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Hands sift through compost ready for use


There are certain materials that could potentially cause problems if they are added to the composting process. There are no hard and fast rules about whether to compost these materials. Use your discretion and common sense when considering if you should add these materials to your compost system. Experience will be your most valuable resource in choosing what to add to the compost pile.

Some of the most common problematic elements are discussed below. Click any item below to learn more (show all items).

Grass clippings

The easiest way to manage grass clippings is grasscycling, (just leave grass clippings on the lawn, see "grasscycling"). However, if you prefer to remove grass after mowing, you can compost grass clippings, if you are willing to actively manage your composting process.

Freshly mowed grass has a high moisture content, and higher nitrogen content than many other materials. The moisture can cause the grass to mat down and clump up, resulting in anaerobic (no oxygen) pockets. This clumping effect, combined with the relatively high nitrogen content of fresh grass clippings and improper composting methods may result in odor problems. However, adding this material can help heat up your compost process and, therefore, adding grass to the compost bin may be desirable. Using proper management techniques can help you avoid odor problems.

To compost grass clippings you will need to:

Add bulky, high-carbon materials (browns)

Fresh grass clippings must be balanced with browns for the compost process to proceed without odor generation. Use browns that provide extra "bulk." Bulk keeps the compost from matting down and allows oxygen to enter the pile. Wood chips, leaves, cut up sticks, all provide bulk. You would probably want to avoid using paper as your primary brown if you are composting mostly grass clippings. While paper is a source of carbon, it has a tendency to mat down and could restrict airflow.

You can use either the "mix it" or "sandwich" methods of pile building. However, the mix it method tends to work better. If you use the sandwich method, make many thin layers, versus a few thick layers. For example, the green layer should be no more than 1-2 inches thick. This will help prevent the grass from matting down and creating anaerobic pockets. Always begin and end your pile with browns to help provide for air to enter the pile at the bottom and absorb odors before they are released from the top of your compost.

You may want to start with twice the volume of browns as grass. However, an actively managed bin can be constructed with less brown material.

NOTE: Bags or piles of grass clippings will go anaerobic very quickly, and whenever you open them up, you will release an "odor bomb" and then have a huge clump of grass to contend with. Therefore, do not store grass for later use. Instead, store browns.

Oxygenate (aerate) the pile

A well oxygenated compost system does not create odors like an anaerobic (no oxygen) situation. Using a bulky brown material will help, but if you are composting grass clippings, you must be committed to turning the pile periodically.  This mixes the materials, thoroughly oxygenates the pile, and prevents anaerobic pockets from forming. You may need to turn the compost system more frequently in the first few weeks than when composting other materials.

If you plan on composting grass clippings on a regular basis, you might consider making or purchasing a rotating drum composting unit. This kind of system is known for its ease of turning and oxygenating the compost. As a result, the composting process is accelerated and you may be able to stabilize the grass clippings in a week. After the material is stabilized, it is less likely to create odor problems. It must be cured for a period to complete the composting process, but will greatly reduce the composting time period and may allow you to keep up with the weekly mowing schedule common during the summer months.

Manage two or more compost piles simultaneously

After beginning composting grass clippings one week, the composting process will continue for several more weeks. What do you do during the summer months when clippings are created each week? You'll have to create a second, or even a third bin.

So you might have a schedule like this:
  • Week 1 – start compost bin 1
  • Week 2 – turn compost bin 1 and start compost bin 2
  • Week 3 – turn and consolidate compost bins 1 and 2 into bin 1, and then start composting again in bin 2
After a couple of consolidations, you will be able to set the compost to the side and let it finish the composting process without much more activity.

Diseased Plants

The concern with adding diseased plants is that the disease may survive the composting process and infect future crops that are treated with the compost. Most, but not all, plant diseases are destroyed when subjected to high temperatures (130-140 degrees F) for a period of 72 hours (three days). These high temperatures may be reached in the pile during hot composting.

However it is usually difficult in a home composting system to mix a pile well enough to bring all material in contact with the high temperatures at the core of the pile and assure complete destruction of pathogens. This being the case, some horticultural experts suggest that it is not advisable to add large amounts of diseased plants to the compost pile if the resulting compost will be used in the garden.

Option 1

You may want to try using compost made with diseased plants in a quarantine area or with different kinds of plants. For example, if you have had problems with garden plant diseases, compost made from the diseased garden plants could be spread on the shrubbery with less chance of transmitting diseases that may have survived the composting process.

Option 2

If you are making compost that absolutely must be free of diseases, you may want to keep diseased plants out of the compost. Experiment, you may find that your composting process is adequate for destroying pathogens.

Weeds and Weed Seed

The concern with adding weeds to the compost is that the weeds or their seeds will survive the composting process and result in new generations of weeds when compost is used in the landscape. Most seeds are destroyed by exposure to 72 hours (three days) of high temperatures (more than 131 degrees F). These high temperatures may be reached in the pile during proper composting. However, it may be difficult in a home composting system to mix a pile well enough to bring all material in contact with the high temperatures and assure complete destruction of weeds.

Think about the type and condition of weeds that you are adding to the compost system. Ask yourself if this is an aggressive weed that you're trying to banish from your yard. If so, maybe it's not worth the risk of adding it to the composting system.

A pointer about weeds:  Tubers, corms and rhizomes tend to survive the composting process, and, therefore, these reproductive parts are more difficult to compost than seed heads.

Herbicides & Pesticides

If you are composting materials from your own yard, then you have some measure of control over whether to use pesticides and herbicides in the first place. Once you have decided to use a pesticide or herbicide, be sure to follow the instructions on the label.

Most of the pesticides and herbicides currently on the market are not as persistent in the environment as pesticides and herbicides of decades ago. Therefore, much of the problematic elements in pesticides and herbicides dissipate over time, regardless of the composting methods employed. Furthermore, the heat and microbial activity of composting helps to break down pesticides and herbicides. So as a general guidance, once a reasonable amount of time has passed since application the organic materials treated with pesticides and herbicides may be composted.  Always check the label for proper handling of pesticides and herbicides.

 Be comforted to know that the composting process uses microbes and heat which greatly reduce and often eliminate the problematic elements.