Low Tunnel Materials for Protection and Phenological Acceleration of Figs

Results & Discussion 2021

Temperature data is only available for site A. The lowest recorded temperature that the fig trees experienced under the inner layer of protection was 13.5F on 1/30/21 when the outside ambient temperature hit -1.3F. On 12/18/20 the trees experienced 15.3F while outside it was 3.6F. I mention that because the trees only had one layer of protection and in mid-December they may not have been at peak cold hardiness.

Snow cover heavily influenced the level of protection. For example, on 2/20/21 site A received around eight inches of snow. On 2/21/21 the outside temperature dropped to 0.7F while under the inner protection the figs enjoyed a balmy 30F. When time permitted I tried to bolster this affect by mounding snow onto the tunnel using a snowblower. In contrast, on 1/9/21 with no snow cover the difference between outside (15.4F) and the inner layer (18.9F) was only 3.5 degrees.

The full data set and charts can be accessed here-- https://docs.google.com/spreadsheets/d/1DZWZkTFkATfm0AITchveZ5xHrkKgEyjMJv-Ui5WZIOI/edit?usp=sharing

For protection from deep winter lows the protection scheme of layers of clear poly worked very well. Fig trees should be able to survive 13.5F with low/acceptable levels of damage that can be worked around by leaving extra buds and canes. Unfortunately the clear poly also resulted in sharp spikes in temperature on sunny winter days that very likely disrupted the cold hardiness of the fig trees and may have also resulted in some trunk cracking. For example, on five separate days in January the innermost layer of protection recorded higher than 90F. In the spring, most trees showed heavy signs of damage. Most died to their trunks (around 12 inches off the ground), while some died to the ground. No trees died outright. The only exceptions were the trees on the outside of the tunnels, which may have resulted from air infiltration due to poor sealing. The trees at Site B with the mylar inner protection layer fared better than the other tunnels as well. I suspect the reflective properties of the mylar mitigated the heat spikes while also reflecting ground heat.

These results motivated the PI to change the materials used for the winter of 2021-2022 to include white poly to decrease the chances of temperature spikes on sunny winter days.

Initial Results, Winter 2021-2022

As noted above data collection for winter temperature readings did not commence until 1/8/22 at Site A and 1/15/22 at Site B. Initial low temperature recordings at Site A are discouraging.

1/11/22, 3am. Outside Temp: 7.0F. Outer Layer: 8.0F. Inner Layer: 12.4F

1/15/22, 3am. Outside Temp: 0.7F. Outer Layer: 5.3F. Inner Layer: 8.5F

1/15/22, 11pm. Outside Temp: -2.1F. Outer Layer: 0.3F. Inner Layer: 5.3F.

So the fig trees at Site A have already experienced a much lower temperature (5.3F) than they did in the winter of 2020-2021 (13.F). Also problematically, the trees have experienced temperatures over 67F on four days already (though only one day over 80F).

From just two days of real data from Site B no interpretations should be ventured. But venture I shall. The daytime data is encouraging. The outer white poly stayed 3-6 degrees cooler than the clear poly outer layer. The mylar inner layers actually read cooler than the outer (between layers) probes, suggesting the mylar material was actually reflecting the coolness of the partially frozen ground. Unfortunately the inner layers recorded abysmal temperatures on the morning of 1/16/22. The south tunnel read 0.9F, the middle -1.8F, and the north 3.6F. If these were true readings it would be likely that all trees will be reset to the ground. I’m holding out hope it was some kind of fluke due to radiative cooling or the sensors hadn’t grown their winter skin yet.

