How Cranberries Grow: Fall Frost

During the fall months, cranberry growers need to monitor their bogs for frost on cold nights. Since cranberry bogs are usually the lowest feature in the landscape, they typically can be 10-20 degrees cooler than the surrounding uplands. The cranberry plants contribute to radiational cooling, which can occur on clear, calm nights in the fall. These conditions can lead to temperatures that can damage the fruit, otherwise known as frost damage.

As the cranberries mature and turn from green to red in color, they are able to withstand colder temperatures. In late summer, green fruit can tolerate temperatures as low as 28°F. As the season progresses, a red blush begins to form on the fruit, providing a visual indication that lower temperatures may be tolerated by the ripening fruit. Each variety of cranberry has a different tolerance, that changes as the fruit ripens. As the fruit turn darker red, their frost hardiness increases. This tolerance only changes if the fruit then become over ripe, which occurs with late season fruit (late October and beyond) and then the tolerance begins to lessen again. The University of Massachusetts Cranberry Station has studied fall frost conditions and has determined the fruit tolerances that growers use as their working guidelines for when to frost protect. According to the published guidelines, the lowest safe temperature that cranberries can withstand is 23°F.

In order to protect the fruit from frost damage, growers run their irrigation system, coating the fruit with a blanket of water. Water protects the plants through the law of physics known as the Heat of Fusion. As the sprayed water coats the plants and berries, it begins to freeze. The physical process of water turning to ice releases heat and this heat warms the plants. As long as water continues to freeze and release heat, the berries are protected. Growers generally start their irrigation systems 2-3° above tolerance to insure that they have correctly established the plant's tolerance and that their temperature probes (place in the cranberry bogs) are recording accurately.

A relatively new technology to the cranberry industry, automated irrigation systems enable growers to automatically turn on and off their irrigation pumps.  These systems use sensors placed within the cranberry vines to monitor temperature and other weather conditions. These systems are fully-controllable via the Internet and growers can log onto a secure site to monitor and manage each pump.  On average, a recent survey showed that growers utilizing automated irrigation systems can save more than 9,000 gallons of water per acre on a frost night.  In addition, there are substantial savings in the amount of wear on the pumps, as well as on labor and fuel costs.

View a slideshow on Fall Frost in cranberry agriculture.

For more information on the first implementation of automated irrigation systems in cranberry agriculture, download the Conservation Innovation Grant Final Report.