Lawn care calendar for Washington: when to fertilize and seed

Dates are WSU's suggested schedule; weather moves them.

Which grasses grow where in Washington

WSU says to skip zoysia, bermudagrass, dicondra, centipedegrass, carpetgrass, St. Augustinegrass and mondograss in Washington, because the climate does not suit them. Buffalograss was being evaluated for central and eastern Washington. The cool-season grasses that work are Kentucky bluegrass, turf-type perennial ryegrass, fine-leaf fescues, turf-type tall fescue and, in the west, bentgrasses kept mown below 3/4 inch.

Mowing heights in the bulletin: fine fescues 1.25 to 1.5 inches west and 1.5 to 2 east; Kentucky bluegrass 2 to 2.5 inches east; turf-type perennial ryegrass 1.25 to 1.5 west and 2 to 2.5 east; tall fescue 1.5 to 2 west and 2 to 3 east. Remove no more than 30 percent of the blade per mowing, and drop the mower no more than half an inch at a time.

Frost dates from NOAA

Dates are the NOAA 1991-2020 normals for the last spring and first fall 32 F freeze at 50% probability. Seattle-Tacoma International averages March 2 and November 23. Spokane International averages April 26 and October 12.

Our arithmetic: that is about 266 days between the dates at Sea-Tac and about 169 at Spokane, a gap of 97 days.

Spring

After a fall nitrogen application, WSU says to hold off on spring feeding until after April 1 unless the grass shows a deficiency. That order of feeding builds root growth and avoids the surge of annual bluegrass. Seed in April and May in western Washington, in April and May or August and September in the east, and keep the surface moist with several small waterings a day in dry, sunny weather.

Lawn soils west of the Cascades are often deficient in phosphorus and potassium, and WSU suggests fertilizers with a 3-1-2 or 6-1-4 N-P-K ratio there; eastern lawns usually need only nitrogen. Under state law, phosphorus on established turf requires a soil test showing deficiency.

Summer

June 15 is the third feeding date. Water by feel instead of by schedule: irrigate when the top 2 inches turn dry and crumbly, and soak to at least 12 inches if the soil is that deep. Early morning, around 4 a.m., limits disease, and late night is the next best time. Sandy loam or heavier soils generally do not need water more than once a week; light sandy soils may.

Light, frequent sprinkling promotes shallow roots. Remove thatch if it builds beyond half an inch, and aerate hard or compacted lawns.

Fall

September 1 is the last feeding date inside the growing year. The late-fall application of 1 pound of nitrogen is the one WSU stresses for the best-quality lawns: after the last mowing and before the soil freezes in the east, between mid-November and December 7 in the west. Seeding is still satisfactory in the west through October 15.

Apply sulfur at 2 to 3 pounds per 1,000 square feet a year, through a sulfur-bearing fertilizer or ammonium sulfate twice a year; WSU says it helps color and some weed and disease control.

Washington's phosphorus law

RCW 15.54.500 bars applying turf fertilizer labeled as containing phosphorus to turf, with two exceptions that matter at home: establishing or repairing grass from seed or sod during the growing season, and an area where a soil test taken within 36 months shows a phosphorus deficiency. It also bars phosphorus on frozen ground and on impervious surfaces, and cities and counties cannot be less strict.

The WSU schedule is written in pounds of nitrogen per 1,000 square feet, and the lawn fertilizer calculator on this site works from the same first number on the bag.