Stump Mulch and New Plants

July 10, 2024 · 4 minute read
Stump Mulch and New Plants

    You just had to take down a large Oak that died in your backyard and now you are left with a giant stump where the tree once stood.  It’s ugly and just takes up space as it slowly rots and puts mushrooms up around the old roots, leaving much to be desired. Wouldn’t it be nice if you could put a tree back in the same place but you have always heard that even if you grind the stump, you will have to wait years to put another plant in the same area.  It feels like you are stuck with a bare spot on your property that will be nothing but trouble, but I have great news!  All those stories are only half right and with a little bit of work, a new tree can be planted immediately without issue.

    First, the stump has to be removed.  If it is like the vast majority of stumps in Richmond, the stump and roots cannot be ripped out with an excavator without doing significant damage to the surrounding property and making a tremendous amount of waste, which rules out this option.  This is easily solved by grinding the stump which is accomplished with a machine that has a large solid steel wheel tipped with tungsten carbide teeth that chew away at a stump until it is all turned into fine wood chips and soil. This looks like an easy solution as the grindings are very easy to dig in, it looks like there is still a lot of soil mixed in with the wood shaving, and people put compost in the ground when planting new trees, so why wouldn’t this be good too.

    It all comes down to something that is dryly called the “Carbon to Nitrogen Ratio” (C:N) which dictates what does and does not do well in the soil.  Simply put, this is the relationship between how much Carbon is present in a given material compared to the amount of Nitrogen present.  Most plants growing in our soils prefer a C:N of approximately 25:1 which essentially means that any product being introduced to the soil has approximately 25 times more Carbon than Nitrogen.

    The reason this all matters is because wood chips, leaf litter, and even manure must go through a breakdown process in the soil before its constituents become available to plants.  This breakdown is accomplished by microbes and fungi that feed on the Carbon in the material.  While a lot of Carbon provides food for these microbes, they also need Nitrogen to build things as basic as DNA up to amino acids and proteins.  If there is not enough Nitrogen in the mix, these microbes and fungi will do what they must to grow and actually begin to pull available Nitrogen out of the surrounding soil as they break down the Carbon rich materials.

    Knowing this as we look toward the C:N of wood chips, it is 450-800:1 for hardwood wood chips and 200-1200:1 for softwood wood chips.  This means that there can be anywhere from 8 to 48 times more Carbon than Nitrogen in these materials which creates a very unbalanced nutrient level and severe lack of Nitrogen.  Remembering high school biology or even your lawn care provider, Nitrogen is the most important nutrient for plants and when our plants are yellowing, it is a good assumption that a lack of Nitrogen is the causal factor.  Now imagine trying to plant a new tree in such an environment.  There is not enough Miracle Grow in the world to balance out these C:N ratios.  But all is not lost.

    While wood chips mixed in the soil are detrimental, wood chips applied on the surface of the soil are extremely beneficial as they are nothing more than mulch which we all know is one of the best things that can be put around a tree.  This is because the wood chips are not incorporated into the soil and only the surface area of the interface between the soil and mulch undergo unbalanced C:N.  By keeping the wood chips/mulch on top of the soil, it allows a slow breakdown by microbes that gradually release nutrients into the below soil.

    This brings us back to the giant pile of stump mulch left after grinding the stump and now we know that planting in it is a pretty terrible idea, leaving two options.  First, the stump mulch can be removed by the contractor performing the stump grinding, leaving a purpose dug hole to plant the tree and add topsoil, as needed, to replace the remaining soil volume lost with the stump grindings.  Optionally, the stump mulch can be simply spread around the surface of the soil and used as mulch, while still cleaning out the freshly ground hole to avoid the addition of too much Carbon into the new soil.

    If you find yourself with a stump in your yard or even a pile of freshly ground stump mulch, it is very possible to replant immediately.  Be sure to remove stump grindings from the hole and backfill any needed soil with good quality topsoil or even soil from elsewhere on the property.  Don’t let an unbalanced C:N make replanting a monumental task and don’t forget to mulch your trees!