Growing Degree Days (GDD), or heat units, are a practical, science-based way to track plant and pest development. Home gardeners can use GDD to better time planting, monitor insect activity, and anticipate key stages like bloom and fruit set. This approach relies on temperature, one of the main drivers of biological growth.
What Are Growing Degree Days?
Growing Degree Days measure heat accumulation over time. Most plants, insects, and diseases develop in response to temperature. GDD captures this process by estimating how much heat is available for growth on any given day.
Each species has a minimum temperature, called a base temperature, below which development slows or stops. By tracking how much daily temperature exceeds this base, gardeners can estimate the progress of a plant or insect life cycle.
How Growing Degree Days Are Calculated
GDD are calculated using daily maximum and minimum air temperatures. To calculate, use this formula:
GDD = ((Tmax + Tmin) ÷ 2) - Tbase
- GDD = Growing Degree Days
- Tmax = the daily high temperature
- Tmin = the daily low temperature
- Tbase = the base temperature for the plant or insect of interest
If the result is negative, it is recorded as zero since no growth occurs below the base temperature. These values are added over the season to create a cumulative total.
For example, if the high is 75°F, the low is 55°F, and the base temperature is 50°F:
GDD = ((75 + 55) ÷ 2) - 50 = 15
That day contributes 15 growing degree days.
Understanding Base Temperatures
Many crops and pests use a base temperature of 50°F, but some require different base temperatures depending on when they start seeing growth. Because species vary in their temperature requirements, different base temperatures are used to improve timing and accuracy.
Common Base Temperatures
- 40°F: Cool-season crops and early spring perennials
- 50°F: Many insects, turfgrass pests, and common phenology models
- 52°F to 55°F: Some fruit crops and insects
- 60°F and higher: Heat-loving plants and warm-season pests
Finding GGD Data for Your Area
Don't worry, you don't need to calculate the GDD for every day and add them up yourself. Several GDD calculators or maps are available. These tools simplify tracking and often include models for common crops and pests.
Iowa State University Growing Degree Day Map
To use this map, select the base temperature from the dropdown menu (base=50 is most common), then select the start date and year. GDD rarely accumulates before March 1st, so the start date can be set for then. The end date can then be set to the current day. Then click the "Update Map" button and zoom in on your location to see the GDD for your area.
Why Growing Degree Days Matter in the Garden
Calendar dates alone can be misleading because the weather varies from year to year. GDD provide a more accurate measure because they reflect actual conditions experienced by plants and pests.
Using GDD helps gardeners:
- Improve the timing of planting and transplanting
- Predict bloom and fruit development stages
- Monitor insect and disease development
- Schedule scouting and management actions
Phenology and Its Connection to GDD
Phenology is the study of recurring biological events and their relationships with weather and climate. Examples include leaf emergence, flowering, insect emergence, and bird migration. While calendar dates vary from year to year, phenological events tend to align closely with accumulated heat.
Phenology and GDD are used in the same way. Penology connects what you see in the garden to underlying temperature patterns, and GDD quantifies those temperature patterns. Over time, combining phenological observations with GDD tracking strengthens decision-making.
Using Indicator Plants
Gardeners often observe “indicator plants” to signal seasonal progress. For example:
- Lilac bloom often coincides with the emergence of certain insect pests
- Crabapple bloom can signal timing for apple pest management
These relationships reflect shared responses to temperature. GDD help explain and predict these observations more precisely.
Using GDD to Track Plant Development
Plants progress through predictable stages based on accumulated heat. By tracking GDD, gardeners can estimate when key stages will occur.
Germination and Emergence
Many seed packets include days to germination, but GDD can refine that estimate. If temperatures are cool, germination may take longer, even if calendar days suggest otherwise.
Bloom Timing
Flowering often corresponds to a specific GDD range. Tracking GDD can help predict when ornamental and fruit crops will flower.
Fruit Set and Harvest
Fruit development also depends on heat accumulation. Gardeners can use GDD to estimate harvest windows and plan picking schedules more effectively.
Using GDD for Pest Management
One of the most valuable uses of GDD is predicting insect development. Many insect pests have well-documented GDD thresholds for egg hatch, larval stages, and adult emergence.
Insect Life Cycles
Insects are cold-blooded, so their growth closely follows temperature. By tracking GDD, you can predict when a pest will be active and time scouting or control measures accordingly. For example, egg hatch of certain caterpillars occurs at a specific GDD accumulation. Beetle emergence may follow a predictable heat unit threshold after that. This allows for timely intervention, which is often more effective than reacting after damage appears.
Timing Control Measures
Applying controls at the right stage is critical. GDD help identify when pests are most vulnerable, such as newly hatched larvae. This improves control success and reduces unnecessary pesticide use.
Using GDD to Monitor Plant Diseases
Some diseases are also influenced by temperature. While moisture is often a key factor, heat accumulation can signal when conditions are favorable for infection or spread. For example, certain fungal diseases develop rapidly within specific temperature ranges. GDD models can indicate periods of increased risk. By combining GDD with weather observations, gardeners can better anticipate disease pressure and act early.
GDD Resources
There are many resources available to provide growing degree days for important phenophases (stages in an organism's life cycle) for a variety of plants and insects.
Degree Days for Common Insect Pests | University of Wisconsin
Degree Days for Common Landscape Trees and Shrubs and Their Insect Pests
GDD of Landscape Insects | University of Michigan
GDD of Conifer Insects | University of Michigan
GDD Tracker for Turfgrass Pests, Diseases, and Weeds | University of Michigan
Limitations of Using Growing Degree Days
Growing Degree Days are a valuable tool, but they have limits. GDD models rely only on temperature, so they do not account for other important environmental factors such as rainfall, soil moisture, day length, and extreme weather events. For example, drought stress or excessive rainfall can slow plant growth or alter pest activity even when GDD totals suggest normal development.
Microclimates within a garden, such as shaded areas or urban heat effects, can cause temperatures to differ from those recorded by nearby weather stations. GDD models also assume average responses, yet plant varieties and pest populations can vary in their development rates.
For these reasons, use GDD as a guide rather than a precise predictor. Combine GDD tracking with direct observation and good recordkeeping to make the most informed management decisions.
More Information
Photo credits: 1: artrachen AdobeStock; 2: Michael O'Neill AdobeStock