Mulch is used around plants to conserve soil moisture, moderate soil temperatures, protect roots, reduce weeds, and help prevent trunk damage from lawnmowers and weed trimmers.
Many gardeners, as a routine garden task, add liberal amounts of mulch to trees and shrubs believing it is the “correct” thing to do.
Over time, mulch can accumulate to depths of five, six, or more inches causing plant problems.
Plant problems caused by too much mulch
Excessive mulch reduces soil oxygen for roots, suffocating them and causing them to die.
Roots in search of oxygen and water sometimes grow into excess mulch. During dry periods the mulch dries out and the roots in the mulch die.
Mulch piled up against the trunks of trees and shrubs keeps the bark moist underneath the mulch causing decay and a possible entry point for diseases and insects. Pull back the mulch about 3 inches so it does not touch the bark.
Light watering may only wet the mulch layer while the soil remains dry.
Too much mulch may delay the onset of dormancy in the fall by keeping root zone temperatures warm. In contrast, excessive mulch may prevent the breaking of dormancy in the spring due to cooler root zone temperatures.
Provides a hiding place for voles, a rodent that gnaws on bark and girdles trunks and the lower stems of shrubs.
Keep organic (bark and wood products, compost, and leaves or grass clippings) and synthetic (plastic, stones, or landscape fabric) mulches 3 inches away from the base of the trunk to prevent damaging the bark by keeping the area too moist.
Organic mulches should not exceed three inches in depth. Although organic mulches break down over time they should only be replaced as needed to maintain their original depth.
Carefully monitor the watering of mulched plants to make sure the water percolates through the mulch layer into the soil.
As old mulch decomposes or before applying fresh mulch it can be gently worked into the planting bed to improve the soil or removed and worked into other garden areas.
Shallow-rooted plants like boxwood should not have more than one inch of mulch around their roots. Acid loving plants like azaleas can be mulched with pine bark or pine needles.
Additional mulch may be added occasionally to supplement and freshen-up the existing covering but do not let it exceed a depth of 3 inches.
Based on publication HG 803 Commercial Landscape Series: Mulch Problems; Authors David Clement and Mary Kay Malinoski (retired), Extension Specialists, University of Maryland Extension
Excessive mulch reduces soil oxygen for roots, suffocating them and causing them to die. Roots in search of oxygen and water sometimes grow into excess mulch. During dry periods the mulch dries out and the roots in the mulch die.
How is excess mulch removed? Hand shovels to gently scrape the excess away, while being careful not to damage the roots. A high pressure air spade can blow away the excess mulch without damaging the roots.
Excessive mulch that is a layer more than 3 inches deep can suffocate and bury plants. In this case, oxygen and water are unable to reach the roots. A layer of 2 to 3 inches of mulch is enough.
Moldy mulch happens when excess moisture is present in the soil or environment around your garden. It can also occur if too much mulch has been added to the area, trapping moisture and promoting mold growth. It can also be caused by old, decaying plants and other organic materials left behind from previous seasons.
If you have a problem with excess mulch, dig through the landscape to see how deep the mulch really is. A light raking of existing mulch may be all that is needed to break through the crusted or compacted layers that can repel water.
For maximum benefits, the mulch layer in your landscape beds should be about 3 inches thick. If the new mulch you're planning to add to your landscape beds will make it exceed that amount, then it's okay to go ahead a remove some of the old mulch before adding the new layer.
About 3 inches of mulch is recommended for beds, and Becker recommends mulching twice a year. Mulching with the right amount each time will help fend off weeds and conserve moisture, which will reduce your need to irrigate.
There are some species that tolerate excessive mulch better than others. Some of the worst effected plants are grassy plants (iris, daylilies, liriope, etc.), Indian Hawthorn, Azaleas and Loropetalum.
Mulches used improperly can have a detrimental effect if: – They make contact with the stem, trunk or crown of the plant causing disease and rot. – They're applied too heavily and keep oxygen from reaching the roots. (1.5 – 2.5″ is usually plenty for perennials, 3″ for trees.)
If it still looks like mulch, you can reuse it. If it resembles dirt, it's lost its functionality as mulch. If it smells of vinegar, rotten eggs, or alcohol (“sour mulch”) it is temporarily unusable until it's rinsed and dried. If the mulch has any signs of disease, fungus, or parasites, it should be disposed of.
There are several products that should never be used as mulch: sawdust, wood shavings and un-aged wood chips. As these materials begin to break down, they consume large amounts of nitrogen, depriving surrounding plants of this vital nutrient.
Excess mulch can hold too much moisture, making the soil waterlogged, which encourages harmful diseases in the plants. Too little mulch doesn't prevent weeds from sneaking through. It's sort of like Goldilocks and the three bears. You want the amount of mulch to be just right.
While the material itself does not draw termites to the area, mulch spread over three inches deep creates an inviting habitat for the pests. Layers of organic matter trap heat and moisture next to the ground. Termites use this thick mulch as shelter from harsh weather. Some kinds of mulch may even repel termites.
If you get a lot of rain, consider spraying off this area with a water hose or using a leaf blower to dry it out faster. You may also want to rake up any leaves or other debris that has been collected in this area as well. Using vinegar. Using vinegar is a good way to kill mold on mulch.
You should spread your mulch to be two to four inches thick. If your mulch is too thin, then weeds can push through. If your mulch is too thick, it prevents water from reaching the soil.
Avoid dousing your newly-mulched area with too much water for at least 24 hours. A light sprinkling for the plants is okay. The goal is to keep water contained within a newly mulched area to prevent seepage.
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