Thatch and Dethatching in Montana Lawns

What is thatch?

MSU Extension's MontGuide on maintaining lawns defines thatch as primarily the accumulation of dead, nondecomposed rhizomes. Colorado State University Extension describes it as the spongy, brown layer of organic matter between the leaf zone and the soil. It forms when grass produces stems and roots faster than soil life breaks them down.

It is not clippings. Both MSU and CSU say grass clippings do not cause thatch; they are mostly water and decompose quickly. Bagging clippings to prevent thatch removes free nitrogen and does nothing about the actual cause.

How thick is too thick?

Half an inch. MSU says a thatch layer thicker than 1/2 inch interferes with the penetration of air, water and nutrients into the turf, and its photos show a healthy layer of no more than 1/2 inch next to one approaching 3 inches. CSU uses the same threshold: over half an inch, it is important to reduce the buildup.

To check, cut a small wedge out of the lawn with a knife or spade, at least a few inches deep, and look at the profile from the side. Green blades on top, soil at the bottom, and between them the brown spongy layer. Measure that layer, not the green. Check several spots; shady corners, sunny slopes and the area by the sprinkler that runs long can all differ.

What causes heavy thatch in Montana lawns?

Mostly too much growth and too little breakdown. MSU says that in a healthy, well-maintained lawn thatch will not accumulate, and that most thatch problems are caused by overvigorous growth and can be corrected by reducing nitrogen and/or irrigation. CSU names soil compaction as the primary factor and lists excessive nitrogen, infrequent mowing, vigorously growing turf varieties and some lawn pesticides as others.

Grass type matters too. CSU's turfgrass selection guide lists Kentucky bluegrass as a thatch-former, turf-type tall fescue as a slow thatch-former, and perennial ryegrass as not forming thatch; fine fescues can also become thatchy. Most Montana lawns are bluegrass-based, so most of them can build thatch if they are pushed with heavy feeding and daily watering.

CSU adds a pattern we recognize: thatch problems are very common on newer sod laid over clay-type soils, where roots never really move into the hard soil below. Our guide to sod or seed explains why soil prep before sod matters.

Why is thick thatch a problem?

Beyond blocking water and air, thick thatch holds roots up in a layer that dries out fast, so the lawn browns sooner in heat. MSU links thatch to disease: its advice for melting out and leaf spot includes removing thatch by aeration in spring if it is more than 1/2 inch, and it lists keeping thatch under 1/2 inch among the controls for Rhizoctonia blight. MSU notes that fairy ring is worst where turf has a dense thatch layer, and that billbug larvae are typically found just below the thatch.

It also gets in the way of treatments. MSU says aeration hastens the penetration of grub control products into heavily thatched turf. Our guide to lawn insects in Montana covers those pests.

Should I dethatch or aerate?

For most lawns, aerate. MSU says that where thatch is heavy or soil is compacted, aeration will provide quicker results than dethatching. It recommends coring machines that remove cores 1/4 to 3/8 inch in diameter and 3 to 4 inches deep, and says that on poor soils a thin (1/4 inch) layer of compost after aerating can improve soil quality. Solid spike devices, MSU says, are not effective and can contribute to compaction.

MSU is blunt about power rakes: mechanical dethatchers should only be used when the thatch layer has become overly excessive, because dethatching causes severe damage to the grass and is roughly equivalent to renovating the lawn. If you are considering that level of work, read our guide to renovating a tired lawn first; it may be the more honest description of the job.

When is the right time to dethatch or aerate here?

Dethatching: MSU says to dethatch in spring before the grass has started to grow, to reduce the impact, and to follow up with reduced nitrogen and/or irrigation so thatch does not rebuild. Aeration: MSU says to aerate when turf is vigorously growing in spring and fall, never when it is dormant, and when the soil is moist enough for the tines to penetrate.

CSU favors late August to late September for aeration because fewer weed seeds germinate in fall, and suggests spring and fall where compaction or thatch is a problem. Our guide to when to aerate and overseed has the western Montana timing in detail.

How do you aerate properly for thatch?

Make a lot of holes. CSU recommends plug holes 2 to 3 inches deep and no more than 2 to 4 inches apart, and its aeration note suggests aiming for holes on 2 inch centers, which usually means more than one pass. Water a couple of days beforehand so the tines pull full plugs; if the soil is too dry, plugs come out short or the machine bounces. Mark sprinkler heads first, since CSU warns it is easy to hit them.

Leave the cores on the lawn. CSU says to leave plugs for a few days so rain and irrigation wash the soil out of them, and that they break down with a few mowings. That soil working into the thatch is part of how aeration helps it decompose. If you overseed, do it right after aerating, when MSU says seed-to-soil contact is best.

How do I keep thatch from coming back?

Change the inputs that built it. MSU's fix is reducing nitrogen and/or irrigation, and its fertilizer guidance splits the year's nitrogen into two or three moderate applications, with the most important feedings in fall. Our guide to fertilizer rates and timing explains how that translates to a bag.

Water deeply and infrequently instead of lightly every day (MSU), mow often enough to take only a third of the blade (MSU and CSU), and leave clippings on the lawn. Annual core aeration on compacted or clay ground keeps the soil life that digests thatch working.

Does thatch change how I should water?

Yes. MSU says thatch thicker than 1/2 inch interferes with water penetration, so on a thatchy lawn more of each watering runs off or sits in the spongy layer instead of reaching the roots. CSU's PlantTalk on efficient watering says heavy clay soil can absorb only about a quarter inch of water an hour, and suggests splitting a long run into shorter cycles (two 10 minute runs instead of one 20 minute run). The same approach helps a thatchy lawn while you work the thatch down.

Check whether water is getting through by pushing a screwdriver or trowel about 3 inches into the soil the day after watering, as CSU suggests. If the thatch is wet and the soil beneath is dry, the lawn is not getting the watering you think it is. Our summer watering guide covers amounts and timing.

Should I rent an aerator or hire it out?

Either works if enough holes get made. CSU notes that aerators are readily available from rental firms and are about as easy to use as a lawn mower, and its aeration note is frank that a single commercial pass can leave holes several inches apart and do little for the turf. Whoever runs the machine, ask how many passes, how deep the plugs will be, and whether sprinkler heads will be flagged first.

Renting a power rake is a different decision. Because MSU compares dethatching to renovating the lawn, plan it like a renovation: early spring timing, seed ready for thin spots, and a watering plan for the recovery weeks.

Thatch help from HJ

We core-aerate (we do not spike-roll) and overseed through our aeration and overseeding service, and we will tell you honestly if a lawn needs renovation instead. Our licensed fertilizer and weed control program is built to feed without overfeeding, and our lawn care crews mow at the right height. This guide is part of our Montana Landscaping, Lawn & Irrigation Guide. Call (406) 215-7319 or request a free estimate.

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