By Dan Burns
OSU Ext. Master Gardener Volunteer
The objectives of using organic mulches such as shredded hardwood bark, animal manures, composts, cobs, shells, hulls, etc. are many and varied.
They decompose over time, resulting in increased aeration, added water-holding capacity and stabilized soil structure. This reduces soil compaction from rain, weight (people walking on the soil) and cultivation. Mulching material such as straw, wood chips and saw dust have a high carbon-to-nitrogen ratio, so nitrogen must be added with the mulch. High carbon-to-nitrogen materials are quickly decomposed by microorganisms, and microorganisms require nitrogen to multiply and survive.
The pH (acidity or alkalinity) of soil can also be changed, depending on the mulch used. Periodic soil testing, available from your extension office, will indicate any changes.
Some gardeners opt for more permanent inorganic mulches such as plastic or woven landscape fabrics topped with gravel or colored stone, used primarily for weed control. Products made from recycled rubber tires are often used in playgrounds and other children’s play areas.
Apply mulch first in the spring (about now) after the soil has warmed and begun to dry out. Mulching too early will result in soil remaining too wet and root damage can occur.
The recommended mulching depth is 2-2.5 inches to provide weed control, moisture conservation and temperature modification. It also will improve the look of your landscape and add to your property value.
Mulch applied at less than 2 inches will not meet those primary objectives. But 3-6 inches or deeper will cause waterlogged soil in wet seasons and prevent proper water penetration in dry seasons. Mulch piled high around tree trunks — “volcano mulching” — leads to constantly wet bark and conditions favorable for disease development. It also encourages stem-girdling roots, which can kill a tree in just a few years.
A heavy layer in winter is a haven for rodents, which will feed on the tender bark of some plants when other food sources are scarce or snow-covered.
Before reapplying mulch to an existing base use a rake or cultivator to aerate the lower layer. This “fluffing” will slow decomposition of the mulch and allow water to penetrate more readily.