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Berry tunnel
In late June 2021, farmers looked at their crops and wondered what, if anything, could be done to protect them from the excessive heat. Some fruits, like blueberries and raspberries, took an unusually harsh beating.
“You can literally grab some of the leaves and they crumble,” David Mutz of Abbotsford’s Berry Haven Farms said of his raspberries at the time. “The fruit size is tiny and we’ve been running overhead [irrigation] guns to try to get some moisture in the plants and some evaporative cooling.”
Every berry grower experienced losses, some more than others depending upon the type of berries they were growing, the varieties and the climate in their micro-region.
The heat is likely to come again
When considering how berries fared in the 2021 heat event, raspberries were hit hardest, with blueberries second, strawberries third and cranberries had the least amount of damage, but were definitely not left unscathed. Unfortunately, while the heat dome was unprecedented, it will probably happen again in the future.
At a workshop in late 2021, Agriculture and Agri-Food Canada agroclimate specialist Trevor Hadwen said BC’s 2021 drought was the most significant and severe on record since the Canadian Drought Monitor began in 2003.
Before climate change, historical data would have indicated such an event would not repeat. But, with climate change, it’s only a matter of time.
“It’s very unlikely to happen again without climate change. It’s very likely to happen again before the end of the century with climate change,” Hadwen says.
With previous years of drought, the soil in many areas was already dry, creating additional plant health concerns. Depending upon the plants and region, these concerns included increased and new pest pressures, increased and new diseases, poor pollination, reduced plant vigor and other factors.
Raspberries and blueberries, like fruit trees, are in the soil for many years, so understanding the soil becomes that much more important. Tools like climate and weather stations, data and modelling will become increasingly necessary for growers to manage drought and extreme heat to save not just crops, but improve long-term plant health.
Reflective tarps to cover picked fruit have shown promise in protecting blueberries, but while this option adapted from the forestry industry helps with harvested fruit in high temperatures, it does nothing to protect unharvested berries.
Cranberry growers make use of artificial cranberries from shapeways.com to stay aware of internal berry temperatures. The berry substitutes last all season and perform similarly to real cranberries. It’s another tool that aids in understanding what is happening in the field.
Drip Irrigation
If you can’t take the heat, use a sprinkler
David Bryla, a research horticulturalist with the USDA’s Crops Research Unit in Corvallis, Oregon, has been exploring ways to mitigate damage in high bush blueberries. Studies primarily conducted by his former PhD student, Joy Yang, helped define variables for effective evaporative cooling practices.
While the studies had been ongoing for a few years in Elliott and Aurora varieties, 2021 was an opportunity to put their feet to the fire, so to speak. Damage is worse when the heat is sudden and plants don’t have the opportunity to acclimate, which is exactly what happened during the heat dome.
“Last summer it resulted in a considerable amount of fruit damage,” he said of the heat events. “It can occur in both green and blue coloured fruit in blueberries and it can occur in any cultivar.”
Fruit damage such as softening, shriveling and necrosis (death of tissue) was seen within one to three days after an extreme heat event when the weather was hotter than 35 degrees Celsius and when berries were seven to 11 degrees hotter than the ambient temperature. Berries at the green stage are more vulnerable than at the blue stage, perhaps due to the formation of wax and cuticle layers.
“Aurora was a worst-case scenario,” he says. “Now we know it is very susceptible to heat damage. Elliot is more resistant to heat damage.”
In some cases, Aurora growers had 100 per cent losses of their crops.
“This last summer we saw considerable benefits of evaporative cooling of blueberries,” Bryla says. “We saw significant effects on berry weight. Larger berries, lower brix, I think that’s dilution. We had a measurable effect on fruit firmness with cooling.”
But most fields are now irrigated through drip systems, so there is a need for a secondary system for cooling.
“With drip there was a considerable amount of damage,” he says. “We lost about a pound of fruit per plant.”
Using overhead sprinklers and micro sprinklers, cooling management should begin when air temperature reaches about 32 degrees Celsius during early stages of fruit development and at about 35 degrees during later stages. Yang found that 20-minute cycles (on for 20 minutes, off for 20 minutes) were the most effective cooling method. This kept berry temperatures at about the same level as constant irrigation but without the high levels of water use and increased pest and disease pressures that accompany wet fields.
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Covered fruit growing
“We don’t have much data at this point on evaporative cooling in raspberries because up until this last summer, it’s not something we really expected to need too much of,” he says.
Future studies will look at specifics in blueberry evaporative cooling and the cost benefit of tools for raspberries.
High tunnels as a solution for raspberries
Preston Farris, production manager with Reiter Affiliate Companies, notes that growing raspberries under poly tunnels protects the crop from a variety of environmental factors and can help with heat mitigation. The organization’s entire raspberry crop is grown in tunnels.
“The plastic type is going to be the biggest variable in heat mitigation,” he said of tunnel choices. “The UV plastic can be one tool to diffuse that UV light and protect the berries. The fruit can take a lot of heat, but what it can’t take is a lot of direct UV.”
With UV being the primary cause of sunburn in raspberries, a plastic that assists with diffusing it can help prevent crop damage. However, like anything in farming, there are numerous trade-offs. Timing of crop harvests will be impacted by the levels of UV protection chosen. While the UV blocking is beneficial in high-heat timeframes, it may have a negative impact on spring or fall crops.
Diffusing the UV light impacts pollinators as well. With the smaller size of bumblebee nests, these can be positioned inside the poly tunnels. Honeybee hives are left outside the tunnels, but this leads to challenges if neighbouring farms have crops flowering in open fields.
Different poly manufacturers have different levels of diffusion, shade and other factors inherent in their plastic. If more shade is needed, the tunnel structure provides a frame for shade cloth, creating a level of temporary protection.
“All of these things are balances,” Farris said. “You take away the sun one day, the next day, you’re going to have less.”
In terms of shape and size, wider and taller tunnels generally allow for more airflow, therefore staying cooler than narrower and shorter structures. A longer tunnel is also going to retain more heat than a shorter tunnel due to reduced airflow.
“It’s really important if you’re going to use poly tunnels to think about what your desired outcome is,” he summarized.
Sadly, what felt like a once-in-a-lifetime event is bound to occur again. Growers may not be able to mitigate all their losses, but with ongoing studies and learning from others, there may be ways to reduce the risk.
