We raise organic apples and berries on our farm in Lafayette County, southwest Wisconsin.  In the past ten years we have transitioned our farm from vegetable to fruit production.  We had used high tunnels (unheated greenhouses covered with polyethylene film) to produce some of our vegetables and we wanted to repurpose these tunnels into a useful part of our fruit farm.  We learned that the high tunnels work extremely well for growing grafted nursery stock, and we decided to also try raising fruiting dwarf apple trees inside tunnels.  Other growers and researchers have found that tunnels work well for growing stone fruit, brambles, and grapes because the tunnels provide a good growing environment with reduced disease pressure and protection from excessive rains.  In our case we were primarily interested in the potential for the tunnels to reduce disease.  Controlling apple scab and other diseases in outdoor-grown organic apples requires resistant varieties and/or an intensive spray schedule.  In high tunnels, the plastic cover keeps the foliage dry and should prevent infection by most fungal and bacterial diseases.

In spring 2019, we planted almost 200 trees, representing seven varieties, on dwarfing G.41 and G.11 rootstocks inside two tunnels.  Tree spacing was 3 feet within rows and 11-12 feet between rows.  Trees were trained on a trellis using the tall spindle method.  We have now grown the trees for three years and picked fruit in both 2020 and 2021.  Here are some of our key observations and conclusions:

The apple trees grew vigorously inside the tunnels and readily filled their space.  Timely notching of the leader and branch training was needed to develop trees with numerous calm branches.  As expected, the trees suffered virtually no disease damage, except for powdery mildew, which appeared in the 2021 growing season.  Powdery mildew is able to infect in the absence of water on the leaves and thus is common in high tunnel cultivation of many crops.  The disease-free foliage in the tunnels, even in late summer, was remarkable – and frankly unprecedented in our ten years of growing organic apples outdoors.

Numerous insect pests did attack the foliage and fruit in our tunnels, including mites and aphids (which rarely or never cause significant damage to our outdoor trees) as well as typical apple pests such as codling moth, plum curculio, and leafrollers.  Overall we feel that high tunnel apples would require a similar insect pest control regime as outdoor grown apples.

One important consideration in high tunnel fruit production is to avoid drastic temperature fluctuations on sunny winter days.  In a tunnel covered with clear plastic, daytime winter temperatures can soar into the 50’s or above, but then crash below zero during the nighttime, which wreaks havoc with trees’ cold hardiness and makes them susceptible to winter injury.  Two strategies for avoiding these drastic temperature fluctuations are to uncover the tunnels in winter or to cover them with an opaque cover (a method popularized by orchardist Dan Sheild at Stone Creek Farms in Minnesota).  We uncovered our tunnels in winter 2019-2020, but covered them with opaque black and white silage tarp in winter 2020-2021.  We preferred the later, silage tarp technique because recovering tunnels on a tight deadline in windy spring weather was difficult and stressful, and because repeated removal and reinstallation exposes the delicate and expensive greenhouse film to damage.

Purchased bumblebee colonies placed inside the tunnels provided excellent pollination and fruitset was generally high, although June drop of fruitlets was also high, possibly exacerbated by the warm temperatures and reduced light levels in the tunnels.  There was significant frost damage inside the tunnels during bloom in 2020, and we recommend that growers consider an “emergency” heat source to keep tunnel temperatures above freezing on cold spring nights.

Heat-related damage during the growing season was a major issue in our tunnels even though we ventilated the tunnels through rollup sides and used Klerks brand koolite plastic, which lowers temperatures inside the tunnels compared to traditional polyethylene covers.  Many fruit suffered sunburn, soft flesh, and off-flavors.  The extent of the damage varied by variety: Suncrisp and Hudson’s Golden Gem were notably unaffected. 

Average yield per tree was about 7 lbs in 2020 and 25 lbs in 2021 (translating to roughly 200 lbs/acre and 750 lbs/acre, respectively), and almost 70% of the fruit were our “#1 grade”, saleable to our grocery store accounts.  Yields varied greatly between varieties.  In one of our tunnels, e.g., Suncrisp trees have so far almost 100 lbs fruit per tree over two years, whereas Ashmead’s Kernal trees produced only 3 lbs! 

High tunnel constructions costs are significant: each of the 3500 square foot tunnels used in this trial cost about $10,000 in materials and 180 labor hours to construct in 2017-2018, and the covers must be replaced every four years at additional cost.  Overall, we feel that it is difficult to recoup the costs of constructing and maintaining the tunnels by growing apples inside.  It would certainly be essential to select high yielding and heat-tolerant varieties for tunnel cultivation. 

A detailed report of our results including growing methods, expenses, and yields, is available online at www.twoonionfarm.com/research/ and I am happy to answer questions by email: twoonionfarm@gmail.com.

Disclaimer: This product was developed with support from the Sustainable Agriculture Research and Education (SARE) Program, which is funded by the U.S. Department of Agriculture – National Institute of Food and Agriculture (USDA-NIFA).  Any opinions, findings, conclusions, or recommendations expressed within do not necessarily reflect the view of the SARE program or the U.S. Department of Agriculture.  USDA is an equal opportunity provider and employer.

This article was posted in Apples and tagged Apples, Chris McGuire, High tunnel, organic, organic apples.