Tropical Style

Cedar Sawdust & Mulch

Cedar sawdust and chips function as both mulches and soil amendments. But you might be avoiding cedar goods due to reports that they leach toxic substances, which in turn make their way to waterways in which they poison fish. It is also a frequent belief that substances in cedar stop seed germination and harm plants. There’s minimal evidence cedar sawdust and mulches are a threat to plants and aquatic creatures, but the insect-repellent and pathogen-inhibiting properties of cedar have been documented, according to Linda Chalker-Scott Ph.D. with Puyallup Research and Extension Center at Washington State University.


Alleopathy refers to the release of poisonous substances that affect plants and animals. Plants in the Cedrus and Thuja genera include chemicals within their heartwood that protects the plants in decay. These very same substances, thujaplicin and thujone, inhibit bacteria and fungi and deter insects, like ants, cockroaches and beetles. These chemical compounds are not water-soluble. Applying cedar mulches in the garden and around the house can have the additional benefit of deterring unwanted pests.

Nitrogen Deficiency

Cedar sawdust mulch attracts microbes to break down the organic material. These microbes utilize nitrogen for their reproduction and growth. Avoid nitrogen depletion by a light application of hydrogen peroxide, around 1 to 2 tsp for each 10 square feet, prior to applying cedar mulches. Sources of nitrogen include blood meal and balanced garden compost mixes. Determine the quantity of fertilizer you will need to get the desired quantity of nitrogen by dividing the first number in the fertilizer analysis, representing nitrogen content, into 10. The result is equal to the quantity of fertilizer needed for 100 square feet.


Hardwood sawdusts, like cedar, acidify soil as they decompose. Acid-loving plants, like blueberries, azaleas and rhododendrons (Rhododendron spp.) , benefit from acid mulch. Sawdust mulch can also be appropriate for trees and lots of woody ornamentals. Plants that grow best in neutral to alkaline soils, however, need additional lime when mulched with sawdust. Use a soil test to find out the quantity of lime needed to raise the pH into the recommended level for specific plants.

Application and Maintenance

Cedar sawdust ought to be aged for a month or two prior use. Use the mulch in spring, after the soil warms up, as mulch keeps soil cooler compared to unmulched soil. Sawdust mulch should not exceed 3 inches deep. Compaction can be a problem, and also a deep layer of mulch can smother origins and prevent air and water getting into the roots and soil. Cedar mulch decomposes slowly and should not be secured until the layer is considerably reduced. Once a year, fluff it up by gently raking the surface of the mulch layer.

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