What to do with grass clippings

Grass clippings are the only free input a lawn produces, and the standard household response is to collect them, bag them and drive them somewhere. Extension guidance disagrees with almost every part of that, and it disagrees with numbers attached.

The case for leaving them

The conversion from a rate to an amount of product is in fertilizer numbers explained. The short version: on a 4,000 sq ft lawn, one pound of nitrogen per 1,000 sq ft from a 24% bag is about 17 lb of product you did not have to buy or spread.

The thatch myth, settled

The most persistent reason people bag is a belief that clippings cause thatch. Extension guidance addresses it directly: leaving clippings on the lawn does not contribute to increased thatch, and clippings break down rapidly and do not accumulate.

Thatch has its own causes and its own threshold. Minnesota Extension treats thatch over half an inch as a problem, and attributes it to vigorous varieties, excessive nitrogen especially in spring, compacted soil, acidic soil, poor drainage and pesticides that suppress the microbial and earthworm activity that breaks thatch down. Clippings are on none of those lists.

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The three cases for removing them

Two of those three are avoidable by mowing more often. Extension guidance is that clippings of an inch or less filter down and decompose quickly, and you only produce longer clippings by letting the lawn get away from you. The interval logic is in how often to mow.

Using clippings as mulch elsewhere

If you have collected clippings anyway, they are a usable mulch in beds and around vegetables — with two published constraints.

• Depth. Do not apply more than 1 or 2 inches of grass clippings at one time.
• Moisture. Avoid wet grass clippings, which can mat down, reducing oxygen and moisture.
• Source. Do not use clippings from a lawn that has been treated with anything you would not want on a vegetable bed, and do not use diseased material.

Dry, thin layers, topped up regularly, is the pattern that works. A single thick wet layer is the one that goes slimy and smells.

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Composting them properly

Grass clippings are the classic compost failure, and extension guidance identifies the reason precisely: a thick layer of grass clippings in a compost pile will lead to bad odors from anaerobic decomposition.

1. Mix them with dry materials rather than adding them as a layer. Leaves, straw and shredded card all work.
2. Add in thin amounts, more often, instead of a full bag at once.
3. Turn the pile after adding a quantity of clippings, to get air into the material.
4. Keep diseased material out of a pile you intend to spread back on the lawn.

Autumn leaves are the ideal dry partner, and the same season produces both. Penn State Extension separately supports mulching a leaf layer of up to about half an inch straight into the lawn — the mower method is in best mower for mulching leaves.

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The decision, in one place

What we cannot tell you

Extension guidance quantifies the fertilizer value as the equivalent of one application per year and does not give a nutrient percentage for clippings, so neither do we. We also cannot tell you how quickly clippings break down in your conditions — that depends on moisture, temperature and soil biology.

What is published: clippings are a nutrient source, they do not cause thatch, an inch or less filters down and decomposes quickly, remove them only when diseased, too long or wet, mulch at no more than 1 to 2 inches, and mix with dry material to compost.