GROWING DEGREE DAYS (GDDs)

If you are a new user or want to learn more about GDD recommendations in wild blueberry management, please scroll down to the introduction section.

We will select several wild blueberry weather stations each season to report their seasonal GDD development. Here are the stations to which we will report their GDDs in 2025. We intend to give updates from April 1 to October 31.





 

 


 

We will begin updates after April 1, 2025.

 

 

 

Figure 1. Selected Wild Blueberry Weather Stations and GDD Updates (April 1- April 1)



Introduction (updated on March 24, 2025):

This GDD page was created to provide wild blueberry growers with information on using GDD and the latest GDD recommendations in wild blueberry management.

What is GDD, and how can GDD support your decision-making in wild blueberry field management?

Growing Degree Days (GDD) is a weather-based indicator that tracks daily temperatures to estimate plant and insect growth and development. GDD is a supporting tool that improves growers’ field activity planning, such as pesticide applications, beehive introductions and harvesting.

In wild blueberry production, key field activities are conducted based on the crop stages. Research projects have been undertaken to understand wild blueberry plant physiology better and provide GDD recommendations on key stages, such as plant emergence, fruit bud F2, bloom, and fruit stages. The following section will detail GDD recommendations for Nova Scotia wild blueberry management.

To get started with the GDD tool and use it to support your wild blueberry management, we recommend growers select the closest weather station for better accuracy. You can choose your station from Perennia’s Farm Data Tools GDD Calculator or view all weather stations in wild blueberry fields through this link: http://www.novascotiawildblueberryblog.com/p/weather.html.

Growers can obtain cumulative GDDs through different means, but we encourage growers to use the GDD Calculator in the Farm Data Tools platform. Each station’s cumulative GDDs can be compared with recommended thresholds to get an idea of the local wild blueberry stage, which requires further confirmation by field visits before making decisions.

GDD is another method of giving growers better estimates on crop stages so you can plan activities by comparing the latest local cumulative GDD to recommended numbers. By employing GDD in your wild blueberry management, growers can improve crop stage estimation and gain confidence before conducting field activities.

Here is a factsheet to help you understand GDD in Wild Blueberry Production: https://www.perennia.ca/wp-content/uploads/2018/03/Understanding-Growing-Degree-Days-in-Wild-Blueberry-Production.pdf.

 

Local Wild Blueberry Growth Stages’ GDD Recommendations

In this section, we summarize the latest GDD recommendations from researchers and quickly explain some key points when using them for activity planning.

Sprout Field

The sprout-year wild blueberry GDD model was initially developed by Dr. Scott White (DAL-AC), who studied plant emergence and tip dieback stages and their GDDs. With his continuous efforts and research in recent years, Table 1 shows the latest GDD recommendations for plant emergence and tip dieback.

Table 1. Wild Lowbush Blueberry Growing Degree Days Model (White 2012)

*GDD start calculation date: April 1; Base T= 0 °C.

Plant emergence GDDs and sprout field management: The status of plant emergence in sprout fields is critical in deciding whether to conduct key field activities (Figure 2), such as herbicide and fertilizer applications. We have the following recommendations to help growers’ planning, but we strongly recommend that you validate them before implementing them.

A close up of a pile of sticks

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Figure 2. Wild Blueberry Plant Emergence

 

·       Wild blueberry plant emergence starts at 213 GDD, and growers are encouraged to apply pre-emergence herbicides between 150-200 GDDs. Most pre-emergence herbicides (Chikara, Ignite, Spartan, Velpar…) should be used before wild blueberry emergence starts or below a low emergence percentage (5-10%) for crop safety and better efficacy.

·       50% plant emergence is another critical point for growers to watch, which can help you decide when to apply post-emergence herbicides, such as Callisto 480 SC, Venture L, Poast Ultra, etc. After 50% plant emergence, most wild blueberry broadleaf and grass weeds are expected to emerge and reach control points. For example, it is effective to apply Callisto to control narrow-leaved goldenrods when pest plants reach about 20 cm or around 400 GDD in local conditions.

·       Fertilizer applications. Growers might consider applying granular fertilizers to sprout fields in the spring. After fields reach 50% plant emergence, wild blueberry plants are expected to require more energy for development. Fertilizer application at this time will benefit plant growth.


Crop Field

In wild blueberry production, more crop stages and activities occur during the crop year than in the sprout year. The industry conducted research projects to understand three crucial wild blueberry crop growth stages and their GDDs to support decision-making, including fruit bud F2 (Figure 3), bloom (Figure 4) and fruit stages (Figure 5).

Perennia and WBPANS conducted the studies from 2022 to 2024. We studied wild blueberry sites across Cumberland, Colchester, and Pictou to establish GDD thresholds for those three growth stages. We present the crop year's GDD recommendations based on this project and local practices. Because of regional variations, we encourage growers to validate the actual field crop stage after referencing the recommended numbers.

Fruit bud F2 stages (Figure 3) and Monilinia Blight Management. According to the industry standard, blueberry fruit buds above 40% F2 are susceptible to blight infection; therefore, fungicides should be applied to protect crops during the susceptible period (40-60% F2).

Figure 3. Wild Blueberry Fruit Bud F2 Stage


Table 2. Wild Blueberry Fruit Bud F2 Stages and GDDs

 
 *GDD start calculation date: April 1; Base T= 0 °C.

Bloom stages& GDDs, and field activities during the bloom period. Fungicide applications and beehive introductions during the bloom period are critical to successful disease prevention and pollination.

·       Early Bloom Botrytis Blossom Blight Control. If your fields require fungicide sprays for Botrytis management, and you want to complete it before bringing bees, 10-25% (353-379 GDD) would be a suitable window.

·       Pollination. Blueberry growers should coordinate well with their bee suppliers to ensure timely beehive placement in crop fields. Table 3 gives growers baselines to track bloom development in their areas, which can be used to determine your bee schedule based on personal preference and beekeepers’ availabilities. 

 

Figure 4. Wild Blueberry Blooms

 

 

Table 3. Wild Blueberry Bloom Stages and GDDs.

      *GDD start calculation date: April 1; Base T= 0 °C.

 

Fruit ripening stages& GDDs. Fruit ripening information can help growers manage insects (SWD and Blueberry Maggots) and determine harvesting schedules. Please refer to Table 4 and the recommendations below for fruit stage management.

·       Insect traps and monitoring. Growers are encouraged to place insect traps when fields reach the early ripening stage, which can help determine insecticide application schedules.

·       Field harvestable point. We recommend growers use 1700 GDD (around 90% ripening) as a reference point to begin harvesting if conditions are favourable. 

 

 


                                                       Figure 5. Wild Blueberries

 

Table 4. Wild Blueberry Fruit Ripening Stages and GDDs


*GDD start calculation date: April 1; Base T= 0 °C.

 

This introduction section was updated on March 24, 2025.