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Growing Degree Days for Lawn Care: What GDD Is and How to Use It

Published July 2, 20269 min readlawn care

Quick Answer: What Growing Degree Days Tell You

Growing degree days (GDD) count the heat your lawn has banked over the season. Each day adds the amount its average air temperature runs above a base temperature, and days below that base add nothing. The running total is a far better guide to plant and pest development than the calendar, because a warm March and a cold one put the lawn in very different places on the same date. This site accumulates GDD at base 50°F, from January 1, using the simple-average method: daily GDD = (high + low) / 2 − 50, never less than zero. Turf managers read the accumulated total to time crabgrass pre-emergent, annual bluegrass seedhead suppression, growth-regulator reapplication, and grub control.

Quick Answer

See your local growing degree days and soil temperature

Enter your ZIP code to see the growing-degree-day total from the nearest NOAA station paired with your current soil temperature, so you have both the season's accumulated heat and what the ground is doing today.

The Model: Base 50°F, From January 1, Simple Average

A growing-degree-day total is only meaningful when it travels with two numbers: the base temperature and the start date. The formula this site uses is the simple-average method that USA-NPN and Michigan State Extension both publish:

Daily GDD = ((daily high + daily low) / 2) − base temperature

If the daily average falls below the base, the day contributes zero, never a negative number. You add up the daily values from the start date to get the accumulated total.

Two choices define our number. The base is 50°F, the standard threshold for turf and warm-season weed development that Cornell's Northeast Regional Climate Center uses for turf degree days. The start date is January 1, the accumulation start USA-NPN uses for its base-50 maps and the one Iowa State uses for its base-50 insect models.

Here is the part that trips people up: the start date is not universal. The widely used Michigan State GDD Tracker starts counting on February 15, and Cornell's turf maps start on March 1. None of these is wrong; they are just different models. What matters is that you never compare a total from one model against a total from another without checking that the base and start date match. A base-50 total from January 1 and a base-32 total from February 15 are different measurements that happen to share a name.

Always Read the Model, Not Just the Number

Michigan State Extension makes the point directly: different crops and pests use different base temperatures and different start dates, so a bare "GDD" figure is ambiguous. The MSU GDD Tracker even runs its crabgrass timing at two separate bases (50°F and 32°F), and its annual bluegrass and growth-regulator models at base 32°F. When you pull a GDD threshold from any source, confirm its base and start date before you act on it. The number on this site is base 50°F from January 1.

Growing Degree Days Versus Soil Temperature

These two signals answer different questions, and this site shows both because both matter.

Growing degree days are an air-temperature accumulation. They measure how much heat has built up over the whole season, a running tally that only climbs. USA-NPN and Michigan State both compute GDD from daily maximum and minimum air temperature. The total tells you where the season is in its development, which is what most pest and plant-development timing depends on.

Soil temperature is a point-in-time reading. It tells you how warm the ground is right now, and it rises and falls with the weather. Soil temperature is the better signal for germination decisions, because that is the layer seeds actually sit in.

The two can diverge in useful ways. Purdue points out that forsythia bloom, a classic spring cue, tracks air temperature, while crabgrass germination tracks soil temperature. That is exactly why an air-GDD estimate of germination carries some uncertainty: the thing you are predicting responds to the ground, not the air. Pairing accumulated air GDD with current soil temperature, which the card on each city page does, gives you both the season's banked heat and today's ground conditions in one view.

How Turf Managers Use Accumulated GDD

Growing degree days are a timing signal read against a model, not a fixed calendar date. Here are the applications that drive turf GDD tracking, with the base each model uses called out, because the base is not the same across them.

Crabgrass Pre-Emergent (Base 50°F, With a Range)

Purdue and Kansas State associate crabgrass germination with roughly 150 to 200 GDD at base 50°F. Forsythia bloom, often cited as the trigger, actually lands around 1 to 25 GDD, far too early; it is a "get ready" cue, not proof of germination. Treat the 150 to 200 range as a window closing in, then confirm with soil temperature before applying, since germination tracks the soil. Note that some pre-emergent timing tools use base 32°F instead, so check which base a recommendation is built on before you compare it to the base-50 number here.

Annual Bluegrass Seedhead Suppression (Base 32°F)

Suppressing Poa annua seedheads with ethephon (Proxy), often tank-mixed with trinexapac-ethyl (Primo), is one of the classic GDD applications. Kansas State and the MSU GDD Tracker time the first spring application to roughly 150 to 200 GDD at base 32°F. The base here is 32°F, not 50°F, a different model from the crabgrass number above, and a clear illustration of why the base has to travel with the total.

Plant Growth Regulator Reapplication (Base 32°F)

Trinexapac-ethyl regulation fades and then rebounds if you wait too long between applications. The well-known model, summarized by Cornell's turfgrass program, reapplies at a fixed physiological interval of about 200 GDD (base 32°F / 0°C) rather than on a set number of calendar days. Tracking GDD instead of days keeps the regulation steady through the swings of spring and fall. Again, base 32°F.

