My huge Norwood Maple tree was damaged and had to be taken down in March 2019, resulting in lots of saw dust. I am told that I need to add 1 lb of nitrogen to 50lbs of saw dust if I wanted to use as a mulch, but I could remove it and store it for a year and then use it as a mulch without adding nitrogen. Is this true?
You had an earlier question this week about verticillium wilt on your elderberry bush and mentioned that you had a Norwood maple (I think you meant Norway maple) nearby. If the maple tree that you removed was taken from an area where the soil might contain verticillium fungus, you should not use the sawdust as mulch, as this would risk spreading the fungus to uncontaminated soil. Also, consider the reason you removed the maple tree – you mention it was damaged – had it been affected by pests or disease? If so, you may want to avoid using the sawdust as mulch.
If your maple tree was healthy when you removed it and you feel confident using the sawdust as mulch, aging it for several months is a good idea. Raw (fresh) maple chips or sawdust contain lots of carbon and cellulose and require both nitrogen and time to decompose. Sawdust has a much higher surface area than wood chips, so is more greedy for nitrogen. If you were to use the sawdust right away, the mulch could draw nitrogen out of the soil, and away from plant roots, weakening the plants. To make up for this deficiency, nitrogen would have to be added to the soil.
By aging the sawdust for several weeks or months and adding nitrogen fertilizer on top to help it decompose, you encourage rotting of the mulch material without robbing the soil of nitrogen. If you add both moisture and nitrogen to sawdust, it should rot in under 6 months, although as we head into the winter (it is early October) it is likely best to age it for a few additional months, to take advantage of warm weather (when it returns!) to speed decomposition. Make sure to fluff the sawdust pile once in awhile, as it can compact, slowing decomposition.
Once the sawdust has rotted, it will be a healthy addition to the soil, and will no longer be a nitrogen-eater, but you may want to incorporate organic fertilizer or other nitrogen source into the soil to make sure there is plenty of nitrogen for the plants – good practice, in any event. Finally, even once it is in place as mulch, the sawdust should be re-fluffed at least once a year to prevent compaction.
Here are some helpful resources:
- SF Gate. Problems using maple tree chips and mulch.
- Washington State University Extension. Master Gardener Online. Wood chip mulch. Landscape boon or bane? This expert highlights myths and facts about woody mulches, providing evidence-based recommendations. This information also applies to sawdust.
Good luck making the best use of your sawdust!