What Are Growing Degree Days (GDD)?

The Short Version

Growing Degree Days (GDD) measure how much warmth has accumulated since January 1. It is not a count of warm days — it is a running total of heat units, measured in degree-days. When you see “GDD: 98” on your Lawn Sense dashboard, that means 98 degree-days of heat have built up over the course of the year so far.

Why does this matter for your lawn? Because weeds, insects, and grass don’t check the calendar. They respond to accumulated warmth. A crabgrass seed sitting in your soil doesn’t “know” it’s April — it tracks how much total heat it has absorbed, and it germinates when that total crosses a biological threshold. GDD is the best way to measure exactly where you are in that process.

How It’s Calculated

Every day, Lawn Sense computes a simple formula using the daily high and low air temperature:

Daily GDD = (Daily High + Daily Low) ÷ 2 − 50

If the result is negative (meaning the average temperature was below 50°F), the day contributes zero. GDD never goes backward — it only accumulates.

The “50” in that formula is the base temperature. Below 50°F, turfgrass weeds and cool-season grasses effectively stop growing, so those cold days don’t count toward development. This base-50 model is the standard used by Purdue University, Penn State, and most land-grant turfgrass programs across the country.

A Quick Example

Say your area had these temperatures over three days:

Day	High	Low	Mean	GDD
Monday	72°F	48°F	60°F	10
Tuesday	55°F	38°F	46.5°F	0
Wednesday	68°F	52°F	60°F	10

Monday contributes 10 degree-days. Tuesday’s mean was below 50°F, so it contributes nothing. Wednesday adds 10 more. After these three days, 20 degree-days have accumulated.

Over an entire spring, hundreds of these daily calculations add up. By mid-May in Zone 5b (southern New Hampshire, for example), you might see around 100–200 cumulative GDD. That total reflects every warm day since New Year’s — January through March contribute almost nothing because daily means are well below 50°F, while April and May are where the accumulation really starts.

Why Not Just Use Soil Temperature?

Soil temperature tells you what’s happening right now. GDD tells you what has happened over the entire season. Both matter.

A single warm day might push soil temps to 55°F, but that doesn’t mean crabgrass is ready to germinate — there hasn’t been enough sustained warmth for the seed to develop. GDD captures that sustained-warmth picture. This is why Lawn Sense uses both: soil temperature as the primary trigger and GDD as the confirmation that enough seasonal heat has accumulated.

University researchers have found that this combination — soil temp for the current snapshot plus GDD for the seasonal trend — gives more accurate timing than either measurement alone.

GDD Thresholds That Matter for Your Lawn

Crabgrass Pre-Emergent (Spring)

Research from Purdue University suggests that crabgrass germination initiates around 200 GDD (base 50°F). That means you want your pre-emergent barrier in place before you hit 200:

GDD Range	What It Means	What To Do
Below 100	Too early for germination — season is still building	Get your products ready
100–200	Primary application window	Apply pre-emergent now
200–300	Germination likely underway	Use dithiopyr (Dimension) — it has limited post-emergent activity on young crabgrass
Above 300	Pre-emergent window has closed	Switch to post-emergent control

Grub Prevention

Preventive grub control (chlorantraniliprole / GrubEx) is most effective when applied between 300–500 GDD. This timing correlates with Japanese beetle and European chafer egg-laying activity.

The Two Academic Models

If you dig into university extension resources, you’ll encounter two GDD models used for crabgrass timing. Both are valid — they just use different math to reach the same practical conclusion:

Base-50 model — Used by Purdue, Penn State, and most extension programs. This is what Lawn Sense uses. Base temperature of 50°F, with pre-emergent targets around 100–200 GDD.

Base-32 model — Used by Michigan State University’s GDDTracker tool. Base temperature of 32°F, with pre-emergent targets around 250–500 GDD. The numbers are bigger because the base is lower (more days contribute non-zero values), but the real-world timing recommendations come out to the same window.

If you’re cross-referencing your Lawn Sense numbers with MSU’s GDDTracker, you’ll see different numbers — that’s expected. The underlying timing guidance is the same.

How Lawn Sense Calculates Your GDD

Lawn Sense pulls daily high and low temperature data from Open-Meteo, a free weather API that sources data from national weather services. Each day’s temperatures are converted to the base-50 formula above, and the running total accumulates from January 1 of the current year.

The number updates daily and is cached for performance. It reflects actual recorded temperatures for your specific ZIP code coordinates, not regional averages or estimates.

Further Reading

The GDD research behind Lawn Sense’s timing recommendations comes from peer-reviewed turfgrass science at major land-grant universities:

• Purdue University Turfgrass Science — “When Will Crabgrass Germinate?” and “Use Growing Degree Days to Better Time Your Applications”
• Michigan State University — GDDTracker and “Timing Crabgrass Preemergence Applications”
• West Virginia University Extension — “Using Growing Degree Days to Predict Germination”
• UW-Madison Turfgrass Science — Crabgrass Pre-emergence Timer
• Cornell Northeast Regional Climate Center — Turfgrass Degree Days