Ice encasement treatments on top of annual bluegrass plants in a low-temperature growth chamber. Clear acetate film was wrapped and taped around the pot, and water was misted onto plants at 26.6 F (minus 3 C) until a thick ice layer developed. Photo by Megan Gendjar
A critical process that needs to occur for perennial turfgrass species to endure winter is called cold or fall acclimation. Acclimation to cold temperatures and reduced light duration and intensity is characterized by major internal changes in plant tissues
(1). Internal changes are needed to prepare cellular and tissue structures for harsh winter conditions. Changes to cellular membranes and the location of water within tissue structures can help reduce the possibility of ice crystal formation within
cells and can also impact the functioning of cellular processes during dormancy in structures that overwinter in grasses, such as crown tissue or below-ground structures like rhizomes.
If winter conditions are severe or internal changes during cold acclimation were not sufficient, winterkill of various turfgrass species can occur, particularly on annual bluegrass putting greens. Fall weather can be variable and does not always provide
optimal conditions for cold acclimation processes, leaving plants susceptible to winter damage. Climate change may increase the incidence of variable seasonal conditions, meaning that an understanding of how critical environmental factors influence
cold acclimation and winterkill stress survival is needed.
A major environmental factor that is poorly understood in association with turfgrass survival of winter is soil moisture levels during fall acclimation. Recommendations for fall irrigation practices or winter preparatory strategies could result from a
better understanding of whether drier, optimal or higher levels of soil moisture influence annual bluegrass overwintering and survival.
Figure 1. Soil water content (%) and leaf relative water content (%) during simulated cold acclimation in growth chamber conditions of annual bluegrass plants. Least significant difference values are represented by vertical bars (P ≤ 0.05) for treatment comparisons on a given day of treatment. Means from 2021 and 2022 are pooled together.
To initiate research on how soil moisture during fall conditions impacts winterkill of annual bluegrass, we conducted a controlled environmental growth chamber condition study that monitored soil moisture levels to determine the effect of soil moisture
on annual bluegrass recovery from ice encasement and physiological health parameters. Ice encasement was used as the winterkill stress because it is a severe and complex stress that creates high stress pressure and serves as a great experimental test
for annual bluegrass endurance.
We allowed annual bluegrass plants grown in deep rooting pots — 2.4 inches (6 centimeters) in diameter by 13.8 inches (35 centimeters) in depth — to acclimate to cold temperatures by stepping temperatures down in growth chamber conditions
from 64 F to 50 F to 39 F (18 C to 10 C to 4 C) for at least two weeks at each temperature.
During these decreasing temperature changes, plants were kept at 8% (dry), 12% (optimal) or 20% (high) soil moisture and were monitored with time domain reflectometry in the top 1.5 inches (3.8 centimeters) of soil.
Relative water content of leaves was also measured to gauge plant water content response to soil moisture treatments (Figure 1). Annual bluegrass plants in pots were then exposed to either 0, 40 or 80 days of ice encasement by misting plants at freezing
temperature (26.6 F/minus 3 C) to develop a 1-inch- (2.5-centimeter-) thick ice layer on top of the pots.
To measure the health of plants following treatments, we determined recovery rates following ice encasement by digital image analysis and Canopeo software (2). We also had separate plants to harvest and measure the health of cellular membranes and sugar
content within leaf, crown and root tissues. Elevated levels of malondialdehyde in plant tissues indicates that membranes have incurred damage.
Sugar content was analyzed as total nonstructural carbohydrates, which is a summation of water soluble (free sugars like glucose, fructose and sucrose) and storage carbohydrates (including starches like fructans, i.e., those destined to be used later
via starch breakdown).
The study was performed in 2021 and repeated in 2022.
Figure 2. Annual bluegrass recovery measured by percent green cover taken via digital images and analyzed with software. Recovery was measured in a greenhouse following low temperature and ice encasement treatments followed by a de-acclimation period in growth chambers. Least significant difference values are indicated by vertical black lines on dates when significance was detected (P ≤ 0.05) on a given day of treatment. Means from 2021 and 2022 are pooled together. Note that scales are different on each x-axis.
This study provides initial evidence that drier soil conditions of putting greens may promote overwintering survival following ice encasement conditions. The lower soil moisture treatment during cold acclimation enhanced the speed and extent of recovery
of plants (Figure 2). The average percent canopy cover after 36 days of recovery in the greenhouse was 71.9% higher for 8% soil water content (SWC) treated plants compared to 12% and 20% SWC treated plants.
This observation of better recovery could be related to physiological factors that were altered by soil moisture content during cold acclimation.
Ice encasement duration impacted crown, leaf and root membranes (Figure 3) and carbohydrate contents (Figure 4). Membranes within these organs became more damaged with longer durations of ice encasement, and sugars were generally reduced by ice encasement,
likely since respiration used up these sugars during dormancy or ice encasement stress caused a breakdown of membranes or sugars. Root levels of carbohydrates and lipid peroxidation were least influenced by the ice encasement treatment among the organs
evaluated, which could be related to the protective properties of the soil.
