Composting leaves, grass and yard trimmings, and the herbicide check

Your yard already makes most of what a pile needs. This chapter covers how to store leaves, what to do with grass, wood and manure, and the one hazard in yard material that composting does not fix: persistent herbicides.

Leaves: the year's browns Understand

Fall leaves are the browns that most yards supply for free. EPA's rule of thumb is to add at least two to three times as much browns as greens by volume, with dry leaves as its example of browns (EPA's Composting at Home page). Convention A household that adds food scraps all year therefore needs a supply of leaves that lasts all year, and leaves fall in about one season. Storing them is the job.

Shred, then store

Shredded leaves break down faster. The University of Illinois Extension says brown leaves break down faster if they are shredded and moistened, and suggests running a lawn mower over raked leaves (University of Illinois Extension on recycling leaves). NC State's Beaufort County center says shredded leaves decompose more quickly and evenly than whole ones, and that whole leaves still work in layers (NC State Beaufort County on fall leaves). So a mower pass is useful and not required. A bag of shredded leaves holds a different volume of leaf than a bag of whole ones, which is one reason the calculator's count is an estimate.

Store the leaves you will use as browns during the year in bags or a bin you can reach in winter. Illinois suggests garbage bags with small holes for air. Illinois also says to keep diseased leaves apart from the ones you recycle; Learn Vegetable Gardening's close-the-season chapter covers what to do with diseased plants at the end of a season, and What goes in holds the stays-out list for compost.

How many bags a year of scraps needs

The arithmetic is short, and the browns calculator does it from your scrap log. Gallons of food scraps a week, times the browns ratio, times the weeks to cover, divided by the size of a bag, rounded up. The ratio is the volume of browns to greens: EPA says two to three times, and other sources give anything from equal parts to three parts. Convention

A cubic foot holds about 7.48 gallons, which is how the table converts. The count assumes loose leaves measured by volume. Leaves settle and shrink in a bag and break down while they wait, so the number of bags you need depends on how much yours settle. Count again after your first fall and write the result on the plan. Convention

Leaf mold and leaves on the lawn

Not every leaf has to go in a bin. Leaf mold is leaves left to decay on their own, without food scraps. Penn State's extension says to form a pile of leaves, pack them down, water them and wait a couple of years, in a wood or wire bin 3 to 6 feet across and at least 3 feet tall, or in a large plastic bag or trash can (Penn State Extension's Leaves Are Treasure, Not Trash). It says shredding, turning, watering or covering the pile can bring that down to 6 to 12 months. UGA Extension's Paulding County center says to let the pile mold for 1 to 2 years, moist but not soaking wet (UGA Paulding County's leaf mold post). Penn State describes leaf mold as not high in nutrients but good for soil structure and water holding. Convention Use it as mulch or mix it into soil, not as a fertilizer.

The other easy option is the lawn. Penn State says a layer of mowed leaves up to about half an inch deep is beneficial to the lawn, and that a mulching mower works best. Deeper than that is more than the source describes.

Grass clippings

Grasscycling means leaving clippings on the lawn when you mow. It is the lowest-attention option in this chapter, and the one this manual uses as the default. Penn State's turf specialist reports that, over three years, Kentucky bluegrass clippings held 46 to 59 percent of the nitrogen applied as fertilizer, so returning them let a lawn get by with less fertilizer (Penn State Extension on recycling turfgrass clippings). The study is on one grass in one place. UGA Extension reports that clippings return as much as 20 percent of the nitrogen in some conditions and that one study found they break down in less than one week (UGA Extension's grasscycling circular). Both say clippings do not cause thatch, the tightly intermingled layer of partly decomposed stems and roots between the green grass and the soil; UGA says it comes from stems, shoots and roots. UGA says to mow often enough to remove no more than one-third of the plant height. Test it If you see clumps of clippings after a wet mow, rake them off the surface.

Clippings in a pile

Fresh clippings are wet and nitrogen-rich, so in a pile they behave like food scraps. UF/IFAS Extension in Sarasota County says wet grass can mat down and clump into anaerobic pockets, and that in a layered pile the green layers should be no more than 1 to 2 inches thick, with bulky material such as wood chips, leaves and cut-up sticks mixed in (UF/IFAS Sarasota County's problem materials page). The default here: if clippings go into a bin, add a thin layer and cover it with leaves. Convention

Did you treat the lawn?

