Aeration & De-thatching is Good Time to Begin Fall Fertilization

Home lawns have once again struggled through another hot summer. Our cool-season grasses like Kentucky bluegrass, tall fescue, and perennial ryegrass have faced disease pressures, drought, insects and the reduction of root mass due to the effects of high heat. Our night temperatures are beginning to dip into the sixties once again and will possibly reach the fifties in a few weeks. As of late, we have been receiving some significant rainfall. Therefore, it’s that time of the year again, when our cool-season grasses start to thrive and recover. It’s also the best time of the year to open up the soil and begin fall fertilization.

Some lawns have a tendency to develop thatch as it relates to mowing frequency and varieties of grasses growing in your lawn. If you notice thatch developing in your lawn, you may need to de-thatch. Lawns with thatch up to ½" are generally OK, but as that thatch layer approaches ¾", it’s time to rent a de-thatching machine from your local rental store or garden center. It may be necessary to de-thatch in two directions, raking debris each time. Set the depth of the machine until the blades barely touch the soil surface. You will notice some thinning of the turf canopy; however, recovery will occur throughout the fall. De-thatching will improve the movement of nutrients and water into the root-zone where they are needed. Fall is usually the best time to de-thatch cool-season lawns, since de-thatching in the spring can disrupt your pre-emergence control for crabgrass.

Aeration is the 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. Soil plugs also spread some soil into the thatch layer where soil microbes can feed on thatch debris. Aeration can be done in both spring and fall and is the very best way to begin a fall fertilization program. Applications of fertilizer after aeration will move nutrients immediately into the root-zone 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 pH is also important as it affects nutrient availability to the plants. Soil test results will also give you the soil pH and any information about lime requirements. A soil pH around 6.6 to 6.8 is optimum. Soil pHs between 6.0 and 7.0 are acceptable. 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. This guide sheet can be accessed through the XPLOR web site @ http://muextension.missouri.edu/xplor/.

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 and 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, 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, de-thatching and fall fertilization can be directed to the MU Turfgrass Research Center at the number below.

Brad Fresenburg, UMC Extension/Research Associate, Turfgrass Research Center, (573) 443-4893