For the physiological factors that were measured, we did not find that soil moisture had an impact on leaf or crown tissue. But for root tissue, soil moisture did influence root sugar content (Figure 4A). Root carbohydrates were influenced by soil moisture
regimes during cold acclimation since Day 0 plant roots exposed to the 8% soil moisture treatment had 143.9% higher total nonstructural carbohydrates and 137.6% higher water-soluble carbohydrates compared with Day 0 plants exposed to 12% and 20% soil
moisture. Whether higher levels of storage and water-soluble carbohydrates could translate into higher survival of annual bluegrass is not known. Another interesting observation is that storage or water-soluble carbohydrates were not greater in crown
tissue compared to leaf tissue at the end of cold acclimation (Day 0, Figure 4B). The crown tissue did have higher carbohydrates than root tissues. The crown, as the overwintering survival organ, would be expected to be prioritized, or, in other words,
provided with ample sugars to prevent ice crystal formation, prevent desiccation, and to serve as starch storage for spring recovery. Field assessments of carbohydrate shifts in annual bluegrass overwintering, studies to compare different cold acclimation
regimes and studies comparing plants overwintering that were not treated with ice encasement to those ice-encased may assist with interpreting this finding.
Dry soil is characterized by higher levels of oxygen than saturated soils since the process of drying causes air to replace water in soil spaces. It is possible that the higher amounts of air in drier soil conditions prior to ice encasement, which can
cause reduced oxygen content in soil and plant tissues, could have played a role in enhanced survival. The enhanced level of root sugars found in plants treated with drier conditions during cold acclimation could have promoted root survival and regeneration
of leaf tissues. Dry fall conditions could have also served as a stress preconditioner to allow annual bluegrass plants to be more winter resilient. However, more research would be needed to evaluate this, particularly studies that monitored oxygen
levels during ice encasement treatment.
While additional controlled studies and field-based evidence should be gathered prior to turfgrass managers adopting any new fall irrigation strategies, if ice encasement is a recurrent problem on some putting green areas, reducing irrigation of annual
bluegrass putting greens during fall cold acclimation could be beneficial. Given that soil moisture did not significantly influence carbohydrate or lipid peroxidation results, except for in roots, additional research is needed to understand the mechanism
associated with the recovery findings described here and if drier conditions may influence crown health physiology in other ways. Simultaneously, we have conducted a related field study using various soil moisture conditions and plant growth regulators.
The results of that study are currently being analyzed and may be important to consider prior to adoption of any watering regime or changes to watering practices.
Figure 3. Malondialdehyde content, which indicates membrane health in plant tissues, of crown, leaf and root tissue following cold acclimation only (0 days) or ice encasement (40 or 80 days). Means from 2021 and 2022 are pooled together and different capital black letters indicate statistically different MDA means in a given plant tissue between dates.
The research says
- Drier soil conditions of putting greens may promote overwintering survival following ice encasement conditions.
- It is possible that the higher amounts of air in drier soil conditions prior to ice encasement, which can cause reduced oxygen content in soil and plant tissues, could have played a role in enhanced survival.
- While additional controlled studies and field-based evidence should be gathered prior to turfgrass managers adopting any new fall irrigation strategies, if ice encasement is a recurrent problem on some putting green areas, reducing irrigation of annual
bluegrass putting greens during fall cold acclimation could be beneficial.
Figure 4. Total nonstructural carbohydrates (TNC) as a sum of water-soluble carbohydrate (WSC) and storage carbohydrate (SC) fractions in root tissues (left) and leaf, crown, and root tissues (right) of annual bluegrass as it relates to 0, 40, or 80 days of ice encasement. Means from 2021 and 2022 are pooled together. Different capital black letters indicate statistically different TNC means and lower case letters represent statistical differences for SC and WSC within their respective bars (P ≤ 0.05).
The authors thank the O.J. Noer Foundation, GCSAA and the Michigan Turfgrass Foundation for funding and supporting this research. The results and figures in this article were adapted from an article previously published by the authors: Leaf, Root, and
Crown Tissue Physiology of Annual Bluegrass after Cold Acclimation at Varying Soil Moisture Levels and Ice Encasement in the Journal of the American Society for Horticultural Science 148(3):99-107 (https://doi.org/10.21273/JASHS05288-22).
- Patrignani, A., and T.E. Ochsner. 2015. Canopeo: A powerful new tool for measuring fractional green canopy cover. Agronomy Journal 107(6):2312-2320 (https://doi.org/10.2134/agronj15.0150).
- Thomashow, M.F. 1999. Plant cold acclimation: Freezing tolerance genes and regulatory mechanisms. Annual Review of Plant Physiology and Plant Molecular Biology 50:571-599 (https://doi.org/10.1146/annurev.arplant.50.1.571).
Megan Gendjar is a research assistant, and Emily Merewitz (firstname.lastname@example.org) is an associate professor in the Department of Plant, Soil, and Microbial Sciences, Michigan State University, East Lansing.