Photos available here-- https://photos.app.goo.gl/ChgKbTAYxUdAEays9

2021-2022 Winter Results and Discussion

Sensor Issues

For this study I used a combination of Ambient Weather and Ecowitt wifi gateways and sensors (they appear to be the same hardware). The gateway unit needs a wifi connection to log the data and the sensors need to be within radio range (around 300 feet). The benefit to using this setup is that it is very inexpensive, can be monitored on the fly, saves to the cloud, and can be setup with temperature alerts (helpful for say greenhouse growers who can turn on a heater, aka do something other than chew on their nails). I ran into several of the drawbacks, however. For one, both of my growing locations were farther than 300 feet. At the Dresden location this meant that the closest tunnel uploaded data the most consistently and the farthest was the least reliable. Using lithium AA was a big help but at low temperature the sensors could become less reliable (just when you want them to be the most reliable).

Using mylar as one of my covering materials caused interference rendering the normal sensors useless. The work around is to use sensors that have a wired probe so you can put the sensor body outside of the mylar but have the probe measure the temperature under the covering. This works great until rodents start chewing on the wires. This happened multiple times over the course of the winter. 

Alternatives to these sensors seem to be coming into more widespread use. SensorPush makes a bluetooth connectable sensor that uploads its data once you come within bluetooth range. At the moment they're too expensive for widespread use ($90/piece, the Ambient/Ecowitt sensors are $15-20/piece), but the price may fall. 

Cold Temperatures and Low Tunnel Materials

The right combination of materials for protection of fig trees (or any sensitive plants) from winter low temperatures remains elusive but mylar is cementing itself as part of the solution. Greenhouse plastic, whether 6 mil poly or 2 mil white overwintering poly, has performed in an underwhelming fashion and the temperature sensors have even reported figures below ambient temperature perhaps due to radiative cooling (where IR waves escape at a disproportionate rate due to the transparency of the plastic). Whereas clear 6 mil or 2 white 2 mil poly had a minor effect on temperature (between -2.2 and +2.6F difference), mylar had a consistently positive effect generally adding 6-7 degrees over ambient temperatures in four different cold episodes between 1/22/22 and 3/4/22. Sensor malfunctions prevented data from all materials being compared in some cases but in all cases some contrast was evident between the temperature under the mylar and outside of the mylar. 

WP=white poly. MY=Mylar. CP=Clear Poly. 

The other consistently positive covering material in this study has been snow. At site B (in Waterloo, NY) I was able to use a snowblower to cover or seal the edges of the low tunnels when snow was available. The results stand out as outliers when looking through the data. Similar cold temperatures were experienced on 1/26/22 and 2/5/22 but the results are markedly different due to the influence of snow cover. Seen below, it looks like even partial insulation by snow could be responsible for as much as a 14 degree increase. 

RC=Row Cover

This effect of snow can also be seen when contrasting the effects of covering materials at the two different sites. Between 2/14/22 and 2/15/22 Dresden experienced a low of 2.7F while Waterloo experienced -1.0F. The cumulative covering materials in Waterloo bumped the temperature up 12.7F, while in Dresden similar materials only boosted the temperature 3.9F--the sole difference being the level of snow built up around each tunnel. 

Overall, while it seems like mylar is part of the solution further research into better insulating materials is needed. Vegetable farmers often use a heavy felt material to cover their fragile greens in the spring, these kind of textiles could work well in tandem with an outside layer of plastic to keep them dry and hoops or a draping wire to support their weight. Organic materials like composting leaves could also play some role, particularly in sealing the edges of the tunnels. 

Hot Temperatures and Low Tunnel Materials

While not as obvious as the damage caused by voles and cold temperatures, damage to fig trees caused by heat should be a strong consideration for materials selection in a low tunnel system. The temperature spikes caused by a greenhouse effect with clear poly can wreck havoc on the cold hardiness of perennials and can also throw off your data analysis. On average, the covering materials for the Waterloo fig tunnel increased the temperature roughly between 7 and 10 degrees over ambient in January 2022. That sounds great. Except that much of that temperature boost may come from the afternoon sun, wasted heat that is possibly detrimental to the cold hardiness of the trees. 

Another way to quantify this is to count the number of times the temperatures under the covers exceeded 60F. While this could be beneficial to getting off to a strong start later in the spring, in the winter it may pull the trees out of dormancy right in time for a killing cold snap. Below is a count (each count represents a four hour increment) of the times the temperature exceeded 60F between January and March. 

