Lawn care calendar for Idaho: when to fertilize and seed

The four-feeding plan is the University of Idaho CIS 846 example for 4 pounds of nitrogen; a soil test sets your annual amount, and the plan uses 1 pound at a time.

Which grasses grow where in Idaho

The University of Idaho’s lawn publications cover cool-season grasses: Kentucky bluegrass, perennial ryegrass, tall fescue and fine-leaf fescues. Its Starting a Home Lawn guide calls bluegrass the most widely adapted lawn grass, spreading by rhizomes in spring and fall, and suggests turf-type tall fescue at 7 pounds of seed per 1,000 square feet, perennial ryegrass at 5 and fine fescues at 4. It also lists buffalograss, which goes dormant and turns gray after a killing frost.

Supplemental irrigation is necessary to establish and keep a healthy lawn in most areas of Idaho, so the calendar here is built on irrigated turf. University of Idaho also covers northern Idaho in a separate fertilizer guide, CIS 911, which I could not read as text, so the feeding schedule below is the southern guide and applies most directly to the Snake River Plain and the Boise area.

Frost dates from NOAA

Dates are the NOAA 1991-2020 normals for the last spring and first fall 32 F freeze at 50% probability. Lewiston-Nez Perce County Airport averages April 6 and October 26, Boise Air Terminal April 24 and October 21, Idaho Falls Fanning Field May 23 and September 20, and Pocatello Regional Airport May 23 and September 18.

That works out to about 203 frost-free days at Lewiston, 180 at Boise, 120 at Idaho Falls and 118 at Pocatello (our arithmetic). Eastern Idaho has about 12 weeks less than Lewiston, so the last feeding and late-summer seeding both have a shorter window there. The University of Idaho says the active growing season shortens as you move east of southwestern Idaho.

Spring

The University of Idaho says most lawns in southwestern Idaho start active growth in late February or early March and keep growing until about mid-November. Its nutrient-ratio strategy gives 0.5 pound of nitrogen per 1,000 square feet for each month of active growth, while the soil-test strategy sets the yearly nitrogen by organic matter: 1 pound for soils at 5 percent or higher, up to 5 pounds for soils under 1 percent.

The first feeding in the four-application plan is early May, with a quick-release nitrogen as grass grows fast in late spring and early summer. The guide says to use no more than 1 pound of nitrogen at a time, or up to 1.5 pounds of slow-release. Mowing heights are 1 to 2 inches for bluegrass, ryegrass and fine fescues and 2 to 3 inches for tall fescue, cut before the grass passes 3 or 4.5 inches respectively; expect to mow twice a week in the spring flush.

Summer

A lawn needs about a quarter inch of water a day in the warmer part of summer, according to CIS 1063. On deep loam, clay loam and clay soil it suggests 2 inches at each watering, once a week; on sandy soil about 1 inch every three or four days. CIS 1157 adds that a smooth-textured soil holds about 1 inch of usable water per foot, a sandy one 0.5 and a silt loam 1.2, and that spray heads put down 1.5 to 2 inches an hour while rotors apply 0.25 to 0.5.

When daily temperatures average above 85 F, University of Idaho says most grasses go inactive unless watered. A catch-can test shows how long your sprinklers need to run, and letting grass grow up to 4 inches gives deeper roots with access to more water. Thatch over half an inch should be removed by power raking or vertical mowing, followed by three to four weeks of good growing weather.

Fall

Plan late-summer seeding for August 15 to September 15, University of Idaho says, because warm days and cool nights suit seedlings; seed a new lawn in late summer or early spring, and seed early enough in fall that the grass can mature before winter. Midsummer seeding is difficult in heat and drought. A starter fertilizer such as 16-20-10 at about 6.25 pounds per 1,000 square feet supplies about 1 pound of nitrogen, with a half rate four to six weeks after germination.

Established lawns get the September and late October or early November feedings. CIS 1063 puts three quarters of the total fertilizer in fall (September to early November) and the rest in late spring, mid-May to mid-June, which is a heavier fall weighting than the evenly split four-feeding example in CIS 846.

Fertilizer rules and soil tests

The Commercial Fertilizers chapter of Idaho Code covers registration (22-605), labels (22-607), inspection and penalties, and I found no limit on phosphorus in home lawn fertilizer in the chapter’s list of sections. Section 22-626 bars any city, county or other local government from regulating the sale, use or application of fertilizer, and makes such local rules void.

The University of Idaho recommends a soil test before establishing a lawn and about every 10 years after, taken 8 inches deep from 15 to 20 spots. Phosphorus is applied only when the soil test is under 15 ppm in its table, and the guide notes phosphorus can encourage dandelions when applied to the surface. The lawn fertilizer calculator on this site turns the 1 pound per feeding into pounds of product for the bag you bought.