Fall Fertilization of Home Lawns

Published: August 1, 2009

Home lawns in most years usually struggle through the perils of summer – high heat, humidity, drought and insect problems. Our cool-season grasses like Kentucky bluegrass, tall fescue and perennial ryegrass have not had a typical year in that we only had a few days with heat indexes above 100 F. Our night temperatures during several weeks in July dipped into the fifties. Several other weeks had nighttime temperatures in the sixties and now we may end up with one of the coolest July’s on record. Rainfall has been very timely in most parts of the state so drought has not been an issue at this time. So while we usually look forward to cooler nighttime temperatures and additional rain in September, they are already here. As one looks around, everything looks great and green as we go into August and it already feels like September. But do not be deceived by all this and forget we still have most of August and September remaining and heat and drought can still be part of the picture. However, it is also not too early to start thinking about fall chores to improve even more on what we already have. It’s time to think about aeration and fall fertilization.

Aeration is a practice of pulling soil plugs to open the soil surface for better nutrient and water movement as well. It is a practice that also helps to reduce compaction and thatch by spreading soil plugs on the surface. Soil plugs are crumbled and fall freely into aeration holes as well as spreading some soil into the thatch layer where soil microbes can feed on thatch debris. Aeration is a practice that can be done in both spring and fall.

Aeration is the very best way to begin a fall fertilization program. Applications of fertilizer after aeration will move nutrients immediately into the rootzone of your lawn. This practice has shown excellent results in the density and color of cool-season turfgrasses on their way to recovery from summer stresses.

Aeration equipment can be found at local rental stores or garden centers as well. A machine that pulls a ½” diameter plug three to four inches deep on four inch centers will do an excellent job. Machines that force hollow tines into the soil are better than pull-type drums with tines. Not all machines will meet these specifications, however any amount of aeration is better than no aeration to kick-off fall fertilization.

Fall fertilization should always start with a soil test to determine what the needs of the soil are, if any. Soil test results will also give you the soil pH and any information about lime requirements. Soil pH is also important as it affects nutrient availability to the plant. A soil pH between 6.0 and 7.0 is acceptable. A soil pH around 6.4 to 6.8 is optimum. MU guide # G6954, “Soil Testing for Lawns” gives information on how to take and submit soil samples to the University of Missouri Soil Testing Labs.

Homeowners have a wide variety of fertilizers available to them for fall fertilization. Many organic fertilizers, such as Milorganite, Sustane and Ringer are available and will provide an excellent source of slow released nitrogen. Organic fertilizers do require soil microbes to release nutrients, therefore as soil temperatures decrease by late Fall, performance of these fertilizers may drop off.

More inorganic types of fertilizers are available to homeowners and can be somewhat confusing. Many products have much higher amounts of nitrogen and most are soluble forms (quick release) of fertilizers. Quick release forms of fertilizers are there and gone after about two weeks. You will get a quick flush of green growth, then a quick tapering off of color and growth. Find fertilizers with a good balance of N-P-K (nitrogen/phosphorus/potassium) with a ratio somewhere around 3-1-2. Also look at the ingredient label on the bag and find a product with 30 to 70% slow-release nitrogen. This way your fertilizer is released over a longer period of time requiring fewer applications and allowing the plants to more efficiently utilize plant nutrients.

Fertilizer rates for fall fertilization give best results if 2.5 to 3.0 lbs of nitrogen can be applied per 1,000 square feet. Amounts should be divided over two or three applications throughout the fall. Possible combinations would include a pound of nitrogen per 1,000 square feet in early September after aeration and/or de-thatching followed by 1.5 pounds of nitrogen per 1,000 square feet in late October. A second alternative would include a pound of nitrogen per 1,000 square feet applied in early September, October and November. Most fertilizers are complete fertilizers including phosphorus and potassium; therefore requirements for those nutrients should be based on soil test results. Soil test results indicating high to very high amounts of phosphorus and potassium may require applications of fertilizers with nitrogen alone.

Winterizing fertilizers are usually recommended as the final application of the fall. Good winter fertilizers will have higher and equal amounts of nitrogen and potassium (first and third numbers of the fertilizer components). However, there are conflicting comments about applications of potassium for hardening off plants before winter dormancy. Plants harden off by reducing the amount of water in plant cells, therefore reducing the threat of winter freezing. It is a practice of higher importance for warm-season (bermuda and zoysia) grasses as opposed to cool-season grasses.

Any additional questions on aeration and fall fertilization can be directed to the MU Turfgrass Research Center @ (573) 442-4893.