Fall fertilize for healthier lawn

Here in Eastern Washington, there are optimal times to apply nutrients to help keep the lawn healthy and vigorous. Washington State University recommends considering fertilizer applications four times per year, May 1, June 15, Sept. 1, and Nov. 1-15 after the last mowing and before the ground freezes. When nitrogen is applied in the fall, avoid early spring applications until after April 1 unless nitrogen deficiency is apparent. Better root growth and vigor are encouraged and can avoid the flush of growth by annual bluegrasses. If one application is missed, do not double up on subsequent rates; just use the recommended amount each time fertilizer is applied. For those who fertilize only once or twice a year, fall is the most critical time to fertilize rather than in the spring. Fall fertilized lawns green up more quickly and look better earlier in the spring than lawns that weren't fertilized in the fall. Spring fertilizer applications tend to promote leaf growth often at the expense of root growth. Except in unusually warm years, once the weather becomes more winter-like, grass goes dormant and the soil is cold, it is too late to fertilize. Nitrogen releasing fertilizers require sufficient soil microbial activity to release nitrogen and won't work in cold soils.

Ideally, soil testing is the most accurate way to determine the number of pounds of fertilizer to add. In general, eastern Washington lawns only need nitrogen though other nutrient needs can be assessed through testing. Sulfur is a nutrient that enhances color and helps control certain weeds and diseases in lawns. Washington State University recommends using a moderate rate of 2-3 lbs. sulfur per 1000 feet per year. So, selecting a lawn fertilizer that contains sulfur or ammonium sulfate should be part of a balanced fertilizer management program. As a rule of thumb, lawns require about one pound nitrogen per 1000 square feet per application for each of the recommended four application times each year. Since both nitrogen and sulfur are soluble in water, applications are recommended every year. By contrast phosphorus is not soluble and stays in the soil much longer.

Percent nitrogen is always the first number listed on the fertilizer bag though checking whether it is controlled release, slow release, slowly available or water soluble is also recommended. Divide 100 by the percent nitrogen on the bag to figure how much is needed per 1000 square feet. For example, a fertilizer with 20 percent nitrogen would require 100/20 or 5 lbs. of fertilizer produce per every 1000 square feet of lawn. Figure out the number of square feet in lawn and divide by 1000. Using the above example, multiply by the pounds of fertilizer needed per 1000 square feet which gives the approximate number of pounds of fertilizer to spread on the lawn. So, if a lawn measures out to 12,000 square feet, divide 12,000 by 1000 = 12. Then multiply 5 times 12 = 60 which is the pounds of fertilizer needed to be spread over 12,000 feet lawn to supply 1 pound of actual nitrogen per 1000 square feet.

Most quality lawn fertilizers offer a balance of fast and controlled release sources to provide quick color (without a big surge of growth) and some longer lasting nitrogen. Using balanced fertilizers like 10-10-10 is not advised for repeated use because of the excessive application of phosphorus which can build up to excessive levels, may lead to potential runoff problems and encourages more weeds in the lawn.

Inorganic sources of nitrogen like ammonium sulfate and ammonium nitrate, usually give better results in early spring and late fall than organic forms. After the soil warms, organic sources like sewage sludge or plant and animal byproducts can do better due to increased microorganism activity in the soil. Natural organic lawn fertilizers are typically very low in nitrogen. If using this type, consider doubling the rates of application since generally only half the nitrogen is available to the grass the first year. Otherwise, for conventional fertilizers, gardeners should apply nitrogen in a slow-release form. Quick release or water-soluble nitrogen is easily leached out of the root zone by irrigation and rainfall. Winterizing fertilizers are not recommended, are often more expensive and contain higher levels of potassium which is only beneficial if indicated by low soil test. However, a complete winterizing fertilizer may be good to use for the early November application. Dry fertilizers should be applied when the grass is dry and irrigated immediately afterwards or if irrigation water is not available fertilize to coincide with an anticipated rain event. If a quick release fertilizer is used on sandy soils, split the one lb./1000' into 2 applications approximately 2 weeks apart to reduce possible leaching of nutrients. Slow-release fertilizers are recommended on sandy soils or close to water sources to reduce the threat of leaching or runoff.

– Contact the WSU Grant-Adams Master Gardeners at 509-754-2011 Ext. 4313.