Organic Apple Production In High Tunnels

We established the high tunnel plantings in April 2019 and we collected data through the 2021 growing season.  (Some of the activities described here occurred before the start of the grant project period and were not funded by the grant, but we are including them here to give a complete context for the project).

Organic.  Our entire farm is certified organic by MOSA (mosaorganic.org) and only organic methods were used in this research. 

Planting Arrangement.  There were thirteen trees of each variety in each tunnel, along with 1-2 border trees at each end of each row.  (Data from border trees is not included in this report).  Within each tunnel, the 91 non-border trees were arranged randomly, with varieties randomly interspersed.  Trees were planted 3' apart within the row, with 11-12' between rows.

Varieties.  We selected seven apple varieties for this study: Ashmead’s Kernal, Grimes Golden, Macoun, Hudson’s Golden Gem, and Golden Russet are less common heirloom varieties which can command a high price because of their flavor and historical interest.  However, all are susceptible to scab and other diseases and we therefore do not raise them outdoors.  Unfortunately, the scionwood which we obtained for Macoun and Grimes Golden was not true to type.  The Macoun trees were actually Calville Blanc d’Hiver.  The putative Grimes Golden trees were of an unknown red/green skinned, sweet, soft-fleshed apple, referred to as Mystery hereafter.  The other two varieties in our study, Winecrisp and Suncrisp, are high-yielding modern varieties popular with local organic growers.  We have raised these varieties outdoors.  Winecrisp is scab-immune, but fairly susceptible to cedar-apple rust and Alternaria blotch.  Suncrisp is moderately resistant to scab, but not immune.  We included these two varieties as a comparison to the five heirloom varieties because they are reliable standards and because they are heat-tolerant and thus may thrive in warm high tunnels.

High Tunnels.  Two high tunnels purchased from Nolt’s Midwest Produce Supplies were used in this study; both allow trees to reach heights of 10+′ and are ventilated via rollup sides and endwall doors:

¹Border Trees are at the ends of rows; no data was collected from these trees
²Experimental Trees are the trees from which data was collected

High Tunnel Coverings.  When the trees were planted in April 2019, the tunnels were covered with a single layer of standard greenhouse polyethylene.  We removed the plastic prior to winter (on 10/25/2019 from tunnel 1, and on 12/5/2019 from tunnel 2).  Based on the experience of farmers and researchers with other tree fruits, it is important to not expose trees to the drastic diurnal temperature fluctuations that occur in winter in high tunnels covered with clear greenhouse polyethylene.  On sunny days, temperatures can rise steeply in tunnels covered with clear plastic, but at night the temperatures inside the tunnel are similar to outdoor temperatures.  Trees cannot remain cold-hardy and acclimated under these constant temperature swings and are likely to suffer cold damage at night.  In spring 2020, we covered both tunnels using a single layer of Klerks SunView Cool polyethylene, which lowers tunnel temperatures by approximately 10 degrees compared to the traditional polyethylene covers (tunnel 1 was covered on 5/2/2020, and tunnel 2 on 5/6/2020).  We selected the SunView Cool cover because the trees suffered leaf burn during warm spells in 2019 and because apple fruits are vulnerable to sunburn at air temperatures above 95 degrees.  For the winter of 2020-21, we elected not to uncover the tunnels because of the time required, the stress and difficulty of re-covering tunnels during windy early spring weather, and the risk of damaging the cover during removal and reinstallation.  Instead, on 11/23/2020 we covered both tunnels with Film-Gard black and white 5 mil silage tarp (available at Farm and Fleet).  The tarp was installed white side out, and completely covered the south, east, and west sides of each tunnel, preventing winter sunlight from entering the tunnel.  We had heard from other growers who raise stone fruits in high tunnels that the silage tarp keeps the tunnel cool in winter and prevents daytime temperature spikes.  The silage tarp was removed on 4/6/2021.

For ventilation, rollup sidewalls and endwall doors were regularly left open during the growing season.  Before mid July, we attempted to close the tunnels during rain to minimize the chance that windblown rain would wet foliage and lead to disease.  Later in summer and in fall we no longer closed the tunnels during the rain.

Tree Planting.  The trees planted were one year old grafts raised in our on-farm nursery or purchased from Cummins Nursery, Trumansburg, NY. We planted trees by hand on April 12th and 17th, 2019.

Mulch and Weed Control.  We completely covered the soil surface in both tunnels with hardwood bark purchased from local sawmills after planting.  Tunnel 1 was mulched in May 2019, but Tunnel 2 was not mulched until November 2019 because of time constraints.  Both tunnels were hand weeded as needed during the growing seasons.  Compared to outdoor plantings, the mulch in the tunnels degraded slowly (presumably because of lack of rainfall), and it was not necessary to replenish the mulch during the first three years of growth.