Grub and Beetle Emergence (Base 50°F)

Adult insect emergence is heat-driven and maps well to GDD. Iowa State Extension puts Japanese beetle adult emergence beginning around 1,030 GDD at base 50°F (accumulated from January 1) and continuing to roughly 2,150 GDD. Adult emergence is the lead indicator for the egg-laying and hatch that preventive grub control targets. Our grub control timing guide covers the soil-temperature side of that decision.

Why the Bases Differ

Base 50°F suits warm-season development like crabgrass and beetle activity, which get going once it is genuinely warm. Base 32°F models start accumulating from near-freezing, which fits cool-season processes like early-spring Poa annua seedhead initiation and growth-regulator physiology that respond to heat well below 50°F. The base is chosen to match the biology, which is precisely why totals at different bases cannot be compared.

Reading Your Number on This Site

The growing-degree-day total shown on each city page and on the growing degree days by state pages is measured, not modeled. It comes from the daily air maximum and minimum at the nearest NOAA USCRN station, accumulated at base 50°F from January 1. Because only USCRN publishes the daily air data GDD needs, the figure appears for locations near one of those roughly 150 stations; elsewhere the page shows soil temperature and planting guidance without a GDD number rather than inventing one.

Use the total as a season-position signal. Early in the year it climbs slowly, accelerates through summer, and levels off as temperatures fall, the inverse of the soil-temperature curve that peaks and then cools. Match your accumulated total against the base-50 thresholds above (crabgrass, beetle emergence), or convert to the right base when a task uses a base-32 model, and treat every threshold as a window to confirm locally, not a date to obey.

Put It To Use

Check your local GDD and soil temperature

Enter your ZIP code for the current growing-degree-day total and soil temperature from your nearest USDA or NOAA monitoring station, then match it against the windows in this guide.

Frequently Asked Questions

What are growing degree days?

Growing degree days (GDD) count the heat a plant or pest has been exposed to over a season. Each day adds the amount its average air temperature runs above a base temperature, and days below the base add nothing. The running total tracks accumulated warmth far better than the calendar, which is why turf managers use it to time temperature-driven tasks. This site accumulates GDD at base 50°F from January 1, using the simple-average method: daily GDD = (daily high + daily low) / 2 − 50, floored at zero.

What base temperature and start date do these growing degree days use?

Base 50°F, accumulated from January 1, computed by the simple-average method from measured air temperature at NOAA USCRN stations. Base 50°F is the standard for turf and warm-season weed development, and January 1 is the start date USA-NPN and several extension models use for base-50 accumulations. Other tools use different starts, so a total is only meaningful alongside its base and start date.

Are growing degree day totals comparable across different tools?

Not unless the base temperature and start date match. Michigan State Extension notes that different crops and pests use different thresholds and start dates, so a bare GDD number is ambiguous. The MSU GDD Tracker even runs crabgrass models at two different bases (50°F and 32°F), and its Poa annua and growth-regulator models use base 32°F. A base-50 total from January 1 is not interchangeable with a base-32 total from February 15. Always read the model, not just the number.

How do growing degree days relate to soil temperature?

They measure different things. Growing degree days are an accumulation of air temperature over the whole season, a running tally of banked heat. Soil temperature is a point-in-time reading of how warm the ground is right now. Both matter: air GDD is the better signal for pest and plant-development timing, while soil temperature is the better signal for germination, since that is the layer seeds sit in. Purdue notes that crabgrass germination actually tracks soil temperature, which is one reason an air-GDD estimate of germination carries some uncertainty. Pairing the two, as the card on each city page does, gives you both the season's accumulated heat and what the ground is doing today.

Can I use growing degree days to time crabgrass pre-emergent?

As a heads-up signal, yes, but treat it as a range, not a trigger date. Purdue and Kansas State associate crabgrass germination with roughly 150 to 200 GDD base 50°F, well after forsythia bloom (about 1 to 25 GDD), which is only an early warning. Because germination tracks soil temperature and local conditions vary, use the GDD total to know the window is approaching, then confirm with soil temperature and local guidance before applying. Note that some pre-emergent GDD models use base 32°F instead of base 50°F, so check which base a given recommendation is built on.

Why does this site only show measured growing degree days near some locations?

Growing degree days here are computed from measured air temperature, and the only network we ingest that publishes daily air max and min is NOAA's US Climate Reference Network (USCRN), about 150 stations. USDA SCAN and CSCAN stations measure soil but their bulk data files carry no air element. So a city near a USCRN station gets a true measured GDD total; a city far from one shows no GDD figure rather than a modeled guess. This keeps every number on the page measured and honest, the same posture as the rest of the site.

Sources consulted