The clippings of a lawn that gets a weed killer carry whatever the product leaves behind. You cannot tell by looking. Two steps cover it. Find the product's name from the bag or from whoever applies it. Then read the composting line on its label. The University of Nebraska–Lincoln says to check the labels of any pesticides used within the last year before composting material from your own yard (Nebraska CropWatch on aminopyralid restrictions). Ask the authority

What EPA says about lawn products is narrow and specific, and we quote it rather than advise on any product. In its interim decisions for the persistent herbicide clopyralid, EPA removed residential turf from all labels. For two other herbicides on residential turf, triclopyr and fluroxypyr, the interim decisions bar using treated plant material for compost until 30 days after application (EPA's registration review of pyridine and pyrimidine herbicides). We cannot tell which active ingredient is in your product. If you cannot find out, leave the clippings on the lawn. Questions about a product go to the National Pesticide Information Center, 1-800-858-7378, or your extension office.

Woody material, pine needles, walnut and ash

Woody trimmings are slow, and chips are better as mulch than as compost. Linda Chalker-Scott's review for Washington State University says wood chips are slow decomposers because of lignin and similar compounds, that studies show woody mulch raises nutrient levels over time rather than tying up nitrogen, and that the nitrogen shortage is at the boundary between the mulch and the soil (WSU Extension's wood chips as landscape mulch). The practical default: chip prunings and spread the chips as mulch. If you add wood chips to a pile, EPA lists untreated wood chips among browns. Treated, painted or pressure-treated wood stays out of every system (What goes in).

Pine needles

NC State Extension's Lee County center says pine straw mulch acidifies soil no more than any other mulch (NC State Lee County on pine needle mulch). NC State's gardener handbook lists pine needles among items that resist decay because of a waxy coating, and keeps them out of its pile list (NC State's Extension Gardener Handbook, Composting). The default: use needles as mulch. Neither source reports harm from a few needles in a pile, but the handbook keeps them off its pile list.

Black walnut

The default here is NC State's: keep black walnut leaves and twigs out of the pile, because the handbook says they release substances that might harm plants. If you have a walnut tree, the comparison with the sources that allow it is under Go deeper.

Manure

Herbivore manure, which is manure from plant-eating animals, is a source of nitrogen and bulk, and a source of risks that come from what it carries. Penn State says cow, horse, chicken, sheep, goat and llama manure are acceptable for vegetable gardens; pig, dog, cat and human waste are not (Penn State Extension's Wise Use of Manure in Home Vegetable Gardens). Iowa State agrees on cat, dog and pig (Iowa State Extension's Using Manure in the Home Garden). Convention Dog and cat waste stay out of every system this manual teaches. What goes in has the reasons.

Raw, aged and composted are three different things

Penn State says aged manure is not the same as composted manure, and that it is not safe to assume the pathogens in an aged pile have been destroyed. Composting means a managed pile that reaches temperature. Penn State says composting at 140°F neutralizes the weed seeds in horse and cow manure, and Iowa State says composting kills many weed seeds and pathogens as long as the pile reaches 131°F or higher and is at least roughly 1 cubic yard. Iowa State also says to wear gloves when touching the compost and to wash your hands when finished.

A manure pile is a pile like any other: The hot pile covers building a batch that heats. A household that uses manure without composting it first is using raw manure.

The 120-day and 90-day rule is for certified organic growers

For raw manure, the USDA National Organic Program sets intervals before harvest: at least 120 days for a crop whose edible part touches the soil, and at least 90 days for a crop whose edible part does not (7 CFR 205.203). That is a rule for certified organic producers. Penn State and Iowa State both pass the same figures to home gardeners as guidance, and Iowa State adds that a spring application leaves too little time for many vegetables, so applying in the fall is one way to fit the interval. We cite the intervals as the reference most extension services use. They are not a rule for a home garden, and a household that follows them is following a convention. Convention

For rates, and what compost or manure does to garden soil, go to Learn Vegetable Gardening's feed-the-soil chapter and a soil test.

