Lawn Calendar & Chemist

Use when planning lawn care timing and products: soil-temperature windows for pre-emergent crabgrass control, N-P-K fertilizer math from bag guarantees, seeding vs pre-emergent conflicts, and a region-adjusted annual calendar. Prevents the two classic mistakes — applying too late and over-fertilizing.

Overview

Lawn care fails on timing and dose, not effort. Pre-emergent crabgrass control works only before germination — which is driven by soil temperature (4 consecutive days ≥55°F at 2" depth), not by calendar dates or forsythia bloom. And fertilizer burn/waste comes from ignoring what the bag guarantee actually contains per bag and per 1000 sq ft. This skill converts your location, grass type, and soil-temp observations (or seasonal estimates) into a dated application calendar, computes exact product dose from bag guarantees, and catches the fatal conflicts (pre-emergent kills new seed; urea + temperature >85°F burns cool-season grass).

When to Use

• "When do I apply crabgrass preventer?" / "Is it too late for pre-emergent?"
• Building the whole-season plan: pre-emergent ×2, fertilizer rounds, seeding, aeration
• Deciding how much product to buy / spreader settings from N-P-K guarantees
• Overseeding timing vs pre-emergent conflict resolution
• "My lawn is yellow/burned" — fertilizer misapplication triage

Don't use for: irrigation design, soil-test interpretation beyond N-P-K basics (send to extension service — the skill tells you when), or pesticide selection beyond pre/post emergent herbicide classes.

The Physics of Timing

• Crabgrass germinates at soil 55°F (13°C) for 4+ consecutive days at 2-inch depth — typically when soil hits 55°F for a week. Air temp is a poor proxy (soil lags air by ~1-2 weeks in spring). Apply pre-emergent when soil is 50-55°F and rising, usually 2-4 weeks before your last frost.
• Soil temp estimation: soil ≈ weekly mean air temp lagged ~10 days, minus 2-3°F. The script takes your 7-day mean air temp + date and estimates soil temp with a seasonal lag model — but a $10 soil thermometer beats any model.
• Pre-emergent + seeding conflict: most pre-emergents (dithiopyr, pendimethalin, prodiamine) inhibit ALL seedling roots for 3-4 months. Rules:
  • Spring pre-emergent → seed in fall (cool-season), not spring
  • Must seed spring? Use siduron (Tupersan, the only seed-safe pre-emergent) or skip pre-emergent and use post-emergent quinclorac later.
• Second pre-emergent round (~8-10 weeks after first) extends the window; prodiamine 65 WG at 0.37 lb/1000 ft² gives ~4 months at 0.65 ai/1000 — the script computes this from label rate tables.
• Fertilizer burn conditions: urea-containing products above 85°F on cool-season (fescue/blue/rye) turf; always water-in 0.25-0.5" unless the label says otherwise.
• Cool-season grass (fescue/KBG/rye): feed fall-heavy (Sep-Nov = 2/3 of annual N); spring N causes topgrowth at root expense + disease. Warm-season (bermuda/zoya/ st augustine/centipede): feed summer-heavy, NOTHING until green-up (soil 65°F+).

Fertilizer Math (bag guarantees)

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N-P-K 20-0-8 means: 20% N, 0% P₂O₅, 8% K₂O by weight
A 50-lb bag of 20-0-8 = 10 lb N, 4 lb K₂O
1 lb N / 1000 ft²  →  50-lb bag covers 10,000 ft² at that rate
cost per lb N = bag price / lb N in bag   (the honest comparison between products)

Slow-release fraction matters: >30% coated/urea-form N allows higher single doses (1.25-1.5 lb N/1000) without burn; all-soluble stays ≤1 lb/1000 per application.

