drummond location due mainly to the Mason high tunnel being larger and draftier and only having a single layer of poly. The high tunnel in Washburn is within 1 mile of Lake Superior, which explains the warmer temperatures. The second chart shows the nightly average temperature difference between the low tunnel and high tunnel at all three locations. On average, the low tunnels by themselves maintained the air temperature 3-4 degrees F warmer at night.
Growing Degree Days
Cumulative growing degree days are used to compare the conditions of the heat treatments in the trials. As shown in the chart below, adding supplemental heat increases the number of growing degree days primarily by maintaining a higher minimum temperature. Not surprisingly the number of growing degree days increased as the days got longer into the late winter. The Mason location tended to have fewer growing degree days due to cold location and to a larger and draftier high tunnel with only a single layer of poly. It is unclear why the 45F low tunnel at the Mason location accumulated more growing degree days in February and March compared to the Washburn and Drummond location.
How Well Does the Heating System Work?
As the growing degree day chart shows, adding the supplemental heat did increase growing degree days, but was the space heater effective at maintaining the minimum temperatures even on the coldest of nights? The first chart below shows the daily minimum temperatures in the 32F Low Tunnel at the three locations. (The red line is the target temperature of 32F). The heater system was effective at maintaining an average minimum temperature of around 33 degrees, but was not effective on the coldest nights in January. (The heater at the Drummond location accidentally got switched off during two nights in early March). The second chart shows the daily minimum temperatures in the 45F Low Tunnels. (The red line is the target temperature of 45F). Even though the space heater switched on at temps below 45F, there wasn't enough output to maintain a minimum temperature of 45F each night nor was it able to maintain an average minimum temperature of 45F. The actual average minimum temperature was around 37F. The data is indicative of the importance of sizing heating equipment to the coldest temperatures likely to be experienced. With a limited budget for this project the space heaters was all that could be afforded.
How much does it cost to run the heaters?
Natural gas, wood, or even propane to fuel a forced air or hydronic heating system is likely a more cost-effective option than electric space heaters as used in this experiment. But, the forced air system with an electric space heater was the least cost method to implement the treatments. The costs were calculated as follows: Total watts consumer for each heater was calculated by multiplying the total number of hours the space heater ran by the 900 watts/hr power consumption standard for the heater units. This number was converted to kilowatt hours and multiplied by $0.13/kwh.