Persistent herbicides

Most of what makes yard material unsafe to compost can be dealt with in the pile, by heat, time or keeping it out. One class of weed killer is the exception. Three herbicides that kill broadleaf weeds, aminopyralid, clopyralid and picloram, are a persistent herbicide problem: they survive the heat and the microbes of a compost pile. Oregon State Extension's publication on herbicide-contaminated compost names clopyralid and aminopyralid as examples that persist through composting (Oregon State's EM 9307). NC State's 2024 revision of its herbicide carryover publication says all three can remain active in manure even after it is composted (NC State's AG-727). Physical principle

How they reach a pile

The herbicides are used on pasture, hayfields, wheat and other grain, rights of way and turf. They pass through animals unchanged, so manure from an animal that ate treated hay or grass carries them. NPIC, the National Pesticide Information Center, says the residues are not toxic to livestock but can pass through the digestive tract unchanged and end up in urine and manure (NPIC's compost FAQ). NC State adds that it takes three to seven days for most animals' digestive tracts to clear. EPA lists four pathways into compost: treated turf clippings, hay from treated pastures, straw from treated wheat and other cereal grains, and manure from animals that grazed in treated areas.

How long they last and what they damage

NC State says the herbicides can be deactivated in as few as 30 days, but that complete breakdown can take several years in some field reports, that hay stored in dry, dark barns has shown activity after three years, and that breakdown is particularly slow in piles of manure and compost. The figures vary by report, and they sit side by side under Go deeper. What the reports share is that a pile is not enough to clear it.

Damage shows in broadleaf plants at very low levels. A 2015 Vermont flyer from state agencies and UVM Extension says plants can show symptoms at 1 part per billion, and that in summer 2012 compost made from contaminated manure damaged over 500 home gardens in Vermont (Vermont's persistent herbicide flyer). NC State's table of sensitive crops includes beans, peas, tomatoes, potatoes, peppers, lettuce, carrots, sunflowers and roses. NPIC describes the damage as cupping or curling of leaves. Oregon State says turf, ornamental grasses, lilies and other nonbroadleaf plants (monocots) are not affected.

What EPA and one label say

EPA's registration-review page for these herbicides says the following mitigation measures are required through its interim decisions for aminopyralid, clopyralid and picloram: a prohibition on off-site use of treated plant materials, and manure from grazing animals, for compost and animal bedding or feed until 18 months after application; a requirement that livestock graze on untreated forage for three days before moving to a site where manure is collected or sensitive crops are grown; removal of residential turf from all clopyralid labels; notification of landowners by applicators; and an updated compost pictogram on labels. EPA's page was last updated on September 8, 2026. These are requirements on labels and on the people who apply the products, so they matter to you in a particular way: they give you questions to ask. They do not tell you the history of a bale of hay or a load of manure.

The label for Milestone, an aminopyralid product (EPA registration 62719-519, accepted 2020), is one example of the wording (the Milestone label on EPA's product label system). It says to follow the Use Precautions and Restrictions section. It bars using hay or straw from treated areas within the preceding 18 months in compost, and bars using manure from animals that ate such hay. It also says manure from animals that grazed or ate treated forage within the previous 3 days may be used only on pasture grasses, grass grown for seed, and wheat, and that no broadleaf crop should be planted on a field that received such manure the previous year until an adequately sensitive field bioassay is done. A newer label for the same product may exist. We quote the label to show what labels say, not to advise on its use.

Ask first

The simplest check is a question. Ask the person who gave or sold you the material what was sprayed, on what, and when. Nebraska says homeowners who receive manure should ask about the products used; NPIC says to ask a compost seller about the source and which herbicides were used to grow the animals' feed or grass. Ask the authority Write the answer in the "herbicide answer" column of the materials inventory: none used, unknown, treated or not applicable.

An answer of "I don't know" is an answer, and it points to the pot test. Oregon State advises being wary of mushroom compost, animal manures and bedding when making your own compost.

If you find contaminated material

NC State says manure and compost with these herbicides can be spread on grass pasture, a grass hayfield or a non-food area, and should not be used to grow sensitive crops without knowing its herbicide history or testing it. NPIC says to plant vegetables elsewhere until bioassays show no damage, which may take more than one growing season, and not to compost cover-crop plants grown to take up residue. Oregon State lists waiting, boosting soil biology, a cover crop that is removed, and removing the soil as options, and says to repeat the bioassay once or twice a year until there are no signs of damage. We do not advise on products for any of this. Your extension office can help with a plan.

