
Weed Control
Best Pre-Emergent: Purdue Says the Active Barely Matters
Purdue's finding is awkward for the aisle: prodiamine, pendimethalin and dithiopyr give equivalent control at the correct rate. The split decides it, not the brand.

Soil Thermometer
★★★★★ 4.6/5
our editor rating how we score4.7/5 from 461 shoppers at Amazon — their number, not ours.
$20–$26
Typical price · Spot readings at seed and root depth; no consumable
Affiliate links — see our disclosure. Commissions never influence verdicts.
The instrument every pre-emergent and seeding decision on this site actually depends on — and the specs are the reason to pick this one.
Almost every timing recommendation we publish is triggered by soil temperature rather than a date: pre-emergent goes down when the top couple of inches hold about 55°F, and seed germinates on soil warmth rather than air warmth. This is the tool that turns those from guesses into readings, and three specifications are why it is the right one. The stem is 6 inches, so it sits at seed and root depth instead of leaving most of its length in the air — a 20-inch compost thermometer cannot do this job, because pushed 2 inches into turf its reading is dragged toward air temperature. The dial is marked in 2°F subdivisions with ±2°F accuracy, which is enough to distinguish 55°F from 60°F, and that distinction is the whole decision. And there is a calibration nut on the back, so it stays honest rather than drifting quietly for five years. Take readings mid-morning, two inches down, in several spots — north-facing ground warms later than a south wall, and the cool spot is the one that decides your date.
Product type
Soil Thermometer
Active ingredient
None — 6 in stainless stem, 3 in glass-covered dial, −40 to 160°F, 2°F subdivisions, ±2°F accuracy, rear calibration nut
Price range
$20–$26
Coverage
Spot readings at seed and root depth; no consumable
Best for
Timing pre-emergent and seeding on soil temperature instead of the calendar
our editor rating
4.6 out of 5
Matched against each state's actual disease, weed and pest pressure.
Application window: Late February on the coast to mid-March in north Alabama — the earliest pre-emergent window of any state we cover
Application window: Largely beside the point: crabgrass and the summer annuals that drive pre-emergent programmes in the lower 48 are not the problem here, and a pre-emergent applied on a mainland schedule is money spent on an absent weed
Application window: Mid-February for summer weeds, and again late September for winter weeds germinating in the overseed
Application window: Early March in the south to early April in the Ozarks at soil 55°F
Application window: Mid-January (Los Angeles/San Diego) to late February (Sacramento/Bay Area) — before the last winter rains water it in for free
Application window: Late April to early May as soil reaches 55°F
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — mid-April along the shoreline, into early May in the hills; forsythia dropping its blooms is the local cue
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — typically mid-March in Sussex and late March to early April in New Castle County, with a second application six to nine weeks later because one pass does not cover a Delaware season
Application window: Early February (Orlando/Tampa) to early March (Panhandle); South FL applies year-round for tropical weeds
Application window: Mid-February (Savannah/South GA) to early March (Metro Atlanta), with a second application about eight weeks later — the state is long enough north to south that one date cannot serve it
Application window: There is no single spring flush to time against — warm-season weeds germinate through much of the year, so pre-emergent here is a repeated programme on label intervals rather than a February or March decision
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — usually April in the Treasure Valley, May in the north and at elevation. Cool-season turf here grows fastest when soil sits at 55 to 65°F, which is the same signal
Application window: Late March at the Ohio River confluence to late April in Chicagoland — the widest single-state spread in the Midwest, and the reason calendar-based advice fails here
Application window: April, on soil temperature — and Purdue's note is that any nitrogen in a pre-emergent product should be slow-release, capped at 0.75 lb per 1,000 sq ft
Application window: Mid-April to early May at soil 55°F — a narrow, late window compared with the states south of here
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — typically late March to mid-April moving northwest, with a second application six to eight weeks later in the wetter east
Application window: Mid-March in the Jackson Purchase to early April in the mountains at soil 55°F — and over Inner Bluegrass karst the barrier is worth splitting into two applications, because the ground drains faster than the label assumes
Application window: Late February to mid-March, before soil warms past 55°F — the earliest window of any state we cover
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — mid to late May on the southern coast, June inland; north of Bangor crabgrass pressure is often low enough that the honest answer is to skip it
Application window: Mid-March on the Eastern Shore to mid-April in the mountains — forsythia bloom is the traditional Maryland signal (soil 55°F)
Application window: Mid-April on the coast to early May in the Berkshires at soil 55°F — forsythia is the working signal here
Application window: Late April (Metro Detroit/South) to mid-May (Up North), as soil reaches 55°F
