The Dance of Time and Temperature: Understanding Phenological Clocks and Growing Degree Days in Apiculture
For centuries, beekeepers have relied on nature’s calendar to guide their hive management decisions. Today, this ancient wisdom meets modern science through two powerful tools: the phenological clock and Growing Degree Days (GDD). These interlinked concepts help beekeepers predict and prepare for the intricate ballet between flowering plants and honey bee colonies.
The phenological clock, nature’s own timekeeper, tracks the sequential blooming of different plant species throughout the season. When paired with GDD calculations—a measure of heat accumulation that drives plant development—beekeepers gain remarkable insight into the timing of nectar flows. This marriage of observation and measurement transforms the age-old question of “When will the flowers bloom?” from guesswork into a more precise science.
In grassland ecosystems, where many important nectar sources flourish, understanding these biological rhythms becomes particularly crucial. From early spring clovers to late summer wildflowers, each plant species requires a specific accumulation of thermal energy to reach the flowering stage. By monitoring GDD sums, beekeepers can anticipate these flowering events weeks in advance, optimizing their colony management strategies and potentially increasing honey yields.
The Science Behind the Sum: Understanding GDD Calculations
Growing Degree Days represent the accumulation of thermal energy that drives plant development. The calculation begins with a simple yet powerful formula: subtract a base temperature (typically 5°C for most grassland species) from the daily average temperature. This base temperature represents the thermal threshold below which plant growth effectively stops. The calculation can be expressed as:
Any negative values are recorded as zero since plants don’t “lose” development progress on cold days. The daily values are then summed throughout the growing season, creating a running total correlating with plant development stages. Consider it a bank account where temperature deposits accumulate until enough “heat units” have been saved to trigger flowering.
The daily values are then summed throughout the growing season, creating a running total correlating with plant development stages. Consider it a bank account where temperature deposits accumulate until enough “heat units” have been saved to trigger flowering.
The following is an example of how to calculate and cumulate the GDD:
The Base Temperature
The base temperature is the minimum temperature necessary for plant growth. It is the temperature below which plant development essentially ceases. For most plants, including those that provide nectar for honey production, this base temperature is 10°C.
Here’s where it gets interesting for honey crops: Different plants exhibit various base temperatures. White clover (Trifolium repens) is a significant nectar source in Irish pastures and meadows, with a base temperature of around 5°C. This means it accumulates growing degree days (GDDs) relatively early in the spring. Conversely, heather (Calluna vulgaris), known for producing distinctive late-season honey, has a higher base temperature of approximately 10°C, requiring warmer weather for growth and development than white clover.
Why does this matter? The base temperature influences when plants begin growing and flowering each spring. A lower base temperature allows a plant to accumulate GDDs earlier in the year. An alfalfa field and a basswood forest in the same region may follow very different development timelines due to their varying base temperatures.
Understanding base temperatures assists beekeepers in predicting when major nectar flows will commence. Knowing the GDDs needed for a plant to flower (its flowering threshold) along with its base temperature, you can estimate flowering time by monitoring GDD accumulation from the start of the season.
White clover (Trifolium repens) is a crucial nectar source in Irish pastures and meadows. With a base temperature of around 5°C, once the daily average temperatures consistently rise above 5°C in the spring, white clover grows and accumulates GDDs.
To calculate GDDs, we take the average daily temperature and subtract the base temperature. For example, if the average temperature on a spring day is 13°C, white clover would accumulate 13°C – 5°C = 8 GDDs that day.
Let’s compare this with heather (Calluna vulgaris), which produces distinctive late-season honey in Ireland. With a higher base temperature of around 10°C, heather requires warmer weather to initiate growth than white clover.
On a spring day with an average temperature of 13°C, white clover, with its base of 5°C, would accumulate 8 GDDs (13°C – 5°C). Meanwhile, the same day for Heather contributes only 3 GDDs (13°C – 10°C).
These differences accumulate throughout the season. White clover may reach its flowering threshold and peak nectar flow while heather is still in its early growth stages. This explains why clover honey is generally harvested in early to mid-summer, whereas heather honey is gathered later, often in August or September.
Here’s a mental exercise to help visualise this: Imagine it’s early spring, and the daily average temperatures are beginning to surpass 5°C. Your white clover starts to grow, albeit slowly at first while accumulating GDDs each day. In contrast, your heather remains dormant, awaiting those warmer days exceeding 10°C.
As the season progresses and temperatures rise, the growth of white clover accelerates. It reaches its flowering threshold, potentially around 800 GDDs, allowing bees to gather its nectar. Meanwhile, Heather is only just beginning its growth spurt.
By mid-summer, the clover nectar flow starts to diminish, while the heather is just beginning to flourish thanks to its higher base temperature. It rapidly accumulates GDDs in the summer warmth, and by late summer, it is in full bloom, providing late-season nectar for the bees.
Understanding these growth patterns, influenced by base temperatures, aids beekeepers in anticipating and managing nectar flows. Around expected bloom periods, you can schedule hive management activities, such as adding honey supers or harvesting.
The interaction of temperature, plant biology, and beekeeping strategy is fascinating. Grasping the concept of base temperature and its impact on the growth and flowering of different honey plants enhances your understanding and ability to predict the beekeeping season’s rhythm.
Of course, this is a simplified model. Factors such as day length, soil moisture, and microclimate also play significant roles. Nevertheless, the concepts of base temperature and GDD provide a valuable framework for comprehending and navigating the seasonal development of honey plants.
GDD sums are relevant to beekeeping and honey production because they help predict flowering times and nectar flows. Here’s how:
- Flower Prediction
- Different plants that bees forage on have specific GDD requirements to bloom
- By tracking GDD sums, beekeepers can predict when important nectar sources will flower
- This helps in planning hive management around major nectar flows
- Nectar Production
- Temperature affects nectar secretion in flowers
- Optimal temperatures (usually between 16-32°C) result in better nectar production
- GDD helps track if conditions are favourable for nectar secretion
- Colony Management Applications:
- Timing colony build-up to coincide with expected nectar flows
- Planning when to add honey supers
- Predicting when to move hives to different forage locations
- Estimating potential honey yields based on flowering predictions
- Specific Plant Examples – GDD Table for Irish Honey Plants
