Mulch, especially if it was spread too deeply this spring, can hold so much moisture in the soil that it is harmful to the health of the mulched plants.
Mulch is a great way to suppress weeds, conserve moisture, and add appeal in a landscape. However, sometimes mulching can go awry and create unexpected challenges to gardeners and professionals alike. One such challenge is the overzealous application of mulch. Perhaps in an effort to squash all the weeds below, mulchers sometimes pile mulch too high around the base of trees and other plants. While some may find this appealing, it actually creates serious issues for the plant. Affectionately known as volcano mulch, these mounds of mulch hold moisture against the base of tree trunks and plant crowns that can lead to decay or disease. These mulch-canos can also result in new root formation from the trunk of the tree or a girdling root condition that will strangle the tree. To prevent such issues, proper mulch depth should be 1 – 3″ deep. This will provide adequate moisture retention, weed suppression, and aesthetics without damaging your landscape plants.
With record setting amounts of rainfall in the last few week’s, it might be a good idea to rake back your mulch or turn your mulch especially around newly planted trees and shrubs to allow soils to dry. Too much water can hinder a plant’s growth as quickly as too little water.
An additional challenge mulch users may face is sour mulch. We all recognize the smell. The pungent, vinegary, rotten smell often signals that new mulch is nearby. This odor may be a sign that the mulch piles were improperly stored or that they were not turned frequently enough during production. Without turning, anaerobic conditions form where bacteria can produce toxic byproducts such as methanol, ammonia, or hydrogen sulfide gas. Those sour smells are signs that these substances are present, which can spell trouble for your newly planted trees and herbaceous plants! These byproducts are highly acidic and can actually “burn” plants. Damage may include yellowing leaves, scorched edges, defoliation, or death. In the event you do receive sour mulch, there is a solution. Odorous mulch can be spread into a thin layer on a tarp to “air out” before use. Spraying down the mulch and allowing it to dry on the tarp can also help. This allows the noxious gas to dissipate and the mulch to cure. Mulch can then be used as normal.
– Connie Smith, Program Assistant, OSU Extension