Using Grass Clippings and Yard Trimmings
for Compost and Mulch
BY JANET HARTIN, Environmental Horticulture Advisor, UCCE
and BY DONALD R. HODEL, Environmental Horticulturist, UCCE
Compost is soft, dark, and crumbly, and results from decomposition of organic materials like garden and yard green waste. Compost is beneficial when worked into the soil to increase drainage and water- and nutrient-holding capacity. It also supplies a small amount of nutrients for your plants.
Spread over the soil surface, mulches help to suppress weeds; control erosion; modify soil temperature; reduce soil compaction and water lost to run-off and evaporation; and keep fruit off the soil so it is less likely to bruise and rot. A two- to four-inch layer of mulch solves the problem of muddy garden paths, too.
Composting
In order for garden and yard green waste to decay quickly and completely into compost, it must be:
1. in a large enough pile to allow the center to warm sufficiently for the decay process to occur
2. kept moist
3. well aerated
Some type of structure is usually needed to keep the pile in place, in the right shape, and neat. Several commercially available models are on the market or you can make a simple one yourself. For example, build a circular container out of wire fencing by using a piece of fencing 10 x 4 feet. Pull the two short ends together to form a circle and hook or secure them to prevent them from separating. The wire fence is easy to remove to turn the pile and allows for excellent aeration.
Put the pile in some place out of the way but still easy to get to and water. Alternate layers of green and dried or brown materials, watering each lightly as you place them in the pile. Multiple containers allow you to have several piles in various states of decomposition, ensuring a year-round supply of compost.
The time required for composted materials to be ready for use is mostly dependent on how often the pile is turned, and it can vary from as little as three weeks to as long as a year. Generally, the more often the pile is turned, the faster the material will decompose. If turned every two days, the pile will be ready in about three weeks. If never turned, the pile will take about a year to decompose. Other factors also affect how fast the pile will decompose. Here are some tips for rapid and successful composting:
- Materials compost best if they are between 1/2 to 1-1/2 inches in size. Watery, succulent material can be larger but twigs, small branches, and other woody material take longer to decay if not chopped into small pieces before adding them to the pile.
- Try to have an equal amount of green material (fresh grass, green prunings, fresh vegetables and fruits) and dried material (dead leaves, dried grass, and woody prunings) in the pile.
- Keep the pile evenly moist. Too much water slows decomposition and encourages strong odors. Too little water slows or prevents decomposition.
- Microorganisms decomposing the materials supply heat, which is critical for rapid composting. To prevent heat loss and to build up the amount of necessary heat (160o F), the pile should be at least 3 x 3 x 3 feet. A plastic cover on top of the pile helps to retain heat (keep the sides of the pile open, though, for aeration).
- Turn the pile at least once a week to prevent it from getting too hot and killing the microorganisms responsible for decomposition. Turn material on the outside of the pile into the inside. Turning also aerates the pile, supplying much-needed oxygen for decomposition and reduction or elimination of odors.
- Once a pile is decomposing, do not add additional materials. Excess or additional materials can be stored for later use if kept dry.
- A pile should heat to the required temperature within 24 to 48 hours. If not, it is too wet or too dry, or there is not enough green material. If too wet, spread it out to dry. If too dry, add water. If neither of these helps then add some green material like grass clippings or add some fertilizer such as ammonium sulfate or fresh chicken manure.
- If there is an ammonia odor coming from the pile, add sawdust or woody prunings.
- Do not add soil, stove or fireplace ashes, and manure from meat-eating animals. Manure from plant-eating animals is okay for the pile.
- Although composting kills most insect, diseases, and weeds, it is still best not to add insect-infested or diseased material or weeds to the pile. Also, do not compost poisonous plants, like oleander and castor bean, and any plants sprayed with pesticides or weed killers.
- Signs of good decomposition include a pleasant, "earthy" odor, heat, presence of white fungi on the material, reduction in size of the pile, and the material becoming dark brown.
- As composting nears completion the temperature drops and little or no heat is produced. The compost is then ready to use.
Mulching
Apply mulches year round. Remove weeds prior to application; then spread material two to four inches thick. Materials settle as they decompose so add fresh clippings, leaves, and trimmings periodically to maintain the appropriate thickness. Keep mulches two to four inches away from trunks of trees and shrubs. For best appearance, materials for mulch should be one to two inches long and not more than an inch thick. Let fresh grass clippings dry out prior to using them as mulch. Avoid using any materials sprayed with pesticides, especially if mulching with them around food plants.
Mulch lawns by "grasscycling," simply leaving the mowed clippings on the lawn rather than bagging them. If you grasscycle, mow at least twice a week at slightly less than the recommended height (see Table 1). The basic grasscycling rule is mow often enough so that one-third or less of the growth is removed each time. Contrary to popular belief, grasscycling does not lead to excessive thatch build-up in your lawn.