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 city or 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.
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 (Two Different Models)
This is the clearest example of why a base has to travel with a number, because two respected models answer the same question on different scales.
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.
Michigan State's GDD Tracker times the pre-emergent application differently: 250 to 500 GDD at base 32°F, counted from February 15. It uses the base-32 accumulation as an indirect read on soil temperature, aiming to get the herbicide down and watered in before the soil reaches the 50 to 55°F range where germination takes off. MSU also publishes a base-50 germination model at 200 to 600 GDD for verifying performance and timing scouting.
These are not competing numbers, they are different models measuring different things: one estimates when germination happens, the other when to apply ahead of it. A base-32 total is roughly two to three times a base-50 total over the same period, so reading 300 base-32 GDD against the base-50 germination range would be badly wrong. Whichever you use, confirm with soil temperature before applying, since germination tracks the ground rather than the air.
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 1030 GDD at base 50°F (accumulated from January 1) and continuing to roughly 2150 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.
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.
The Model Suite: Every Window, With Its Base and Start Date
The state pages on this site read your accumulation against the published turf models below. Each row is a complete model: a base temperature, a start date, and a target range. None of these numbers means anything without the other two, which is why they always travel together here.
| Model | Base | Starts | Target range | For |
|---|---|---|---|---|
| Crabgrass pre-emergent timer | 32°F | Feb 15 | 250 to 500 GDD | Homeowners |
| Crabgrass germination timer | 50°F | Feb 15 | 200 to 600 GDD | Homeowners |
| Spring broadleaf herbicide (ester) | 50°F | Feb 15 | 110 to 150 GDD | Homeowners |
| Spring broadleaf herbicide (ester or amine) | 50°F | Feb 15 | 150 to 200 GDD | Homeowners |
| Spring broadleaf herbicide (amine) | 50°F | Feb 15 | 200 to 600 GDD | Homeowners |
| Japanese beetle adult emergence | 50°F | Jan 1 | 1030 to 2150 GDD | Homeowners |
| Poa annua seedhead suppression (Proxy/Primo) | 32°F | Feb 15 | 200 to 500 GDD | Turf professionals |
| Poa annua seedhead suppression (Embark) | 22°F | Feb 15 | 680 to 1050 GDD | Turf professionals |
| Poa annua seedhead flush | 22°F | Feb 15 | 1050 to 2900 GDD | Turf professionals |
| Early-season dollar spot DMI application | 50°F | Feb 15 | 140 to 175 GDD | Turf professionals |
| Growth regulator reapplication interval | 32°F | Feb 15 | About 200 GDD between applications | Turf professionals |
Two rows need a note. The growth regulator interval is a spacing between applications, not a point in the season, so a season-to-date total cannot tell you where you are in it; the site shows it as a pace instead of a verdict. The legacy 50/86 model (base 50°F with an 86°F ceiling, so a 95°F day counts as 86°F) is listed on the state pages for people whose other tools report on it, but it carries no target window of its own.
Michigan State ground-truthed the models above in the Great Lakes region and states plainly that it does not recommend them for the mid-south transition zone or the South. Outside the Upper Midwest and Northeast, read them as a guide and confirm against your state extension service. The Japanese beetle and growth regulator models come from Iowa State and Cornell respectively and are not subject to that particular caveat, though local conditions always matter.
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 city or 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.