Gleaning through this data, it's immediately evident how detrimental clear poly can be while mylar has a strong mitigating effect on temperature spikes. It's also readily apparent how the sun picks up steam in the second half of February and has a very strong effect in March. Growers should consider venting their tunnels on sunny days late in February and certainly in March. There may be a point in March in which killing winter temperatures for figs is passed (mid-March), in which case removing the materials altogether is an option. This would prevent early wake up (when desired) and any preserve any remaining cold hardiness.

There is a material used in greenhouses that consists of alternating strips of clear poly and foil to retain heat. For a low tunnel it could also be used to reflect away excess sunlight. Another option is to spray on an overwintering paint, another technique used by some greenhouse growers. The overwintering white poly had a lesser effect than expected so outside of mylar the search continues. 

Vole Protection Revisited

While we're only a few months into the winter of 2022-2023 the results of our redesigned vole protection have been encouraging. First, it's important to note that rabbits and voles are not the same (:P). Rabbits have continued to do damage to the fig trunks regardless of whether they were painted or not. While rabbits do not seem to dig under the tunnel materials, and thus are excluded by them, they often do their damage in the fall before the covers are applied. The good news is that the stones seem to be discouraging the voles from digging around the root mass and the paint is repelling them from eating the trunks. WOOOOO. There is evidence of vole tunneling, especially around the bait traps, suggesting they are taking the bait. Several vole corpses have been discovered too, RIP. All that said I haven't had a chance to check the high pressure tunnel (landscape fabric remains), and there are plenty of months to go. It also looks like the paint is starting to degrade and there is evidence of the voles nibbling on trunks that did not get painted. We shall see. The first picture is of a shallow vole hole in the no 2 stone. 

The Fig Market

While the in ground fig trees that are the subject of this study contributed very little to fig sales in the fall of 2022, the potted trees and in ground in the greenhouse trees had a banner year. Around 40 pints were sold through a winery tasting room, on display in a case next to cheese and white wine bottles. About 25 quarts were sold to a local restaurant who used them mainly for a fig, cheddar and bacon flatbread (yum). A couple bags of fig leaves and around 6 quarts were also sold to a winebar, which made a fig negroni by soaking the leaves in gin and using the fig fruit in a sweet vermouth. With the production potential of in ground figs, wholesale opportunities will have to be explored in the future. Collaborations with local breweries and wineries--maybe a Fig Field Day in the early fall--are another possibility. Fig cutting sales in the winter of 2022-2023 have been particularly strong and represent a nice no waste opportunity--and help spread the fig gospel to future growers. 

Winter Results 2022-2023

Initial Snapshot, coldest temperature recorded:

S=South tunnel

Mid=Middle tunnel

WP=White poly 3mil

CP=Clear poly 6mil

M or Mylar=4mil mylar

While there may have been marginally more snow at the Waterloo location than at the ARWC site, the only other difference between the two protection schemes would be the 6mil clear poly. Evaluating the trees in the spring, there was slightly better bud survival in Waterloo, supporting the survivability difference in temperatures between the two sites. Overall it seems mylar and clear poly have value while the thinner white poly may not be worth the effort. Decreasing the temperature swings inherent in using clear poly may be better accomplished using a white washing product (commonly used in some greenhouse setups). 

Average temperatures, overheating:

The mylar layer is coated on both the inside and outside. Thanks to this dual layering it is effective both at reflecting ground heat to increase the lowest temperature experienced by the fig trees, and at reflecting the sun and decreasing the highest temperature the trees experience. 

A sharp increase in heating can be observed between January and February due to the longer day length and higher position of the sun in the sky. Around February 8th the amount of day light passes 10 hours for the day, this mark is used by vegetable growers to signify when many plants begin growing again after their winter dormancy. 

