Phoenix
Apply ASAP69°F↓ cooling
Soil has cooled to 75°F, so winter weeds are sprouting. Pre-emergent still stops later flushes.
Winter weeds are sprouting now
Arizona Weed Watch · updated Nov 3, 1:18 AM CT
When to apply pre-emergent in Arizona
Live soil temperatures for Phoenix, Tucson and Flagstaff, updated every three hours. Weeds sprout by soil temperature, not by the calendar, so the right week moves from year to year.
Most Arizona lawns get pre-emergent twice a year: in spring for crabgrass and in fall for winter weeds. In Flagstaff, cool summers mean fall seeding, so it is spring only there.
Spring pre-emergentNext season
Typically Jan 14 – May 27
Stops crabgrass and other summer weeds before they sprout.
Fall pre-emergentThis season
Typically by Oct 18 – Oct 26
Stops winter weeds (annual bluegrass, henbit, chickweed) before they sprout.
Skip it if you are overseeding this fall. Not in Flagstaff.
Right now · fall pre-emergent season
Winter weeds are germinating — apply now if you haven't: Phoenix and Tucson. Broadleaf season over: Flagstaff
Soil is 44–69°F now. They keep coming up in flushes into winter, so pre-emergent still helps. Overseeding 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. Shaded = below 75°F, where winter weeds sprout — pre-emergent has to be down before the line gets there.
Tucson
Apply ASAP68°F↓ cooling
Soil has cooled to 75°F, so winter weeds are sprouting. Pre-emergent still stops later flushes.
Winter weeds are sprouting now
Flagstaff
Done for now44°F↓ cooling
Done for this year. Spot-treat on a mild day, or wait for spring.
Broadleaf window Sep 1 – Oct 16
Arizona 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). Fall pre-emergent stops winter weeds: have it down a week before soil usually cools to 75°F, when they start germinating.
| City | Spring pre-emergent (crabgrass) | Fall pre-emergent (winter weeds) |
|---|---|---|
| Phoenix | Jan 14 – Jan 28 | Apply by Oct 26 |
| Tucson | Feb 2 – Feb 11 | Apply by Oct 18 |
| Flagstaff | May 6 – May 27 | None (seeding season) · treat broadleaf Sep 1 – Oct 16 |
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.
Recommended gear
Fall pre-emergents
Granular or liquid. Follow the label rate, and water it in.
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Scotts Halts Crabgrass & Grassy Weed Preventer
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Dimension 2EW (Dithiopyr)
Liquid concentrate; 64 oz half gallon; mix with water and spray
Dithiopyrliquidcovers ~91,428 sq ft / 2.1 ac$158.00as of Nov 2, 10:00 AM UTC · may change
What people in Arizona are identifying
We record the state each identification comes from. Arizona'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 Arizona this fall
Questions about Arizona
When should I apply crabgrass pre-emergent in Arizona?
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 Jan 14 – Jan 28 in Phoenix; Feb 2 – Feb 11 in Tucson; May 6 – May 27 in Flagstaff.
When should I apply fall pre-emergent in Arizona?
Winter weeds such as annual bluegrass, henbit and chickweed start germinating as soil cools to about 75°F in late summer and fall, and most come up once it is below 70°F. Have fall pre-emergent down before they start: typically by Oct 26 in Phoenix; by Oct 18 in Tucson. Skip it if you are overseeding this fall, because it stops grass seed too. In Flagstaff, cool summers mean cool-season lawns and fall seeding, so skip fall pre-emergent there and treat broadleaf weeds instead.
What is the soil temperature in Arizona right now?
As of Nov 3, 1:18 AM CT: Phoenix 69°F, Tucson 68°F, Flagstaff 44°F (5-day average, about 2.4 inches down). Winter weeds are germinating — apply now if you haven't: Phoenix and Tucson. Broadleaf season over: Flagstaff.
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.