Irrigation.  Both tunnels were irrigated using drip tape with a flow rate of 0.34 GPM per 100', with a single tape line by each tree row.  Irrigation sessions were generally for 8-12 hours on 4-7 day intervals, depending on weather.

Training.  Trees were planted 3′ apart within rows, grafted on dwarfing rootstocks (G. 41 or G.11) and trained using the tall spindle method; these are common practices in outdoor high-density apple orchards. 

In the 2019 growing season, we trained trees on a minimal wire trellis suspended from the high tunnel structure.  Trees were secured to the wires with tree tie tape, and vigorous branches were tied below horizontal using training wires. 

In April, 2020 we installed a permanent support stake next to each tree (Best Angle Tree Stake model PA3120 - 10' high angle iron stake 1-1/4" x 1-1/4"); stakes were pounded 2.5' into the ground, with 7.5' above ground.  We secured trees to the stakes with AgLok chain and continued to train vigorous branches below horizontal using training wires.  On 4/2/2021, we dormant pruned the trees, removing several larger branches from each tree.  In 2021, we continued to train trees to the metal stakes, but did minimal branch training.

To encourage branching, we notched the cambium above dormant buds on the leader in all three  years (4/22-4/29/2019, around green tip; 5/5-5/8/2020, just before bloom; and 4/16-4/17/2021, at green tip to tight cluster).   On one year old wood, we notched by running the saw blade on a Leatherman tool backwards to abrade the cambium just above the bud.  On two year old wood, we used a box cutter knife: we pressed the blade straight into the hardwood just above the bud, then made a second cut down at a 45-degree angle to meet the straight cut and we removed the angled notch.  Extensive blind wood on leaders is unacceptable in high density apple production; notching is highly effective at promoting branching.

In general, vegetative vigor was high, and we were initially concerned that the trees might outgrow their space over time, but fruiting definitely reduced vigor in the second and third years.  Most leaders were close to the roof of the high tunnel by the end of 2020 and were pressing against the roof by the end of 2021.  Repeated rounds of branch training were required in both 2019 and 2020.  The variety Suncrisp was particularly troublesome because of its propensity to grow strong upright branches.

Insect Pest and Diseases.  We scouted each high tunnel weekly for disease and insect pests.  In addition we installed delta traps baited with pheromone lures for codling moth, red-banded leafroller, and oblique-banded leafroller in tunnel 1.  Virtually no signs of plant disease were noted in the tunnels. In 2020, a few cedar apple rust lesions occurred on early spur leaves of Golden Russet and Hudson's Golden Gem; it is likely that these infections may have occurred shortly before the tunnels were covered in early May.  In 2021, occasional cedar apple rust lesions were noted on many varieties, especially near the edges of the tunnels, and we suspect that infections occurred when the rollup sides were left open during a spring rain event.  In both years, the cedar apple rust levels in the tunnels were vastly lower than in our outdoor orchard.   Fireblight, scab, Elsinoe leaf spot, alternaria blotch, sooty blotch and flyspeck were all absent or vanishingly rare.  All of these diseases occurred in our outdoor orchard during 2020 and 2021.  The virtually pristine foliage on tunnel trees, even in mid to late autumn, was remarkable and unprecedented in our experience growing organic apples outdoors.  The one notable exception was that in late summer 2021 we observed low levels of powdery mildew in both tunnels on numerous varieties.  Powdery mildew is generally common in high tunnels because warm, humid conditions favor its development, and because it does not require wet foliage to infect leaves. This disease is absent or very rare in our outdoor orchards, presumably because the conditions there are less favorable and/or our intensive spray program controls it.  We suspect that powdery mildew might continue to develop and spread in our tunnels in future years and that potassium bicarbonate sprays might be needed to control it.

In general, the tunnels did not appear to greatly reduce insect pest damage compared to outdoor orchards.  This observation agrees with what growers and researchers have found for other fruit crops grown in high tunnels.  The table below details our observations of insect pest damage in the tunnels and sprays used to control insect pests.

Spray Schedule.  We sprayed insect control products at several times.  In general, we made no disease controls sprays.  We sprayed micronutrients on the same schedule as used for outdoor apple plantings.  Sprays were applied using a Rears Pakblast 50 tractor mounted airblast sprayer, except for safer soap which was spot-sprayed on aphid-infested foliage.  All sprays are shown in the table below.  The high tunnel plantings were certified organic and all sprays were organically approved.  We were unable to spray the outside of the outside rows in both tunnels because the tractor would not fit there.  I.e., in Tunnel 1, which had three rows of trees and two drive aisles, the middle row was sprayed from both sides but the outer two rows were sprayed from one side only.  In Tunnel 2, which had two tree rows and one drive aisle, both rows were sprayed on one side only.