Presented another way, the number of times the temperature went over 60F was reduced by the mylar material and generally increased as we approached spring. While the white poly didn't heat the tunnels as much as the clear poly did during the day, the reduction was slight and disappointing. The thinness of the white poly (4 mil vs 6 mil for clear) may explain some of this reduced effect. Alternative approaches or materials, like spraying on a temporary white coating onto the clear poly as some green houses do, a layer of shade cloth, or climate screens containing alternating strips of clear poly and reflective foil, are certainly worth exploring but were outside the parameters of this study. 

Vole Protection Results

In the fall of 2022 a new vole protection scheme was enacted. This scheme included surrounding the root ball of the fig trees with number 2 stone by replacing dirt with stone, and painting the trunks with a mixture of interior latex and castor oil. 

Overall, the results were promising if not perfect. Rabbits were completely undeterred by the trunk paint mixture and girdled several trees at the Waterloo location. If rabbit pressure is observed, tree guards are the only option. The landscape fabric was left on the north tunnel (12 trees) at the ARWC growing site as a high pressure experiment. Those trees experienced moderate vole feeding and a small amount of tunneling through the roots. On the other two tunnels (24 trees) where the landscape fabric was removed, there was only some slight feeding (no whole trunk girdling) on trunks where the paint had started to fade. While the paint held up fairly well, my recommendation would be to put the initial coat on during a dry spell in the fall then add a second coat just before putting the covering materials down. 

Trunk painting looks to be a viable protection strategy against voles, not rabbits, in low to moderate pressure situations. In growing sites where the population has grown to large levels, trunk painting may not be enough. Note that the regular Zinc Phosphide in PVC bait trap program was still in place. It's also possible that removing the landscape fabric, which cannot be used when growing figs in ground, caused the vole populations to relocate from the south and middle tunnels to the north tunnel exacerbating the situation. 

Low Low tunnel Failure

The north tunnel at the ARWC was also where we tried to tarp clips and vineyard wire alone, without EMT hoops, to make a low, low tunnel after the wind demolished the previous EMT setup. This did not work well. Without the rope ties across the "ribs" of the EMT hoop structure, or purlin wire through the center of the hoops, the wind was able to get under the plastic and compromise the tunnels. If a more comprehensive approach was taken, namely burying the edges of the plastic rather than relying on tarp clips and landscape staples, this low, low setup may have stood a better chance at success. 

2021-2023 Low Tunnel Materials and Winter Temps Comparison

This study was focused on evaluating different materials for use in a winter protection scheme with low tunnels and fig trees. Ultimately all fruit trees and vines have a temperature below which their ability to fruit or survive will be compromised. In this study no fig trees died due to winter damage but a large majority could not set or ripen a crop due to winter damage. The temperature at which fig trees experience bud death isn't well agreed upon but seems to be somewhere between 5F under ideal winter conditions with cold hardy varieties, and 15-20F in winters with large temperature swings and not well suited varieties. So, let's use 10F as our marker at below which your fig crop is critically compromised. 

CP6=Clear poly 6 mil. CP4=Clear poly 4 mil. WP4=White poly 4 mil. RC=1.5oz row cover. MY=4 mil mylar. 

Looking at three years of data using only the coldest ambient temperature recorded at each location, we see that only after the 2020-2021 winter did our fig trees experience a temperature above 10F. That was the year with the least extreme low temperature. So, more or better materials are needed to  ensure a viable crop in winters that drop below -5F (aka zone 6a). That said, the materials we did evaluate show some promise in tempering the extremes if we look at the delta between the ambient and inner protection temperatures. 

While our fig trees clearly suffered some winter damage from simple low temperatures, they also may have suffered from the temperature spikes created by the poly materials once the sun started to pack some punch (roughly mid February according to the data). The mylar material was the best at mitigating these temperature extremes, while the clear poly made it the worst. The white poly was slightly better than the clear in keeping the temperature gain less extreme but was overall disappointing. Heavy textile or row cover materials may do a better job at dissipating heat, or temporary spray on products exist that could be used in conjunction with the clear poly.