Dethatching and power raking are both performed with the same machine. The difference in the two services is how deep the cutting knives are set on the dethatcher, as well as the type of cutting knives used. Dethatching generally involves the use of flail type blades that are designed to rip out excessive thatch. The dethatching machine is set low so that the blades reach into surface of the soil. Dethatching is best done in late summer/early fall. By waiting until late summer there will also be less competition with a variety of weeds that germinate in the spring.
Power raking generally involves using fixed knife type blades that slice the thatch as opposed to ripping it out. The dethatching machine is set high so that the blades just skim the surface. Power raking is not as hard on your lawn as dethatching might be, and your lawn recovers rapidly and maintains its density. Power raking can be done in the spring or fall. If you are power raking to remove winter debris it is recommended to do it before the lawn starts growing to avoid setting the lawn back during the critical spring time growth. If power raking is being combined with overseeding we recommend doing it in early spring to allow the new seedlings plenty of time to get established before the summer drought and heat.
Late summer/early fall remains the optimum time for any overseeding because you avoid the summer heat and drought and the new seedlings do not have to compete with the spring germinating weeds.
Mowing heights that are too short will result in weakened grass plants and weed encroachment. Short grass is susceptible to disease and drought damage, as well as ants and other insects prefer to live in grass that has been scalped short.
Individual grass plants provide healthy shade for themselves and each other at taller heights, as well as shading the topsoil and roots better. Your yard will need less watering and will stop weeds from germinating.
A shorter cutting in the fall helps grass to keep from lying down or matting during snowfalls, and prevents fungus, mold, and mildew from forming in the spring. Field mice like to nest in longer grass, under the snow, during the winter. They nibble grass leaves down to the stem and leave unsightly trails. Another reason is that in the spring your lawn usually sees excess water, which should run off of a short lawn but can pool up in long grass.
Continue mowing as long as your lawn keeps growing. Grass un-mowed late in the season can become matted under snow, making the turf more prone to snow mold diseases. You may continue to return grass clippings to the lawn if you’d like. Your last mowing of the season should be a closer cut than during the growing season.
Grass roots continue to absorb and utilize nutrients as long as the leaf blades are still green and the soil is not frozen, so a late fall fertilization is important. This extends fall color retention, allows early spring lawn green-up, encourages root growth, and provides increased stress tolerance. About the time of the season’s last mowing, bluegrass and tall fescue lawns should receive 0.5-0.75 pounds of Nitrogen per 1,000 square feet, using a fertilizer that contains mostly water soluable nitrogen, such as urea. Do not apply fertilizer after the turf has become brown or if the soil is frozen.
A thick layer of leaves left on the lawn can become wet, matted and smother the underlying grass, making conditions more conducive for snow mold development. Therefore, keep up with the raking.
If the lawn does not have thatch problems then a light layer of leaves, up to two inches, can be chopped up with a mower and allowed to filter back to the soil surface. Heavier layers of leaves should be removed, and could be added to a compost pile as a great source of carbon-rich organic matter. (Pieces of info taken from Sarah Browning, UNL Extension Educator)