Mulch benefits your garden year-round, but is especially beneficial during the heat of summer, says University of Missouri Extension horticulturist David Trinklein.
To keep the soil cool, add 2 to 4 inches of organic mulch, which promotes root growth and reduces solid water loss. Mulch also prevents weeds from germinating by blocking sunlight, says Trinklein. Finally, organic mulch improves soil structure as it breaks down and adds nutrients to the soil.
Trinklein says there are many different mulch materials to choose from, including dried grass clippings, shredded leaves, pine needles and crushed conifer bark.
Some gardeners are following a recent trend of crushing and recycling shipping pallets, which are made from less valuable hardwoods such as cottonwood and sycamore, to use as mulch.
“Wood chip mulch tends to decompose faster than softwood mulches such as pine bark or shredded cypress,” Trinklein said.
Trinklein recommends pine bark or shredded cypress over hardwood mulch for best results, especially in annual flower beds. Dry mowing also works well, but be careful if the lawn has been sprayed with herbicides, Trinklein said.
Missouri has a large hardwood products industry that produces bark as a by-product. Hardwood bark is also available as mulch and is most often sold in bulk, which also tends to decompose quickly.
Soil microbes use up nitrogen when they break down organic mulch, and as a result, they take nitrogen away from your plants as they eat the organic matter, Trinklein says. Without enough nitrogen, your garden plants' leaves may turn paler, they may grow weaker or slower, and their lower leaves may even turn yellow and fall off.
Gardeners most often encounter nitrogen deficiency problems when planting over an already laid layer of mulch. When planting, the mulch can fall into the planting hole, providing more organic matter to decompose. This allows soil microbes to fix more nitrogen.
To prevent this, apply a high-nitrogen fertilizer to the soil surface before applying mulch, or add more nitrogen fertilizer to existing mulch as it breaks down during the growing season.
Acidic mulch has become a problem in recent years, Trinklein said.
“Sour mulch is basically mulch that has broken down improperly,” he says. “The result is a mulch that gives off a foul odor and often releases compounds that are toxic to plants. Sour mulch can cause symptoms like leaf burn, leaf drop and plant death.”
Sour mulch is uncommon, but hardwood mulches decompose faster and are more likely to become sour than softwood mulches. Good mulches smell like fresh cut wood or good garden soil. Sour mulches smell like ammonia, sulfur, vinegar, or silage.
“The benefits of mulching far outweigh the concerns about damage to plants from acidic mulching,” Trinklein says, “but be aware of the issues and check before applying mulch.”
Once mulch has established itself, it may not be necessary to add new mulch every year. This is especially true in areas where it is applied thicker initially, such as around trees and shrubs. However, adding a thin layer of new mulch often improves the appearance of the landscape. In most cases, a new layer of mulch equal to the amount of decomposition that occurred the previous season is sufficient. In an age of environmental awareness, mulch can help make gardening more sustainable, says Trinklein. Mulch's ability to conserve water, slow weed growth, and ultimately nourish the soil makes it a reasonable way to reduce the inputs needed to grow an attractive garden.
