Treasure Valley lawn guide
When to fertilize your lawn in Idaho
University of Idaho Extension says southern Idaho lawns need 3 to 5 pounds of actual nitrogen per 1,000 square feet each year (CIS 846). The trick is splitting that into the right number of feedings at the right times.
How much nitrogen per year
Plan on 3 to 5 lb of actual nitrogen per 1,000 sq ft per year, split across several applications (CIS 846). Over-applying raises the risk of nitrate leaching into groundwater, so more is not better.
To convert: a bag labeled 30-0-4 is 30% nitrogen, so about 3.3 lb of product puts down 1 lb of actual nitrogen per 1,000 sq ft.
A simple Treasure Valley feeding calendar (estimate)
- Late May: about 1 lb N/1,000 sq ft, quick release while grass grows fast.
- Early September: about 1 lb N/1,000 sq ft to fuel fall root growth.
- Late October to early November (after growth slows, before the ground freezes): about 1 lb N/1,000 sq ft, the 'winterizer.'
- Optional early summer feeding with slow-release nitrogen for high-traffic or sandy lawns.
Slow release vs. quick release
Quick-release sources (urea, ammonium sulfate) work best during rapid growth in late spring and early summer. Slow-release (water-insoluble nitrogen) products suit sandy or fast-draining soils and the slow-growth periods of very early spring and late fall (CIS 846).
Yellow lawn? It may be iron, not nitrogen
High-pH Treasure Valley soils often cause iron chlorosis, which yellows new growth. University of Idaho Extension notes iron chelates are the most effective soil-applied fix, and foliar iron gives a quick but short-lived green-up (CIS 846).
Common questions
Should I fertilize my Idaho lawn in summer?
Go light or skip it during peak heat. Spring and especially fall feedings do more good for cool-season grass.
Is a soil test worth it?
Yes. University of Idaho Extension recommends a lawn soil test about every 10 years to set phosphorus and potassium rates.
Related LawnWise guides
- The Only Two Times to Fertilize a Treasure Valley Lawn (And Why Summer Isn't One)
- Yellow Stripes in Your Treasure Valley Lawn Aren't a Watering Problem — It's Iron Chlorosis
- Alkaline Soil in the Treasure Valley: What It's Doing to Your Grass and What to Actually Do About It
- When to Fertilize, Spray, and Seed a Treasure Valley Lawn: The Year-Round Calendar
Sources
- University of Idaho Extension CIS 846 — Southern Idaho Fertilizer Guide: Southern Idaho Lawns
- University of Idaho Extension — Lawn and turf
Figures marked "estimate" are LawnWise rules of thumb for the Treasure Valley, not extension data. Updated 2026-10-30. By LawnWise, from Seljaas & Sons LLC, Middleton, Idaho.