The Washington State University Extension office recommends fertilizing lawns in eastern Washington in early November, early September, early May and mid-June based on plant and root growth. I have recommended this same schedule made easier to remember by associating these dates with holidays. They are Halloween, Labor Day, May Day and Father’s Day.
Turfgrass experts at WSU Extension and other universities had long indicated that late fall (Halloween) fertilization is critical because grass top growth is slowing down and fertilizer applied at that time promotes root growth. In addition, the experts indicated fall fertilization leads to greener grass during the winter months and earlier green-up in the spring.
University researchers decided to take a second look at these older research-based recommendations. They found that nitrogen uptake in late fall is lower compared to uptake earlier in the season. They believe that this occurs because the absorption of the nitrogen is driven by the evaporation of water (transpiration) through the grass blades, and the absorption of water by the roots to replace that water. As the weather cools, transpiration slows and nutrient uptake via water absorption by the roots also slows down. This is accentuated in our region because the soil often lacks enough available moisture to help with the absorption of nitrogen and other nutrients in the soil once irrigation water is turned off.
While late fall fertilization does not appear to promote late fall root growth, it does encourage root growth in early spring and leads to earlier spring green-up. Early fall fertilization actually provides for greener turf through fall and early winter.
Researchers have found that nitrogen from a late fall application of fertilizer may either stay put in the soil or be lost through leaching due to precipitation, especially on sandy soils. If high levels of nitrogen remain in the soil until top growth begins in early spring, it can lead to the excessive lush growth that uses up carbohydrate reserves needed by the turf to withstand summer stress.
Additional research at Cornell University was aimed at determining the value of applying a fertilizer high in potassium or what is called a “winterizing” formulation with the purpose of decreasing the turf’s vulnerability to winter damage from cold temperatures. You may have seen “winterizing” lawn fertilizers available for sale in the fall. Based on recent and older research, there is no proof that high-potassium fertilizers applied in the fall improve the cold tolerance of cool-season grasses like those planted in our area. In fact, the Cornell research indicated that these fertilizers increase turf susceptibility to snow mold.
The result of this more recent research has not greatly changed lawn fertilization recommendations. The holiday timing is the same as I mentioned earlier. There is still no proof that turf-winterizing fertilizers provide any benefits and they may actually harm turf by encouraging snow mold. The only thing that has changed is the type of nitrogen applied.
In early fall (Labor Day), turfgrass experts recommend using a product that contains 50 percent or less of nitrogen in slow-release form. In late fall (Halloween), they recommend using one that contains more quick-release nitrogen. These two fall applications are still the most critical ones of the year, so if you have not applied your “Labor Day” fertilizer yet, do it now.
Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.