Throughout the year, various threats to all or parts of the cranberry plant may translate to a less than optimal crop. One key aspect of the grower’s role is to anticipate these threats, monitor the marsh and its environment, and take action when necessary. Sometimes, the action is relatively simple—such as turning on sprinklers at the right time—but the decision-making behind it is complex. In springtime, for example, there is no one temperature that signifies that a frost will threaten the marsh. Instead, the tender buds—which are vulnerable to damage by frost—have different temperature tolerances based on how far they have developed. The more advanced they are, the less tolerant they are of cold (and so they must be protected at higher temperatures). Growers must know what is happening on the marsh and the overall development of the buds in order to identify when dropping temperatures pose a problem. In addition to the bud development stage, other factors determine risk of damage, including location and variety:
Location: Temperature on the planted area of the marsh is critical, as opposed to the temperature of upland areas. For example, the lower areas of the marsh in which the vine is planted can be subject to temperatures ten or more degrees Fahrenheit colder than the upland areas. Special monitoring is therefore required.
Variety of cranberry: While all buds become more sensitive to lower temperatures as they develop, different varieties are at risk of injury at different temperatures. In addition, different phases in development also determine each variety’s particular temperature sensitivity. To manage a threat, growers spray the marshes with water for the duration of the intolerable temperatures (a few hours, a day, etc.). Although at first it may seem counter intuitive to spray water on the plant—after all, it will freeze —this is effective because as water freezes, the ice-water system stays at a constant temperature: 32 degrees Fahrenheit. Thus, the constant water bath provides a buffer from even colder temperatures. In addition, the ice itself is a wind barrier, creating a second protection for the plant. As long as water is continually applied to the vines, a small air layer is trapped between the plant layer and the ice layer. If the pumps stop suddenly and the spraying ceases, the air layer is lost and the frost adheres directly to the plant causing damage.
It may seem a good idea to regularly spray the plants with water so that growers need not monitor temperatures. In fact, this is not the case. Too much water, especially in the spring, can foster disease and pest growth. Also, water is a precious resource and must be managed carefully. It is of prime importance, therefore, for growers to know if and it is truly necessary to spray water on the plants. They must realize when the temperature is a definite threat to the buds.
Growers must use their knowledge of the bud’s development, weather forecasts, and a bit of foresight to determine the trigger temperature at which they will turn on the water. Usually, this is about 3-5 degrees Fahrenheit higher than the minimum temperature the buds can tolerate at their given stage of development.
Some growers are fortunate to have high-tech resources to assist them. These resources might include automatic temperature alarms, temperature-triggered irrigation systems, and/or remote access to control valves that set the watering system to respond to pre-determined conditions. Automatic phone call systems set up by agencies that support agriculture can also serve to alert growers to the need for action. Less convenient options require more personal monitoring and a physical presence on site.
If all goes well, the buds make it through this vulnerable time. If, however, there is damage, it can be recognized by plant color as well as actual bud condition. Walking the marsh beds is an integral part of keeping tabs on what’s happening, so that growers can adapt to the on-site, in-the-moment conditions.
The following information addresses the differences between the advanced version and the simplified versions of the challenges:
1. For older students (advanced version), the resources provide information on upland temperatures as well as the temperatures in the planted areas. In the real world, temperatures on the lower, planted regions can typically be lower (by 10 degrees F or even more) than the upland. This level of complexity will make the situation more challenging for students, and will require that they practice the skill of sifting through more information than they need for what is relevant to a problem. For younger students (simplified version), this detail is not added. Students are provided with planted area temperatures only.
2. For older students (advanced version), the different marshes are planted with different cranberry varieties. The tolerance of different bud stages varies with variety, so students must attend to detail to manage the situations properly. This difference between marshes can also add an opportunity for deeper discussion in the Growers’ Association meeting. For younger students (simplified version) all students examine the same data, for the same variety of cranberry. This enables you to run a streamlined whole class discussion to support students as they work.
3. For older students (advanced version), the same temperatures may or may not mean the same risk to both marshes, because the marshes are growing different varieties. The advanced challenge offers an opportunity for students to experience a more realistic model of the real world. For younger students (simplified version), there are no differences between the two marshes, and therefore the appropriate response is the same, making it more likely that all students will be able to track the classroom information and discussion without confusion.
Note: For additional detail, please review the information in the Learn About Cranberries Web resources listed for this lesson.