On your website you recommend 3 -4 inches of mulch under trees. If I use...

Asked August 22, 2013, 9:36 AM EDT

On your website you recommend 3 -4 inches of mulch under trees. If I use landscape fabric under the mulch, does that change the recommended depth? We have 12 - 36 inch oaks in our woods and lawn, and have very sandy soil.

Isanti County Minnesota mulch selection

1 Response

The answer to your question is more complex than it seems. Following are excerpts from two Extension bulletins that address the question:

"Characteristics of Inorganic Mulches: Inorganic mulches include lava rocks, pebbles, plastic or landscape fibers which neither break down and improve soil structure nor add nutrients to the soil. If a site requires renovation, inorganic mulches can be difficult to remove, whereas organic mulches can be easily incorporated into the soil. Dark-colored inorganic mulches, such as plastics, warm the soil more than organic mulches. Black plastic will raise the soil temperature to approximately five degrees higher than uncovered soil. Clear plastic will warm the soil even more, to 10 degrees higher than bare soil. Plastic is an effective mulch material for some crops, such as strawberries or tomatoes, where the plastic is removed each year. Plastic is not an effective mulch, however, for landscape plantings containing trees or shrubs, as it restricts movement of gases between the soil and the atmosphere. An adequate root system to support vegetative growth will not develop under plastic, which prevents adequate root intake of oxygen. Regulation of the proper level of soil moisture is also difficult under plastic. A drip irrigation system located under this mulch is necessary to efficiently and effectively apply water. Unlike plastic, landscape fabrics (geotextiles), are porous. Oxygen and other gases and water enter and exit these fabrics, making them a suitable mulch for trees and shrubs. There are, however, several disadvantages to using landscape fabrics as mulch--when the pores in this fabric become filled with soil or other organic material, for example, weed seed can germinate above the fabric. If the fabric is covered with an organic mulch to hide its appearance and extend its life, the roots of woody plants can also become intertwined with the fabric, and considerable root loss can occur when these plants are transplanted. Always cover landscape fabrics with an inorganic mulch such as pebbles. Another disadvantage of landscape fabric is its tendency to slide off of sloped surfaces."

Go here to read the entire bulletin:

How Deep Should the Mulch Layer Be?
Because many factors should be considered when deciding
how deep the mulch layer should be, general
recommendations are not possible. For example, the
optimal depth of mulch will vary depending on soil
texture, climate, type of mulch, age of plants, and
management objectives.A thin layer of mulch does not
suppress weeds or conserve moisture as effectively as a
deeper layer. A thin layer also needs to be replenished
more often, which increases maintenance costs. On the
other hand, an excessively deep mulch layer can pro-
mote waterlogging of heavy soils, decrease soil oxygen
levels (see below), result in shallow rooting, and keep
soils too warm during winter. Mulch can be applied
under the drip line of mature trees to a greater depth
than in a bed containing annual and perennial
herbaceous plants. A 3" to 4" layer of mulch applied to
well-drained soils in seasonally dry climates can help
minimize drought stress. However, a 2" to 3" layer is
more appropriate on heavy soils in regions where rain-
fall is more frequent.

Go here to read the complete bulletin: