The one-third rule says you shouldn’t cut more than one-third of grass’s height per mow. Hence, it’s best to mow when the turf is 50% taller than ideal. Doing so helps lawns look lively and lush.
Our article explains all the ins and outs of one-third mowing, including how it works, why it matters, and how to apply it each season. Using reputable research, personal practice, and expert interviews, we left no stone unturned. Now, we break it all down in an easy-to-follow guide.
Understanding the one-third rule
The one-third rule of mowing tells you not to trim more than ⅓ of the turf’s height. So, at least ⅔ of the grass blade tissue remains. For example, if the grass is 3 inches tall, don’t cut off more than 1 inch. It’s simple yet vital to maintaining a healthy lawn.
About 60 years ago, U.S. Department of Agriculture scientists figured it out. The one-third rule has remained the gold standard ever since. Thus, it is the first step in understanding how to mow the lawn correctly.
Why the one-third rule matters
The way a good stylist helps your hair, proper mowing matters to your lawn. Following the one-third rule strengthens grass blades and invigorates growth.
I asked Dr. Grady Miller, a distinguished professor in Turfgrass Management at NC State University, why the one-third rule is important.
“First, it is an easy-to-explain concept. Using it can help prevent (shock) when a significant amount of top growth is removed at once. This often cuts into the stem of the plant, rather than just leaf material. Turfgrass plants may not recover as quickly from the stress of those severe (low) cuts as well.”
Following this rule has significant benefits.
✔ Fosters photosynthesis
Plants get energy from photosynthesis like people get energy from eating. Lacking stomachs, grass turns light into storable energy by absorbing sunlight and carbon dioxide through its blades.
Cutting off too much blade tissue is like giving the grass a gastric sleeve. The butchered blades can’t soak up as much sun and, thus, produce less energy. Following the one-third rule helps your lawn get the energy it needs to thrive.
✔ Aids mineral absorption
Plants must also absorb water and minerals through their roots to photosynthesize. The most important minerals are NPK — nitrogen (N), phosphorus (P), and potassium (K). Grass with extensive roots slurp up hearty helpings of these nutrients.
Since stunted grass blades can’t store much energy, it uses most of its might to regrow blade tissue. This leaves roots unenergized, inhibiting their growth. As a result, grass absorbs less water and minerals.
✔ Creates natural mulch
The one-third rule turns mulched grass clippings into healthy organic fertilizer. The clippings quickly decompose into the soil, providing turf with essential minerals. It’s one of the best types of lawn food, and it takes minimal effort to fertilize the lawn—just leave the clippings on the grass.
I asked grass guru Chris Webber of Heritage Landscape Supply about the benefits of fertilizing turf with organic ingredients. Webber says they:
- Help water move through the soil
- Add structure to the soil
- Feed beneficial microbes that also helps support beneficial bacteria that helps with nutrient breakdown
Conversely, cutting more than one-third of the grass’s height creates excess clippings that harm the lawn in the following ways:
- Smothers the turf so the grass can’t synthesize the sun
- Increases thatch accumulation, which prevents water, air, and nutrients from reaching the roots
- Retains excess water in your lawn, which invites pests and encourages fungal growth
- Creates a habitat for pests
| My tip: The best mulching mowers make the finest grass clipping mulch, and the ones with side discharge distribute the fertilizer quickly and evenly. A regular mower also works if you replace the standard blades with gator blades, but the clippings won’t break down as rapidly. |
✔ Increasing immunity
Just like people, plants need proper energy to quickly recover from stress. Following the one-third mowing rule fortifies grass to fight its foes, including:
- Adverse weather, like extreme heat and extended drought
- Fungal infections, like powdery mildew and leaf spot
- Weeds, like dandelions and crabgrass
- Pests that damage grass, like chinch bugs and sod webworms
Additionally, grass that’s too short can’t shade the soil. When exposed to the love of the sun, lawn weeds are more likely to sprout.
How to apply the one-third rule
Properly applying the one-third rule maximizes the appearance and health of your lawn. Here’s what you need to know:
Know the ideal height for your grass species
Each type of turf has its own Goldilocks height. Thus, identifying your lawn’s grass type is paramount to knowing when to mow it.
