Sweet cherries under cover

Why cherries crack

Fruit cracking is one of the main causes of losses in sweet cherry growing. Oregon State University Extension (EM 9227, 2019) states that in the past as little as 20–25% cracked fruit made a lot unmarketable. Thanks to new optical sorters, a crop with up to 40% cracking can now be harvested profitably.

According to that publication, cracking has two causes:

1. Water on the skin of the ripening fruit. The fruit takes it up through the skin until the skin splits.
2. The tree’s water balance. When the soil is too wet, the fruit takes up water from the roots, even when the skin is dry.

Oregon State University also lists factors that increase the risk: very heavy rainfall (more than 1.5 inches, about 38 mm), especially when the soil is close to saturation. The risk also rises with poor fruit set and large fruit, and after gibberellic acid is applied at the straw-colour stage when there is heavy rain. Susceptibility also depends on the cultivar, that is on the structure and thickness of the skin.

IO (Instytut Ogrodnictwa) lists low susceptibility to cracking in the rain among the desirable traits of cultivars for integrated production. Instytut Ogrodnictwa – PIB (the Polish National Institute of Horticultural Research) is also breeding dessert cherries tolerant to cracking. Cracked fruit rots faster: IO notes that with brown rot and grey mould, cracked fruit is the first to rot.

What orchard covers achieve

Row covers and high tunnels

Oregon State University describes two solutions:

• Retractable covers over the rows. They can be very effective, but they are expensive and labour-intensive. In sunshine they have to be rolled back so as not to overheat the trees, to allow air movement and not to limit photosynthesis.
• High tunnels. They prevent cracking because they shield the fruit from rain. They are popular in some European cherry-growing regions and have been studied at Michigan State University.

According to the same source, the wooden structures used in Norway and other European countries break in strong winds. Steel structures are less prone to wind damage if they are well designed and anchored.

Research results

Study	What was tested	Result
Børve, Stensvand, Norway (2003)	covering from flowering, 6–7 and 3–4 weeks before harvest, no cover	in 6 of 8 trials covering significantly reduced fruit rot; in the sprayed orchard 48% of uncovered fruit rotted and 6–11% of covered fruit
BC Climate Change Adaptation Program, Canada	vented orchard covers	fewer cracked fruit; humidity 20–30% higher, irrigation 30% lower; the covers did not give full protection from rain and hail, but clearly improved it
Oregon State University (2019)	covers and humidity	humidity under the cover does not usually affect cracking, but can favour powdery mildew and brown rot

In the Canadian research no powdery mildew was found under the covers. Oregon State University warns that humidity under a cover can favour it. Results therefore depend on the climate and how the orchard is managed. Ventilation and disease monitoring remain essential.

When to cover cherries

In Børve and Stensvand’s work, the trees were usually covered 3–4 weeks before harvest, mainly to prevent cracking. The authors compared this with longer covering:

• covering 6–7 weeks before harvest did not reduce rot more than covering 3–4 weeks before harvest, but could save one fungicide application,
• covering during rainfall from flowering to the end of harvest made it possible to leave out fungicide sprays against fruit rot entirely,
• longer covering did not reduce fruit quality.

Oregon State University adds two practical points:

• Winter. Where there is a lot of snow, or where the cover would hold heat and make it harder for the trees to harden off, the film is taken off in autumn and put back on in spring, after pollination.
• Pollination under cover. If the covers are on during flowering, hives are placed at least 15 feet (about 4.5 m) from the tunnel, with the entrance facing the morning sun, on a stand about 3 feet (about 0.9 m) high.

A cover does not replace other treatments. As one way of reducing cracking, IO recommends calcium fertiliser sprays: routinely 3, 2 and 1 week before harvest, or just before expected rain, starting 3 weeks before harvest.

Water from the roof: gutters and drainage

This point is easy to overlook. Oregon State University warns that in heavy rain, water running off the cover can waterlog the soil under the tunnel. Up to 60% of the fruit can then crack, even though there is no water on its surface.

Recommendations from that source:

• a tunnel gutter or drainage along the rows of legs to take water away from the root zone,
• planting trees on ridges or raised beds,
• drip irrigation that keeps the soil close to field capacity, without waterlogging, from the straw-colour stage to harvest.

Our multibay tunnels have gutters between the bays that collect rainwater from the roof. Plan how to carry it away from the orchard together with the tunnel design.

Height: the tunnel must be taller than the orchard

A cherry is a tree, not a bush. A tunnel over an orchard must fit the canopy, leave clearance under the film and room for the sprayer, platforms and picking.

• we build tunnels up to 5 m high on upset anchors (1.25–2.0 m),
• bay width 6–11 m, round or gothic arch,
• in multibay tunnels, legs 2.5–3.5 m with a Y-connector, oval tubes 80×40 or 90×50 mm (90×50: +29% strength at a similar price),
• legs and anchors in E370+Z275 steel (about 10% stronger than S350+Z275, at the same price), hoops in S350+Z275 steel,
• single film 150–200 µm with a 3-year warranty.

We decide it during a visit to the orchard.

See our cherry tunnels or start with the configurator.