Grass clippings are a readily available and nutrient-rich organic material that can significantly benefit the composting process, but they require special management. Properly managed, this material accelerates decomposition and contributes to a final product that improves soil structure and fertility. Composting grass clippings allows gardeners to recycle yard waste and create a valuable soil amendment.
Why Grass Clippings Require Special Management
Grass clippings, when freshly cut, are classified as a “green” material because they contain a large amount of nitrogen and very high moisture content. This composition presents challenges in a compost environment. Fresh clippings typically have a carbon-to-nitrogen (C:N) ratio ranging from 12:1 to 25:1, which is significantly lower than the microbial ideal of 30:1. This imbalance causes microorganisms to consume the nitrogen rapidly.
The combination of high moisture and low carbon causes the material to quickly compact into dense, wet layers. This compaction restricts oxygen flow, leading to anaerobic conditions. When oxygen is depleted, decomposition slows down, and excess nitrogen is released as ammonia gas, resulting in a strong, foul odor. Allowing the material to mat also generates excessive heat that can kill the beneficial microbes necessary for decomposition.
Balancing Your Compost Pile
To successfully compost grass clippings, the primary focus must be on balancing the carbon-to-nitrogen ratio and ensuring sufficient aeration. The goal is to integrate the high-nitrogen grass with bulkier, carbon-rich “brown” materials to achieve a final C:N ratio close to the desired 30:1. Common carbon sources include shredded dry leaves, wood chips, straw, or shredded cardboard.
A practical approach is to mix two to three parts of brown material with every one part of fresh grass clippings by volume. This mixing prevents the grass from clumping and introduces the necessary carbon to stabilize the nitrogen content. Another effective technique is to pre-treat the clippings by thinly spreading them out to dry for a day or two before adding them to the pile, which reduces moisture and prevents compaction.
When adding the material, it should be thoroughly mixed into the existing pile rather than simply placed on top in a thick layer. Consistent aeration, achieved by turning or mixing the pile every few days, maintains the oxygen supply and prevents anaerobic pockets. This strategy keeps the decomposition process aerobic, eliminating ammonia odors and ensuring efficient breakdown.
Grass Clippings That Should Not Be Composted
While most lawn trimmings are acceptable for composting, two specific categories should be excluded to maintain the integrity of the finished product.
Treated with Herbicides
The first exclusion is grass that has recently been treated with persistent herbicides or pesticides. Some common lawn chemicals, particularly those containing active ingredients like aminopyralid or clopyralid, are known to survive the composting process. These residual chemicals can remain active for months and may damage sensitive vegetable and ornamental plants when the finished compost is applied. A safe practice is to avoid composting clippings for three to six weeks, or after two to three mowings, following any chemical application. Always consult the product label for specific composting restrictions.
Contaminated with Weeds
The second exclusion involves clippings heavily contaminated with aggressive perennial weeds or weeds that have gone to seed. Weeds like crabgrass, dandelions, or those with extensive rhizome systems, such as quackgrass, can survive if the compost pile does not reach and maintain high temperatures. Most home composting systems do not generate the sustained heat of 140°F to 160°F required to kill all seeds and roots. Adding these weeds risks introducing them directly into the garden when the compost is used. Heavily seeded or persistent weedy clippings should instead be bagged for municipal disposal.