Why Is My Lawn Mower Spitting Out Grass

You’re mowing your lawn, aiming for those perfect stripes, but instead of a clean cut, your mower is leaving behind messy clumps of grass. It’s frustrating, and it can leave your lawn looking worse than when you started. Understanding why your lawn mower is spitting out grass is the first step to fixing the problem and getting back to a beautiful, even cut.

This clumping, often called “grass vomiting,” happens when cut grass isn’t properly dispersed back onto the lawn. It can smother your grass, leading to brown patches and fungal disease. But don’t worry, the causes are almost always simple to diagnose and fix yourself.

That’s the core question. The answer usually falls into one of four categories: your mowing technique, the condition of your lawn, the state of your mower, or the weather. We’ll break down each of these in detail so you can play detective and find your specific culprit.

The Usual Suspects: Wet Grass and Dull Blades

These two are the most common reasons for clumping by far. They often work together to create the perfect storm for a messy mow.

Mowing Wet Grass: This is public enemy number one. Wet grass clippings are heavy and sticky. They cling to each other and to the inside of your mower deck instead of being blown out the side chute or chopped finely by the blade. The result is large, wet clumps that get dumped on your lawn.

Always wait for the grass to dry after rain or morning dew.
If you must mow damp grass, cut only a small portion of the blade height.

Dull Mower Blades: A sharp blade makes a clean cut. A dull blade tears and shreds the grass. Torn grass blades release more moisture, making them clump together instantly. They also take longer to decompose if you’re mulching.

A dull blade stresses the grass plant, making it more susceptible to disease.
You’ll notice a ragged, whitish tip on the grass blades left behind.

Mowing Technique Mistakes

How you mow is just as important as when you mow or what you mow with. A few bad habits can guarantee a clumpy outcome.

Cutting Too Much at Once: This is known as “scalping.” If you let your grass grow too long and then cut it back to your regular height, you’re removing too much leaf material at one time. The mower can’t process that volume, so it spits it out in chunks.

Follow the “One-Third Rule”: Never remove more than one-third of the grass blade height in a single mowing.
If the grass is very long, mow it in stages over a few days.

Mowing Too Fast: Pushing your mower too quickly doesn’t give it time to properly lift, cut, and disperse the grass clippings. The deck gets overloaded, and the excess gets dumped.

Take your time, especially in thick or tall grass.
A steady, moderate pace gives the mower’s airflow system time to work.

Lawn Mower Maintenance Issues

A poorly maintained mower is a clumpy mower. Several key parts need to be in good shape for clean cutting and dispersal.

Clogged Mower Deck

The underside of your mower deck is a clumping headquarters. Grass, mud, and thatch build up here, restricting the crucial airflow that lifts the grass for a even cut and carries clippings out of the deck. This buildup also traps moisture, creating a pasty environment perfect for clumps to form.

Clean your mower deck after every few mows, and always before storing it for winter.
Use a scraper, putty knife, or a dedicated deck cleaning tool.

Faulty or Missing Discharge Components

Your mower is designed to handle grass in a specific way. If parts are altered or missing, the system fails.

Side Discharge Chute: If your mower has one, it must be attached. Running the mower without it causes immediate clumping and is dangerous.
Mulching Plug: If you’re using a mulching kit, the plug must seal the deck completely. If it’s not installed correctly or is worn out, clippings will escape improperly.
Bagging System: For rear or side baggers, ensure the bag is properly attached and not overfull. A torn bag or a loose connection will drop clumps.

Engine Speed & Airflow

Your mower needs to run at full throttle to generate enough blade speed and suction under the deck. Running it at a slower speed reduces lifting power and chopping ability.

Always engage the blade at full engine speed.
Check your owner’s manual; some mowers have a recommended “mowing” setting on the throttle control.

Lawn Health and Grass Type Factors

Sometimes, the problem isn’t your mower or technique—it’s what’s growing in your yard.

Thick, Lush Growth: A very healthy, dense lawn produces a lot of clippings. Even under ideal conditions, a super-thick lawn can sometimes overwhelm a mower, especially if the grass is a bit damp.

Certain Grass Types: Some grasses, like Kentucky Bluegrass or perennial ryegrass, have a finer blade and cut cleanly. Others, like tall fescue or St. Augustine, have broader, fleshier blades that contain more water and can be more prone to clumping if not managed carefully.

Excessive Thatch: A thick layer of thatch (the spongy layer of dead roots and stems) can prevent clippings from falling down to the soil surface. They sit on top of the thatch and form mats.

Step-by-Step Troubleshooting Guide

Follow this checklist in order to find and fix your clumping problem.