Numbered Workflow

1. Collect: grass type (cool/warm), location or 7-day mean air temp + today's date, lawn size (measure: length × width minus house/beds — or pacing), goals (crabgrass? seed? just green?), bag guarantee of products on hand.
2. scripts/lawn_plan.py soil-temp --air-mean 52 --date 2026-04-10 → estimated soil temp + pre-emergent window verdict (GO / WATCH / TOO LATE — with post-emergent route).
3. scripts/lawn_plan.py calendar --grass cool --region transition → dated annual plan (pre-emergent windows, N rounds in lb N/1000, seeding windows, aeration).
4. scripts/lawn_plan.py fert --npk 20-0-8 --bag-lbs 50 --price 32 --area 6000 --n-rate 1.0 → lb/bag needed, coverage, cost per lb N, burn-risk flag by date/temperature.
5. Check conflicts: script refuses calendar entries that pair seeding with non-siduron pre-emergent, or high-N summer applications on cool-season grass.

Pitfalls

1. Applying pre-emergent by calendar date. A warm February triggers germination 3 weeks "early"; a cold spring pushes it late. Soil temp is the gate.
2. Pre-emergent then seeding 6 weeks later. Root inhibition kills the seeding; the label says 3-4 months. Seed in fall or use siduron.
3. More N = more green (until it isn't). >1 lb N/1000 soluble in one pass burns; spring-heavy N on cool-season lawns invites brown patch and shallow roots.
4. Ignoring the slow-release fraction. Cheap all-soluble bags limit dose and frequency.
5. Watering-in failure: pre-emergent MUST be activated with 0.5" water/irrigation within days or it degrades on the surface; fertilizer sits on leaves → burn.
6. Weed-and-feed timing mismatch: granular herbicide needs the weed leaf wet (morning dew) AND the fertilizer schedule may not align with the germination window.
7. Not calibrating the spreader — label rates assume it; a mis-set spreader doubles or halves everything. Calibrate: weigh what falls in 100 ft².

Example

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$ python3 scripts/lawn_plan.py soil-temp --air-mean 54 --date 2026-04-10 --region transition
Estimated 2" soil temp: ~52°F (lag model; verify with $10 thermometer)
Crabgrass gate: germination starts at 55°F × 4 days
VERDICT: WATCH/APPLY — in the 50-55°F pre-window; apply NOW so the barrier activates first.
 Too late already (soil 70°F)? → switch to post-emergent: quinclorac on 1-3-leaf crabgrass.

$ python3 scripts/lawn_plan.py fert --npk 20-0-8 --bag-lbs 50 --price 32 --area 6000 --n-rate 0.75
Bag: 10.0 lb N | need 4.5 lb N for 6000 ft² @ 0.75 → 22.5 lb product (0.45 bag)
Cost: $3.20 per lb N ($14.40 this application). Coverage check: bag covers 13,333 ft².
DATE FLAG: if applying after mid-May on cool-season grass, use ≤0.5 lb N/1000 + water in.

$ python3 scripts/lawn_plan.py calendar --grass cool --region transition
... Sep 01  fall N 1.0 lb/1000 (feed-height of fall)
... Sep 15  AERATE + overseed KBG/fescue @ soil 60-75°F falling (NO pre-emergent since spring)
... Oct 15  fall N 1.0 → root season; Nov 15 final N 0.75 ("winterizer" while still green)
... Feb 20* soil 50-55°F rising: pre-emergent #1 (prodiamine 0.37-0.65 lb ai/1000)
... Apr 25* pre-emergent #2 (8-10 wk after #1)  *dates shift with soil temp — use the gate

Verification Checklist

• Grass type confirmed (cool vs warm season drives everything)
• Lawn area measured honestly (pacing × 0.9 for house/beds)
• Soil temp from thermometer or the lag model + region
• Pre-emergent window GO/WATCH/TOO-LATE verdict with activation watering
• N rates per application ≤1 lb/1000 soluble (≤1.5 slow-release), annual total by grass
• Seeding vs pre-emergent conflict resolved (fall seed / siduron)
• Spreader calibrated (100-ft² catch test)