Application window: Mid-May, as soil warms — the season opens later here than anywhere else we cover
Application window: Late February on the coast to mid-March in the north
Application window: Late March in the Bootheel to mid-April along the Iowa line — soil 55°F moves south to north across about three weeks
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches, which is generally May and can be late May at elevation — and crabgrass pressure in much of Montana is low enough that this is a decision rather than a default
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — usually late April in the south-east, into mid-May in the north and west
Application window: February in Las Vegas to April in the northern high desert
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — late April to early May on the Seacoast, and up to three weeks later in the north; crabgrass pressure itself falls off with elevation
Application window: Mid-April in the south to early May in the northwest, as soil reaches 55°F
Application window: Crabgrass and summer annual pre-emergent as soil holds 55°F at 2 inches — February to early March in the southern valleys, April to May in the north. The state spans two months on this decision
Application window: Mid-April (Long Island/NYC) extending into early May (Buffalo/Upstate), as soil reaches 55°F
Application window: Early to mid-March in the Piedmont, when soil temps approach 55°F — earlier along the coast
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches, usually mid to late May — and crabgrass pressure here is genuinely lower than in the states to the south, so this is a decision rather than a routine
Application window: Late March along the Ohio River to late April in the Lake Erie snow belt at soil 55°F — a month of spread across one state
Application window: Early March in the south to early April in the panhandle at soil 55°F; on western sand, split the rate rather than trusting one pass to hold
Application window: Late February to March west of the Cascades — the earliest cool-season window anywhere, because the valley barely stops growing
Application window: Mid-April around Philadelphia to early May on the northern plateau at soil 55°F — three weeks of spread across fifty miles where the ridges rise, which is why a neighbour's timing is not yours
Application window: Crabgrass pre-emergent once soil holds 55°F at 2 inches — usually the second half of April on the bay and coastal towns, into early May inland
Application window: Early March on the coast to late March in the Upstate, before soil warms past 55°F
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches, generally mid-May — and pressure is lower here than in the corn-belt states, so it is a judgement rather than an annual purchase
Application window: Early March in Memphis to late March in Knoxville — soil 55°F sweeps west to east
Application window: Mid-February (Houston/San Antonio) to mid-March (Dallas/Panhandle), soil temps 55°F for crabgrass
Application window: Late March to mid-April along the Wasatch Front at soil 55°F
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — usually mid-May in the Champlain Valley and later in the hills; in the coldest towns crabgrass pressure is low enough that the question is whether to bother
Application window: Early March (Hampton Roads/Tidewater) to late March/early April (Northern VA/Richmond), as soil reaches 55°F
Application window: Mid-April to early May as soil reaches 55°F; crabgrass pressure is low west of the Cascades, where annual bluegrass and moss matter more
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches — typically late March to early April in the valleys, into late April at elevation, with a second application six to nine weeks later
Application window: Late April to mid-May, as soil temperatures reach near 60°F for several consecutive days — forsythia bloom is the usual reminder
Application window: Crabgrass pre-emergent as soil holds 55°F at 2 inches, generally May and later at elevation — pressure here is low enough that in much of the state this is a decision rather than an annual habit
Common questions
Premium Stainless Steel Soil Thermometer (6 in Stem, 3 in Dial) typically runs $20–$26, covering Spot readings at seed and root depth; no consumable.
None — 6 in stainless stem, 3 in glass-covered dial, −40 to 160°F, 2°F subdivisions, ±2°F accuracy, rear calibration nut.
Costs more than the throwaway plastic dial thermometers, which is the point
Real feedback
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Weed Control
Purdue's finding is awkward for the aisle: prodiamine, pendimethalin and dithiopyr give equivalent control at the correct rate. The split decides it, not the brand.

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Lawn Care Guides
Penn State: crown hydration is the most common and destructive winter injury, and it happens in late winter after thawing and freezing — not in the deep cold.

Lawn Care Guides
Most spring lawn damage is done in March by people being helpful. Spring lawn care runs on soil temperature, not the calendar — and the order matters.

Lawn Care Guides
When to overseed: late summer beats spring for cool-season lawns, most warm-season lawns should not be overseeded at all — and pre-emergent blocks your seed.

Weed Control
Crabgrass is a timing problem — but one pre-emergent pass is not a whole season. UF/IFAS: season-long control takes a second application 6–9 weeks later.

Weed Control
Spring weed control is decided by soil temperature, not the calendar — a pre-emergent applied two weeks late does nothing at all. The trigger, and the plan.
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