In my September 2014 sweet cherry column, I discussed the potential to reduce rain-induced fruit cracking with regards to orchard covering systems, specifically high tunnels. The key first questions to be asked by growers considering any sort of covering system are: What are the specific marketing opportunities and anticipated returns for their operation? What is the likelihood and extent of crop loss from rain?
Since cherries can be grown under various types of protective covers, from high tunnels that cover multiple rows per cover to cable-and-pole tent-like structures that cover individual rows, to retractable-roof greenhouse-like structures that cover entire orchards, the differential strengths, weaknesses and costs must all be considered.
A covering system that is only needed to protect the crop from rain during ripening adds a single benefit to production. A system that also protects from frost and/or promotes earlier ripening and/or facilitates pick-your-own customers on rainy weekends adds additional return-on-investment benefits.
High Tunnel Benefits
High tunnels vary in price across and within manufacturers, generally based on the gauge of steel used for hoops and legs, the amount of steel bracing (particularly important in areas with strong wind gusts), auxiliary components such as doors or gutters, and the durability of plastic for covers.
There are numerous manufacturers of high tunnels, the most well-known being Haygrove Tunnels from the United Kingdom. Material costs per acre for such tunnels tend to range from about $39,000 to $48,000. As with all covering systems, labor quality and site characteristics (stony soils, wind exposure, etc.) can add additional costs to establishment.
Our 10 years of research on high tunnel fruit production — which expanded recently to include apricots, nectarines, and plums/pluots — has shown reductions in sweet cherry rain cracking and bird netting quantity. It is only needed for doors, sides, and valleys between tunnels rather than the entire orchard, and some diseases and insects, while promoting healthier trees, protecting from frosts — with the use of supplemental heaters when doors and sides are added, and advancing harvest date through earlier spring heat unit accumulation.
The greatest challenge in high tunnels is the venting of excessive daily heat that accumulates on sunny days during fruit ripening, which can quickly and sometimes irreversibly reduce fruit quality by causing fruit to stop growing or accumulating sugar, and softening.
On The Lower End Of The Cost Spectrum
Pole-and-cable tent-like row covers (Fig. 1) are the least costly covering system, generally ranging from $15,000 to $23,000 per acre. As with tunnels, prices vary based on types of posts used (wood, steel, concrete), the amount and gauge of cable for cross-bracing, the type of plastic for covers, and functional mechanisms such as seasonally fixed or manually/daily retractable operation. Bird netting may be reduced if the plastic tents connect closely between rows; otherwise, netting the entire orchard above the tent covers may be needed.
Closely connected tent covers also trap heat during ripening if they are fixed rather than retractable, though this trait can help in early spring for minor frost protection. Tent covers with wide gaps between rows trap less heat in the summer, but this also renders them less useful for spring frost prevention, advancing bloom and harvest, or wind protection.
Pole-and-cable tent covers often are engineered locally on an orchard-by-orchard basis, but one international manufacturer, VOEN from Germany, has become well-known for its proprietary plastic-and-net panels as well as its engineered support structure. The plastic flaps overlay netting panels, therefore allowing hot air and wind to vent through the netting while the plastic flaps close for protection during rain.
Many Features With Retractable Covers
Automated retractable orchard-wide covers are the most costly type of system, generally starting from $65,000 per acre. These systems approach greenhouse-type operations for environmental modification, with the ability to open and close the roof as well as optional sides based on inputs from weather station sensors for rain, temperature, wind, etc.
Our research experience with this structure, manufactured by Cravo from Canada, has demonstrated the greatest suite of benefits, as one should expect from the greatest investment. The system retains heat from solar radiation or supplemental propane heaters in spring for the most effective protection from spring frost.
The roof opens for optimized honeybee activity during bloom on sunny days and closes for improved honeybee activity during rainy days. The roof is programmed to close to optimize heat units to advance bloom and fruit development for earlier ripening, while opening to eliminate excessive heat accumulation that would adversely affect fruit quality.
The most expensive design uses a peak-and-valley roof with gutters to shield the orchard from rainfall. The least expensive design uses a flat roof with small T-shaped slits in the plastic spaced in rows to capture rainfall and channel it to specific drainage areas throughout the orchard, requiring the use of fruiting wall-type tree architectures to avoid wetting of the fruit (Fig. 2).
Fruiting Walls And Covering Systems
The development of fruiting wall architectures is explained in a new joint extension bulletin from Oregon State University, Michigan State University, and Washington State University (PNW 667). Performance of such training systems after six years of trials will be discussed in my next column.