The pea or bean pot test

A bioassay is a test where you grow a sensitive plant in the material to see if it is damaged. Chemical laboratory tests exist, but Oregon State says lab testing might not indicate herbicide contamination, and NPIC says some plants are damaged at levels too low for a lab test to detect. A pot of peas is cheap and reads in three weeks. This is the version Oregon State publishes, adapted from a protocol by WSU Extension in Whatcom County. Test it

1. Take samples. NC State says to take random, representative small shovelfuls from throughout the pile, including deep inside, and mix them thoroughly. It suggests at least 20 samples per pile. Run a separate test for each source.
2. Fill six pots. Three control pots hold potting mix without the test material. Three test pots hold two parts of the test material to one part potting mix, by volume. Use the same pots and the same potting mix for all six.
3. Plant three pea seeds in each pot, about an inch deep. Beans work too, and NC State uses peas, beans or tomatoes. Keep the pots warm and in light. WSU's 2002 draft protocol asks for about 12 hours of light and nights not below 50°F.
4. Grow until three sets of leaves appear. Oregon State gives 14 to 21 days after the seeds germinate, so expect three to four weeks in all, as NPIC says.
5. Compare the test pots with the controls. Damage looks like cupped, curled or distorted leaves and growing points, and stunted plants. Write the result on the pot test sheet.

What the result means:

WSU's draft says the test is valid only if at least two seeds in each of two pots germinate. It also says that damage can come from salts, immature compost, pests or disease, and NPIC says lack of nutrients can look like herbicide damage. If the controls do badly too, the test says nothing about the material. For hay or grass clippings, NC State describes a variant: fill a pot with commercial potting mix and spread a thick layer of the hay or clippings on top. NC State also notes that tomatoes used as the test plant may not show damage until they fruit, five to seven weeks in. Peas and beans show sooner, so a clean pea result says nothing about tomato fruit.

See it: Tomás, Ingrid, and Hannah and David See it

Practice: count, decide, ask Practice

Minimum: bag or cage your leaves and write the count, write where the clippings go, and write a herbicide answer for each outside material you have. Time: about 30 minutes once for the questions and the count; the leaves take as long as raking takes, and a pot test takes three to four weeks to read. Materials: bags or a wire cage, the browns calculator, the materials inventory and the system plan; for a pot test, six pots, potting mix, pea seeds and the pot test sheet.

1. Bag or cage the leaves and count. Write the number of bags and the bag size on the inventory, with where you keep them. If your scrap log has a week in it, run the calculator and write how many bags it gives.
2. Write where grass clippings go. The default is on the lawn. If your lawn gets a product, write its name, or "ask" and the date you will ask.
3. List every outside material. Hay, straw, manure, clippings, bagged compost. Write the herbicide answer for each: none used, unknown, treated or not applicable.
4. For each "unknown" that you want for vegetables, start the pot test. Write the date potted on the sheet and the date to read it, three to four weeks later.

Your system Your system

Check your work Check your work

Criteria

• Does the inventory show how many bags or what cage you have, and where they are?
• Does the plan say where grass clippings go?
• Does every outside material on the inventory have a herbicide answer, or a pot test started?
• Is the answer an actual answer ("none used, the neighbor says," "unknown"), not a blank?

Worked example

Tomás's inventory shows 9 thirty-gallon bags of maple leaves behind the shed, a count of about 6 to 8 needed, and "on the lawn" for clippings. He has no outside materials, so the herbicide column says "not applicable." Ingrid's inventory lists the manure as "unknown" with a pot test and the result, and the hay as "unknown, not used." Hannah and David's lists the lawn product's name and "composting line read." All pass. The pot test sheet has a row for Ingrid's manure with damage in test pots.

Common mistakes

• Treating "aged" as "composted." Penn State says they are not the same, and aging does not remove a persistent herbicide.
• Reading a clean pot test as a clean pile. It says no damage in that sample.
• Leaving the herbicide answer blank. A blank is not "no."
• Skipping the question for bagged compost or mulch. The same materials go into them.
• Using a number for a product. This manual quotes EPA and a label and does not advise on products.

What is still unresolved

Whether your leaves cover a year is settled by counting again after the fall. What was sprayed on someone else's field is settled by the person who owns it. Whether a lawn product's label allows composting clippings is settled by the label and, for questions, by NPIC or your extension office.

Knowledge check and scenario

Checklist

• The materials inventory shows my stored browns, and every outside material has a herbicide answer or a pot test started.
• I wrote where grass clippings go on the plan.
• I read how the herbicide sources differ on persistence and test mixes. (Go deeper; optional)