The table below shows the ideal grass height for the most common types of turf.
| Grass type | Mow at this height (inches) | Cut to this height (inches) |
| COOL-SEASON GRASSES | ||
| Tall fescue | 3 to 6 | 2 to 4 |
| Kentucky bluegrass | 3 to 4.5 | 2 to 3 |
| Fine fescue | 2.25 to 4.5 | 1.5 to 3 |
| Perennial ryegrass | 2.25 to 4.5 | 1.5 to 3 |
| Bentgrass | .38 to 1.5 | 0.25 to 1 |
| WARM-SEASON GRASSES | ||
| Bahiagrass | 4.5 to 6 | 3 to 4 |
| St. Augustine | 3.75 to 6 | 2.5 to 4 |
| Buffalograss | 3 to 6 | 2 to 4 |
| Centipedegrass | 2.25 to 3 | 1.5 to 2 |
| Zoysiagrass | 1.5 to 3.75 | 1 to 2.5 |
| Carpetgrass | 1.5 to 3 | 1 to 2 |
| Bermudagrass | 1.5 to 3 | 1 to 2 |
Measure grass height
I recommend using the one-third rule as a guide, not a mandate. Miller reminds us that one-third rule is “a general guide to use. It does not mean that the grass will die if it is not followed.” So, please don’t stress over exact measurements. Instead, stick to a routine mowing schedule that keeps grass from growing too tall.
However, sometimes measuring the grass helps homeowners get a feel for timing. It might also aid those who struggle with spatial awareness. To measure the grass height, use a ruler or yard stick. Then, place the ruler level on the ground and measure to the top of the grass blades.
Remember to look at the average height of the lawn; don’t get distracted by a few overly grown areas.
Adjust the mower
Before mowing, don’t forget to adjust your lawn mower’s cutting height. Most mowers have one or two levers that adjust the height, but refer to the user manual for specifics on your model.
Do you want your hair cut with dull scissors? Or a surgeon operating with a dull scalpel? Probably not. And with good reasons. Dull blades make messy cuts, so they’re prone to damaging the things they slice.
Stay on top of sharpening lawn mower blades to reap the rewards from one-third mowing. Blades typically need sharpening after about 25 hours of use, meaning most mowers need it done around twice yearly. Otherwise, dull mower blades will likely tear up the turf.
Read more: Fall Lawn Care Tips
FAQ about the one-third rule
Yes, grass can recover from being cut too short if adequately cared for. That includes proper mowing and watering. You also have to learn from your mistakes. If you continually scalp the lawn, eventually, it can die.
If your grass grows more than 50% taller than the upper end of its ideal range, don’t cut more than ⅓ of its height per mow. Also, increase your mowing frequency until you get the grass back to its ideal range.
Whenever possible, it’s best to obey the one-third rule. However, when I lived in South Florida, many people didn’t follow it because the St. Augustine lawns grow super fast there. Since South Florida’s climate is so favorable to that turf, most lawns still thrived as long as they weren’t cut too short.
With that being said, Florida’s turf science professors and landscaping pros still recommend sticking to the one-third rule. If you decide to stray, do so at your own risk.
When to hire a lawn care pro
When in doubt, hire a pro. Lawn Love connects you to a network of vetted lawn care pros. They’re affordable, experienced, and insured. We back their quality by offering customers a 100% satisfaction guarantee. It only takes a moment to get an online quote, and you can manage your services via our user-friendly phone app.
Sources:
- Becky Bowling, agricultural and resource economics professor and extension specialist at the University of Tennessee. Personal interview.
- Chris Webber, manager at Heritage Landscape Supply. Personal interview.
- Clint Waltz, turfgrass extension specialist and professor at the University of Georgia. Personal interview.
- Grady Miller, ENVU Distinguished Professor of Sustainability & Extension Specialist in Turfgrass Management at NC State University. Personal interview.
- “How to control thatch in your lawn.” By Sam Bauer, extension educator, Deborah Brown, contributor, and Don Taylor, contributor. University of Minnesota Extension.
- “The One-Third Rule.” By Bob Vavrek, regional director. United States Golf Association.
- “The Story of the 1/3 Rule.” By Marcus Jones, graduate research assistant. Iowa State University Extension and Outreach.
Main Image Credit: Zedspider | Adobe Stock Free | License
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