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

Home lawns have once again struggled through another hot, droughty summer. Our cool-season grasses like Kentucky bluegrass, tall fescue and perennial ryegrass have faced some disease pressures, insects and a lengthy dormancy. We have gone from the sixth driest July on record to the wettest August on record. Our night temperatures are beginning to dip into the sixties once again and possibly the fifties in a few weeks. We have seen significant recovery in many of our lawns, however some parts of our lawns may not be coming back and therefore require some over-seeding. With all this in mind, it’s that time of the year again to open up the soil and begin fall fertilization. One additional step to include this year will be over-seeding.

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, 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. Spreading grass seed after aeration is also an excellent practice in lawns that have thinned considerably from summer drought.

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.

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 inch are generally okay, but as that thatch layer approaches ¾ inch, 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.

A de-thatching machine is also an excellent piece of equipment to prepare seedbeds prior to over-seeding. The removal of thatch, with thinning of the turf canopy exposing soil is a perfect situation for dropping seed into a lawn improving seed/soil contact.

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 give you nutrient levels, soil pH and any information about lime requirements. A soil pH around 6.5 to 6.8 is optimum. Soil pH 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 EXPLORE Web site at http://muextension.missouri.edu/explore.

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 to three 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 analysis label on the bag and find a product with 30 to 70 percent 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 or lower amounts of P and K.

Winterizing fertilizers are usually recommended as the final application of the fall for cool-season grasses. 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 additional potassium for hardening off plants. Additional potassium does not increase plant tissue potassium if amounts of potassium in the soil are already high to very high. Application of winterizing fertilizers simply insures potassium levels will be sufficient for plants to harden off. Plants harden off by reducing the amount of water in plant cells, therefore reducing the threat of winter freezing. If you regularly soil test and know that your potassium levels are high, then a winterizer fertilizer will not provide additional benefit for you. It is a practice of higher importance for warm-season grasses (zoysia and Bermuda) in late summer (early September) as opposed to cool-season grasses in late fall.

Any additional questions on aeration, de-thatching and fall fertilization can be directed to Brad Fresenburg at (573) 442-4893.

Brad S. Fresenburg
Turf Specialist,
MU Division of Plant Sciences
573-442-4893