Image: The cycle of growth, fall acclimation, and dormancy that a perennial fruit crop goes through each year. These stages are all necessary for its growth, reproduction, and survival, and can provide a lens that helps growers better understand fruit production. Infographic by Madeline Wimmer.
Words to be familiar with:
- Fall acclimation: A series of physiological changes triggered by external and internal factors that prepare a perennial plant to survive the winter and have enough carbohydrate reserves to support next year’s bud break, before mature, photosynthesizing leaves have developed.
- Dormancy: the portion of a plant’s life cycle when a number of physiological changes occur that allow the plant to overwinter in a less active state.
- Endodormancy: Also referred to as true dormancy, this state happens when plants first acclimate and go into dormancy. This state usually remains until a certain number of chilling hours have accumulated.
- Chilling hours: Hours where the ambient temperature is between 32 - 45 degrees fahrenheit. The internal clock that keeps the plant on track to exit dormancy during the spring.
- Ecodormancy: This is the dormancy state plants enter after endodormancy. This state is more sensitive to external factors like temperature, after which point, the plant exits dormancy and bud development begins.
- Paradormancy: This state of dormancy is influenced by other plant parts. Buds that develop during the growing season that will not break until the following growing season, are an example of paradormancy.
The method by which it does this involves a gradual change in various plant growth regulators (e.g., abscisic acid/ABA, gibberellins/GA, and Ethylene) that are involved in entering and maintaining dormancy. This and other physiological processes that occur when ambient temperatures are between 32° and 45 ° F and is referred to as chilling. Below this temperature range, biological activity is slowed down, and the chilling process becomes less effective. Warmer temperatures above this range will also disrupt the chilling process.
The amount of chilling hours a specific fruit crop variety needs is variable, and chilling hours are best used as an estimation, rather than an exact science. The range of chilling hours required for temperate fruit crops can be as little as 250 hours (e.g., non-hardy, Vitis vinifera grape varieties) or as long as 1000 hours or more. Typically, fruit varieties that are successful in colder climates will have a longer chilling period requirement, which prevents early bud-break—a phenomenon that is especially a concern in regions and years with variable winter temperatures.
It can be helpful to note that plants that cannot fulfill their chilling requirements also have challenges and can exhibit uneven budbreak; this is why some warm regions in the world are also limited in varieties they can reasonably grow. When selecting varieties, some nurseries may have resources listing chilling requirements for different varieties. Because information is often limited, growers can refer to USDA growing zone requirements, which correlate with chilling requirements, to guide good plant selection for cold climate regions. Additionally, knowing the average date when bud break occurs can be helpful.
After a fruit tree, vine, or shrub has accumulated enough chilling hours by late winter, or early spring it enters ecodormancy, a state where plants are more sensitive to external factors as a cue to exit dormancy and begin bud development. Temperature plays an important role in this, but day length may also contribute to bud break in some fruit crops like cold hardy grapes. When bud development begins, its rate of development is impacted by temperature, where cold temperatures slow down bud development. As a general trend, less developed buds are the most hardy and gradually lose hardiness as they continue to develop.
The ideal situation is for buds to develop at a rate where, by the time bloom or shoot growth begins, the plant is out of harm's way from damaging frost and cold temperatures. When bud development begins too early, this makes a plant more vulnerable to being damaged, and potentially lead to crop loss, which is why a fast accumulation of chilling hours can be concerning.
Knowing about the processes of fall acclimation, dormancy, and chilling requirements can help growers understand the variation in dormancy, winter survival, and a number of other factors. Keeping note of when your fruit crops fully enter dormancy, chilling hour accumulations starting from that day, and also noting when bud development begins is a way that you can start to understand the trends from year to year.
The Midwest Regional Climate Center website offers information about chilling hour accumulations, as well as departures, or differences of the current season as compared to previous years. Their website is currently transitioning data, and right now offers an alternative chilling hour tracking tool growers can use. Be sure to check the start and end dates when using the tool. Additionally, this tool is limited in that it only captures data from a select number of areas in Minnesota. Growers who have a weather station on site and are interested in tracking their own hours, or are able to access hourly data from a weather data service—like this one from Visual Crossing Weather—can import data into a spreadsheet to calculate chilling hours. Those who are making their own calculations can refer to the Visual Crossing Weather resource below for specific formulas that can be used in Excel.
Resources:
- Calculating chill hours for fruit plants and trees - Visual Crossing Weather
- Escaping spring frost in the Upper Midwest - UMN Fruit and Veg News
Author: Madeline Wimmer, UMN Extension fruit production educator.
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