Philadelphia
Treat broadleaf55°F↑ warming
Fall is the best time to treat dandelions and clover: they pull weed killer down to their roots.
Broadleaf window Sep 25 – Nov 17
Pennsylvania Weed Watch · updated Nov 3, 1:18 AM CT
When to apply pre-emergent in Pennsylvania
Live soil temperatures for Philadelphia, Pittsburgh, Harrisburg and Erie, updated every three hours. Weeds sprout by soil temperature, not by the calendar, so the right week moves from year to year.
In Pennsylvania, pre-emergent is a spring job: it stops crabgrass. Fall is seeding season here, so fall weed control means treating broadleaf weeds instead.
Spring pre-emergentNext season
Typically Mar 26 – Apr 25
Stops crabgrass and other summer weeds before they sprout.
Fall: no pre-emergentThis season
Treat broadleaf weeds Sep 1 – Nov 17
Fall is seeding season here, and pre-emergent would stop grass seed too.
Right now · fall (seeding season)
Treat broadleaf weeds now across Pennsylvania
Soil is 52–57°F now. In fall, dandelions, clover and creeping Charlie pull sugars — and weed killer — down to their roots. Seeding this fall? Skip pre-emergent; it stops grass seed too.
Soil temperature by city
Soil 2 inches down, 5-day average: solid line = the last two weeks, dashed = the six-day forecast.
Pittsburgh
Treat broadleaf57°F→ steady
Fall is the best time to treat dandelions and clover: they pull weed killer down to their roots.
Broadleaf window Sep 11 – Nov 3
Harrisburg
Treat broadleaf52°F↑ warming
Fall is the best time to treat dandelions and clover: they pull weed killer down to their roots.
Broadleaf window Sep 16 – Nov 2
Erie
Treat broadleaf57°F→ steady
Fall is the best time to treat dandelions and clover: they pull weed killer down to their roots.
Broadleaf window Sep 1 – Oct 28
Pennsylvania pre-emergent calendar
Spring pre-emergent stops crabgrass. The window runs from the typical day soil warms to 50°F to the day it holds 55°F, when crabgrass starts germinating (2016–2025). In fall Pennsylvania lawns are in seeding season, so there is no fall pre-emergent; the window shown is for treating broadleaf weeds instead, from September until two weeks after soil usually cools to 55°F.
| City | Spring pre-emergent (crabgrass) | Fall (seeding season) |
|---|---|---|
| Philadelphia | Apr 5 – Apr 23 | None (seeding season) · treat broadleaf Sep 25 – Nov 17 |
| Pittsburgh | Mar 26 – Apr 16 | None (seeding season) · treat broadleaf Sep 11 – Nov 3 |
| Harrisburg | Apr 5 – Apr 16 | None (seeding season) · treat broadleaf Sep 16 – Nov 2 |
| Erie | Apr 14 – Apr 25 | None (seeding season) · treat broadleaf Sep 1 – Oct 28 |
These are typical-year dates (the median of 2016–2025); any one year can run a couple of weeks earlier or later, which is what the live readings above are for. For your exact yard, check soil temperature by ZIP.
What people in Pennsylvania are identifying
We record the state each identification comes from. Pennsylvania's list appears here once it has at least 40 identifications spread over 7 or more days — so no single scan stands out. Until then, see the national list, or add yours:
Identify a weed from a photoWeeds to watch in Pennsylvania this fall
Questions about Pennsylvania
When should I apply crabgrass pre-emergent in Pennsylvania?
Put crabgrass pre-emergent down as soil warms to 50°F, before it holds 55°F and crabgrass starts germinating. In a typical year (2016–2025) that window is Apr 5 – Apr 23 in Philadelphia; Mar 26 – Apr 16 in Pittsburgh; Apr 5 – Apr 16 in Harrisburg; Apr 14 – Apr 25 in Erie.
Should I put down pre-emergent in the fall in Pennsylvania?
Usually not. Fall is the best time to seed a cool-season lawn in Pennsylvania, and pre-emergent stops grass seed as well as weeds. Fall is the time to treat broadleaf weeds such as dandelion, clover and creeping Charlie, once the summer heat breaks and while they are still growing.
What is the soil temperature in Pennsylvania right now?
As of Nov 3, 1:18 AM CT: Philadelphia 55°F, Pittsburgh 57°F, Harrisburg 52°F, Erie 57°F (5-day average, about 2.4 inches down). Treat broadleaf weeds now across Pennsylvania.
How the readings and dates are made, and their limits: Weed Watch method. Soil data: Open-Meteo.com (CC BY 4.0); typical dates: NASA POWER.