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Dethatching Lawns

You can help manage thatch by creating conditions that accelerate decomposition, such as maintaining proper pH, avoiding excess nitrogen, topdressing and aerating. Thatch is a dense mat of roots, stems, and grass clippings that accumulates on lawns, blocking the flow of vital nutrients. However, once you have a deep thatch layer, your only practical alternative is to remove it with mechanical dethatching equipment. Dethatching equipment includes three basic types: vertical mowers, flexible-tine dethatchers and multi-purpose dethatchers.

Dethatching a lawnTo combat thatch, use a dethatching implement or vertical mower that cuts through the thatch to the soil surface. As it passes through the grass, dead and living organic material is cut, torn loose, and deposited on the surface. If ½" or more of thatch is present, it may be necessary to cross the lawn two or three times in different directions. Operation of dethatching machines is relatively easy, since they are partially self-propelled. Most difficulty occurs on steep slopes or when soil is very dry.

Dethatcher bladesA good thatching machine has fixed knives or sling blades; spring tines are not effective. Lawn Dethatchers are power rakes designed for fast, easy removal of thatch, promoting healthy growth. Dethatchers utilize specially designed, free-swinging, steel flail blades mounted on a horizontal reel to penetrate the lawn surface, breaking up thatch and promoting vigorous growth. The surrounding grass is left intact. Dethatching also helps prepare the lawn surface for reseeding. Cutting depth is adjustable to match lawn conditions.

Select a dethatching machine that cuts with knives or blades. Some machines have flexible, leaf rake-type tines that are ineffective in removing thatch. Spring tines that can be attached to your rotary mower blade are not good for dethatching either, and can cause severe mower damage. Dethatching equipment can often be rented from lawn equipment rental companies.

The organic material dislodged by the dethatching machine should be removed and composted or discarded. It can be raked into piles or onto plastic sheets or a tarpaulin and placed in garbage bags or dumped in a truck. Several garbage bags or a full pickup load of thatch can be removed from just a small lawn.

Regular dethatching in your lawn

If you didn't dethatch your lawn in the spring, then another good time to get this chore done is early fall if you live in western Oregon, according to Tom Cook, turf grass specialist with the Oregon State University Extension Service.

If you live east of the Cascades, wait until spring, just as the grass starts to green up, generally about April for most areas.

Thatch is a layer of living and dead grass stems and roots. It is the natural consequence of a healthy lawn, explained Cook. For best results, dethatch your lawn about every 1 — 2 years, in March or April or in the early fall. Bentgrass lawns, the most common type in the Willamette Valley, are best maintained with an annual dethatching.

Regular dethatching forces buds to grow near the base of the grass stems, preventing the grass plants from being dead underneath and only green on top. Thatching frees new grass shoots to grow in thick and lush.

The easiest, most economical way to dethatch is to rent a dethatcher. Two types are available— the flail-type and the solid knife-type. The solid knife-type is better for bentgrass lawns, but may not be as readily available for rental except in larger, metropolitan areas.

Small dethatchers, sold as lawn mower attachments, are also available, but Cook doesn't recommend those, because they put tremendous strain on the lawn mower engine.

The old-fashioned, elbow-grease method to dethatch is to use a thatching rake.

Once you have rented your dethatcher, set the blades high enough so they are about 1/8" — 1/4" above the ground when placed on a hard surface such as a sidewalk.

"You don't want to destroy your lawn in the process of dethatching," warned Cook.

Dethatching should not pulverize the soil surfaces. Adjust the blades to about a quarter-inch above a concrete surface. Make between one to five passes through your lawn, until most thatch is removed.

After dethatching, fertilize the lawn with a nitrogen fertilizer to stimulate regrowth.

Homeowners who dethatch their lawns every 1 - 2 years will end up with about 1 - 3 pick-up loads of thatch from an average-size lawn.

"Thatch is only a problem when homeowners wait too long to dethatch," said Cook. "Dethatching regularly is just a little more work than mowing a lawn. But if you wait for too many years, removing thatch becomes a long, agonizing process. It needn't be."

The thatch can be composted or used for mulch if it is herbicide-free. If you have used a weed killer or "weed and feed" treatment in the month before dethatching, then do not use the removed thatch to make compost or mulch.

Never use clippings or thatch debris for mulch or compost if you have used a weed killer containing clopyralid. Even after composting, clopyralid remains active and can injure your ornamental plants.

If I couldn’t count, I would swear that thatch is the dirtiest four-letter word in a homeowners turf dictionary. Many homeowners seem obsessed about managing thatch and making sure that their prized lawn does not accumulate too much of it. There was a time not long ago when folks picked up clippings in order to avoid the risk of thatch accumulation.

Well, by now we all know that clippings if managed properly do not contribute to thatch accumulation. Before I get to tactics for managing thatch problems, let’s make sure that we understand what thatch is. Thatch as defined by Dr. James B. Beard is “a tightly intermingled layer of dead and living stems and roots that develops between the zone of green vegetation and the soil surface.” In the most basic sense thatch is what gives the turf that “spongy” feeling when you walk on it. If you ever walk on a recently established turf area you might notice that it feels very firm. That is because a thatch layer has not developed yet.

Just like beer, a little thatch is good but a lot can really cause a headache. A thin thatch layer of a half inch or less on lawns is normal and helps insulate the growing point of the plant, lower soil temperatures and retain moisture. However, when the thatch layer starts to accumulate to greater than a half inch, especially if it gets to be over an inch in depth, then it’s time to get off the sofa and take action. Excessive thatch accumulation can result in decreased rooting, increased potential of scalping and possibly increased insect and disease occurrence.

There are two basic procedures to manage thatch, and both likely involve a trip to the local rent-all store. Dethatching and aeration are the two methods you can use to reduce thatch accumulation.

Dethatching is the removal of thatch that has accumulated in the turf and is a very aggressive procedure that will leave piles of debris on the lawn that will then need to be raked and removed – a procedure not for the faint of heart. Dethatching equipment goes by many names including but not limited to: power rake, dethatcher, lawn comber, vertical mower, and slicer.

Aeration is the removal of soil plugs that will leave your lawn looking like you have rabbit problems. Aeration, besides removing thatch, offers the added benefits of reducing soil compaction and opening up channels in the soil for water infiltration and gas exchange. Renting aeration equipment is much easier; they simply go by the name aerators. The best time of the year to do aerating or dethatching is in the fall of the year, but make sure to do it early enough that the turf has at least 30 days to recover before the soil freezes and winter sets in.