Pollination.  A Koppert Natupol Excel Startup bumblee colony was placed in each tunnel near the beginning of bloom in each year (on 5/7/2020 and on 4/27/2021) to ensure adequate pollination.  We noticed that bumblebee workers from the introduced colonies regularly foraged outside the tunnel, and appeared to actually prefer foraging in the nearby outdoor field orchard.  However, bee activity was high inside the tunnels on rainy days.  Overall, initial fruitset was high and pollination appears to have been adequate.

Spring bloom and frost damage.  In 2020, high tunnel trees bloomed synchronously with field-grown trees, with first flowers opening on 5/12-5/13.  (Note, however, that the tunnels were uncovered before 5/2-5/6).  Nighttime temperatures in our outdoor orchard reached 28 degrees F on the morning of 5/9 under clear skies with a calm wind, shortly before bloom and when flower buds were very susceptible to cold damage.  On this cold night, we kept endwall doors and rollup sides on the tunnels completely closed in an attempt to retain warmth.  We found no damage to flowers in the field orchard from this cold event.  However, king blooms on most high tunnel varieties were killed (pistils were blackened).  In addition most Calville Blanc d’Hiver fruits in the high tunnel which did develop showed blossom end russeting which is commonly associated with frost damage at bloom.  We did not have a thermometer in the tunnels but we suspect that the temperatures were actually colder in the tunnel than outside.  Other high tunnel growers in our region have noted that nighttime temperatures can be lower in tunnels than outdoors during radiation frost events.  Tunnel 2 also occupies a slightly lower position on our hillside than our outdoor orchards, which may also have contributed to the frost damage in that tunnel.

In 2021, high tunnel trees bloomed synchronously with field-grown trees, with first flowers opening on 4/27.  (When we removed the silage tarps from tunnels on 4/6/2021, we noted that trees in the tunnels were behind outdoor trees in their stage of development.  Thereafter tunnel trees gradually caught up with outdoor trees in their developmental stage.)  When both outdoor and tunnel trees were in tight cluster stage, on April 21, outdoor temperatures reached a low of 25 degrees under clear skies with a calm wind.  During this cold night, we kept end wall doors and rollup sides in the tunnels completely open.  After this frost event, we observed very low levels of frost damage to blooms in the outdoor orchard, and no damage in the tunnels.  We have heard speculation that on calm nights, temperatures in tightly closed tunnels can be lower than outdoor temperatures and it is best to leave tunnels open on calm cold nights, and our experiences appear to bear that out.

Summary of Spring Phenology and Bloom in 2020 and 2021

Thinning.  In 2019, we removed any developing fruit after bloom.  In 2020, we hand-thinned fruits, aiming for 1-3 (occasionally 4) fruits per branch, depending on branch length, and 1-4 additional fruits on the leader.  Thinning was performed on 6/1 and 6/9, about 1-2 weeks after petal fall.  In 2021, we hand-thinned on 5/20-5/29, about 2-3 weeks after petal fall, with an additional follow-up, corrective thinning on 6/11 and 6/15; we aimed for approximately 4-6 inches between fruits.

In both years, we noted a very heavy natural June drop of fruitlets in the weeks after bloom.  Qualitatively, the drop appeared to be higher than what we typically see in outdoor orchards.  In general, warm temperatures and low light levels favor fruit drop, and we speculate that compared to outdoors the tunnels tend to impose conditions favoring drop: the plastic covering reduces light levels and tunnel temperatures are higher than outdoor temperatures.

Heat stress damage on foliage.  There was occasional heat stress damage seen on foliage which was at a young developing stage during extremely hot weather.  The Mystery variety was particularly susceptible to foliar damage.  This damage was much more common in tunnel 1.  Fruit sunburn symptoms, discussed below, were also more pronounced in tunnel 1.  We often that note that tunnel 1 feels warmer than tunnel 2, and we presume that temperatures are higher in tunnel 1 for some or all of the following reasons:

1. Tunnel 1’s larger width (34′ vs 24′) makes ventilation less effective
2. Tunnel 1 is oriented east-west, in line with prevailing winds, whereas tunnel 2 is oriented north-south, which is perpendicular to prevailing winds and improves the effectiveness of ventilation through the rollup sides. 
3. Tunnel 1 it is located about 12’ north from another tunnel (not used in this study) which reduces natural ventilation through the rollup sides.