Step 1: Check Your Mowing Conditions

Is the grass wet? If yes, stop mowing and wait for it to dry.
Has the grass grown too tall? If it’s more than 3 inches above your desired height, plan a staged cut.
Is the thatch layer over 1/2 inch thick? If so, consider dethatching as a separate lawn care task.

Step 2: Inspect and Clean the Mower Deck

Disconnect the spark plug wire for safety.
Tip the mower on its side (check your manual for the correct side—usually with the air filter up).
Scrape away all caked-on grass and debris from the underside of the deck.
Rinse it with a hose (avoid direct spray on the engine) and let it dry completely.

Step 3: Examine the Mower Blade

With the spark plug still disconnected, check the blade for sharpness and balance.
Look for nicks, dents, or a rounded edge. A sharp blade will have a distinct, fine edge.
If it’s dull or damaged, remove it (wear gloves) and sharpen it with a file or grinder, or replace it.
Remember to balance the blade after sharpening by hanging it on a nail through the center hole; it should sit level.

Step 4: Verify All Mower Attachments

Ensure the side discharge chute is firmly clicked in place and not cracked.
If mulching, confirm the mulching plug is installed and sealing the exit.
If bagging, check the bag for holes and ensure it’s clipped on securely. Empty it before it gets completely full.

Step 5: Adjust Your Mowing Practice

Mow at the correct time: late afternoon when grass is dry is often best.
Slow down your mowing pace.
Overlap your mowing rows slightly to ensure even cutting and dispersal.
Alternate your mowing pattern each time you mow to prevent soil compaction and grain.

Preventing Grass Clumps in the Future

An ounce of prevention is worth a pound of cure, especially when that cure involves raking up wet grass clumps. Here’s how to make clumping a rare event.

Regular Mower Upkeep Schedule

Sharpen Blades: Sharpen your mower blade at least twice per mowing season, or more often if you have a large lawn or hit debris.
Clean After Use: Get in the habit of brushing or hosing off the deck after each mowing session before grass dries and hardens.
End-of-Season Service: Before winter storage, give your mower a full tune-up: change the oil, replace the air filter, and service the spark plug.

Smart Lawn Care Habits

Stick to the One-Third Rule religiously. This is the single best habit for lawn health and clump prevention.
Mow more frequently during peak growing seasons (spring and early summer).
Consider mulching mowing. A dedicated mulching mower with a sharp blade chops clippings so finely they disappear into the lawn, providing free fertilizer. This only works if you mow regularly and the grass is dry.

When to Consider a Different Mower

If you’ve tried everything and still have chronic clumping, your mower might be part of the problem. Older mowers or low-horsepower models may struggle with thick grass. A mower with a higher horsepower engine, a “high-lift” blade designed for better suction, or a dedicated mulching system might be a worthwhile investment for your specific lawn.

FAQ: Solving Lawn Mower Clumping

Q: Why does my mower leave clumps of grass even when it’s dry?
A: The most likely cause is a dull mower blade. A dull blade tears grass, releasing moisture that causes clumping. The second most common cause is a clogged mower deck restricting airflow. Check and sharpen your blade and clean the deck thoroughly.

Q: Is it bad to leave grass clumps on the lawn?
A: Yes, it’s not recommended. Grass clumps block sunlight and air from reaching the grass blades underneath. This can kill the living grass, creating brown, dead patches and promoting fungal diseases like snow mold.

Q: How do I fix clumps after mowing?
A: The best tool is a simple garden rake. Gently rake the clumps to break them apart and spread them thinly over the lawn. If the clumps are large and wet, you may need to pick them up and compost them or add them to your yard waste bag.

Q: My mower is brand new, so why is it spitting out grass?
A: New mowers can sometimes have a manufacturing coating on the deck that makes grass stick. Also, the blade, while new, may not be perfectly sharp from the factory. Clean the deck and consider sharpening the blade slightly. Also, double-check that all discharge chutes or mulching plugs are installed correctly per the manual.

Q: Should I mow with or without the bag to prevent clumps?
A: It depends on conditions. If the grass is damp or too long, bagging is the safer choice to avoid clumps. If the grass is dry and you haven’t removed more than one-third of the blade, mulching (no bag) is excellent for lawn health. Side discharging can help in tall grass but requires raking afterward.

Q: Can grass type really affect clumping?
A> Absolutely. Grasses with broad, soft blades (like tall fescue) hold more water and can clump more easily than fine-bladed grasses (like Bermuda). Adjust your mowing schedule to ensure these types are completely dry before you cut them.

By working through these steps, you’ll likely find that the answer to “why is my lawn mower spitting out grass” is something you can fix in an afternoon. Consistent maintenance and good mowing habits are the true secrets to a clean cut and a clump-free lawn. Your grass will thank you for it with healthier, greener growth.