Power raking a lawn, also called dethatching, is a great way to remove the buildup of excess dead plant material, improve water and nutrient flow to the roots and to stimulate new grass growth. Heavy thatch can choke grass plants, protect weeds and insects from the chemicals that fight them, and increase runoff from rain and watering.
Rake When Growing Is Good
Power raking untangles dead grass from the living plants around it, a process that can bruise and even kill the live plants. Power raking should be done either in the spring or fall, when grass is growing vigorously but heat stress is not a problem. This will give plants a chance to recover before they become dormant in winter or midsummer. Experts say fall is better than spring.
"The increased moisture and cooler temperatures at that time of year cause grass to grow rapidly and recover quickly," said David J. Robson Extension Educator for the University of Illinois, adding that "Spring is the second best season to take care of thatch." If you choose a spring dethatching make sure the ground is at least 55 degrees, the temperature where new grass begins to grow.
And When Soil Is Dry
Power rake when the soil is dry but the grass is not brittle. Wet soil will not hold live grass plants well, and more will get pulled out or torn during the raking than when the soil is drier.
Before Seeding or Topdressing
Many people start the growing season by adding fresh grass seed to their lawns. This overseeding works best when the seeds reach the ground. Sometimes seeds can sprout within the dead grass layer, called the thatch layer, but those plants are weak and usually die. Another spring maintenance task is called topdressing. People will rake a thin layer of fresh soil atop the lawn. Topdressing can help decompose a thin layer of thatch, but in most cases it is better to apply the soil after the thatch layer has been power raked.
Preparing for Aeration
If you plan to aerate your lawn, or have holes punched into it to improve the soil, dethatching before hand is a good idea. Thick thatch layers make aeration less effective and there is little risk that reducing thatch before aeration can harm the lawn. Dethatch several days before aeration to let grass plants recover.
When Not to Thatch
It is a common misconception that all thatch is bad. A healthy thatch layer protects soil from erosion, reduces heat stress on grass, and composts dead plants into nutrients the living plants can use. Thatch is not a problem until it exceeds one-half to three-quarters of an inch. Before that point, leave thatch alone unless you have a need for clear access to the topsoil --- say you want to topdress or overseed.