Dr. Amaya Atucha’s presentation at July’s Virtual Cranberry Brown Bag addressed soil water content and irrigation information during cranberry fruit set, development, and ripening. She referenced research done at Laval University and published in 2016 and 2017 as the source for many of her observations, and I promised to review those papers to reacquaint growers with their results.

As we dive in, the researchers note that their research has been done on beds with 6 to 15” of sand over an impenetrable layer. The critical growth stages apply to all cranberries regardless of bed construction, but those with deep sand or those with peat beds should monitor and use judgement before adapting the exact kPa tensiometer cutoffs to their beds.

Cranberries, like all vascular plants, have an optimum range of soil water potential under which they grow best and produce highest yields. In beds similar to those tested by [1], [2], [4], and [5], the overall “wettest” soil that still lets cranberries flourish is -4 kPa. The overall “driest” soil that still lets cranberries flourish is -7 kPa. This is a narrow range! Cranberries can survive conditions wetter than -4 kPa or drier than -7 kPa, but they will not produce top yields.

Too Wet

Cranberries can tolerate too-wet conditions more or less successfully, depending on growth stage. Experimental results show bud elongation and bud set are especially vulnerable to lowered yield from too much moisture. Our understanding of the vines’ nutrient and carbohydrate demands during these stages lines up with these experimental results.

In [4], Pelletier et al. put cranberries in a growth chamber, and saturated the root zone of different plants during 3 different growth stages. They measured photosynthesis directly, and found that bud elongation is when plants suffered most, and most quickly, from lack of oxygen in the root zone. During bud elongation, just 1 day of soil saturation decreased photosynthesis 28% (compared with the unsaturated control). After 5 days of saturation, photosynthesis was down 46%. During flowering, photosynthesis gradually slowed when the root zone was saturated, but was only significantly lower than the control by the 5^th day of saturation. During fruit development (2 weeks after fruit set), photosynthesis was not significantly different from the control, even after 5 days of saturation.  While this study looked at photosynthesis instead of yield because photosynthesis is more direct, it’s a pretty safe bet that the vines which were only photosynthesizing half as well as their neighbors will have lower yield, and so growers should take to keep the water table low, especially during bud elongation.

The other period to keep a sharp eye on drainage and maintain the water table well below the root zone, is during bud formation. In [5], Pelletier et al. measured yield directly, as well as fruiting uprights per area and marketable berries per area. Across 2 years, regions of 3 production fields were maintained all summer using “wet”, “control”, and “dry” watering triggers, as measured with tensiometers. (“Control” was the grower’s usual method, as these growers were all experienced at timing their irrigation based on tensiometer measurements.) 2012 was a wet year, and so the researchers returned to the treatment sites in 2013 to check fruiting uprights per area. The control treatments had 60 fruiting uprights per area the following year, the dry treatments had 57 fruiting uprights per area—but the wet treatments had 38 fruiting uprights per area. Bud initiation seems to drop off when roots are too wet, so make sure roots are well drained as next year’s buds begin to form.

Too Dry

Cranberry vines suffer in predictable ways from being too dry: reduced fruit set [5], heat stress [1], wilting [5], and salt accumulation [1].

Fruit set is when the cranberry upright decides how many berries it can support through harvest. If the plant is stressed by insufficient water during this stage, we measure fewer berries per upright, fewer berries per flower, and fewer berries per area, than in unstressed vines. Monitoring soil water content during fruit set and not letting it become drier than -7kPa will encourage high fruit set.

Heat stress hurts fruit development (yield and quality) when the canopy temperature reaches 90°F and higher. Pelletier [2] recommends turning on irrigation for 20 minutes when canopy temperatures reach this threshold. This irrigation is to cool the leaves, not to recharge the soil water profile, so there is no need to run for longer durations.

During dry periods or when the water table is held too low (below 24”, as measured [1]), capillary rise may slow or stop providing moisture to be taken up by roots. If needed, water table rise or overhead irrigation can be used to recharge capillary action. Capillary action may sometimes be too slow to provide the 2.5mm to 7.5mm of water needed each day to support evapotranspiration–when soil tensiometers read -7kPa (in 6-15” sand), water should be provided to cranberries to prevent wilting.

For those who use subsurface irrigation, it is important to watch salinity levels. Salts from fertilizer applications can wick up in the soil during irrigation, and affect vine development and yield under salty conditions. If you use subsurface irrigation and salt concentrations are increasing in your beds, use sprinkler irrigation to flush the salts lower.

One final note: when Pelletier measured berry weight, the result was U-shaped: overwatering results in smaller berries, AND underwatering results in smaller berries. Berry size is maximized when soil water is kept in the ‘sweet spot’ of -4 to -7 kPa, making tensiometers pay for themselves in 5 to 20 months (depending on farm size and market price) [4].

From [1], the risks that too-wet and too-dry conditions can present to cranberry yield.

From [1]: Areas in which continued research is necessary. Dr. Atucha’s lab is devoting work in 2021 and future seasons to the growth stages and plant needs, nutrient-related needs, during each stage of fruit development, and will share results as they become available.

[1] Guidelines of irrigation and drainage management strategies to enhance cranberry production and optimize water use in North America. Caron, et al. 2017.

[2] Critical irrigation threshold and cranberry yield components. V. Pelletier et al. 2016.

[3] Payback period in cranberry associated with a wireless irrigation technology. Jabet et al. 2016.  https://cdnsciencepub.com/doi/10.1139/CJSS-2016-0011

[4] Cranberry Gas Exchange under Short-term Hypoxic Soil Conditions. Pelletier et al. 2016. https://www.hortau.com/wp-content/uploads/2021/02/17.-Cranberry-Gas-Exchange-under-Short-term-Hypoxic-Soil-Conditions-.pdf

[5] Critical irrigation threshold and cranberry yield components. Pelletier, et al. 2014.

This article was posted in Cranberry and tagged Allison Jonjak, Cranberries, Irrigation.