Mulches can help garden soil stay cool during the heat of summer

by Cathy Bylinowski, MU Extension Horticulture Instructor

     Help your plants beat the heat with mulch. Maintain 2-4 inches of an organic mulch to keep the soil cool, University of Missouri Extension horticulturist David Trinklein advises. This promotes root growth and curbs soil moisture loss. By blocking sunlight, mulch also prevents weeds from germinating. Finally, organic mulches improve soil structure as they decompose and add nutrients to the soil.
     Remember, do not create mulch “volcanoes” especially around trees or shrubs. While mulch “volcano” around the trunk of trees can be seen around town, it is not a recommended practice and can jeopardize the health of your plants. Instead, create a mulch “donut” or “bagel” around the roots of the plant. Expand the mulch bed to the drip line or beyond if possible. Do not place mulch directly on the stem or trunks of trees and shrubs because this can encourage pests such as mice and voles to chew on the stem, cause plant diseases, and create soil health problems.
     Good mulch materials include dried grass clippings, shredded leaves, pine needles and ground softwood tree bark. Bags of cotton burr compost can also be used for mulch in vegetable or flower gardens.
     Gardeners may see problems with nitrogen deficiency when they plant into existing mulched areas. Mulch can fall into the planting holes, where soil microbes will break down organic matter in the mulch, consuming nitrogen in the process. Nitrogen deficiency often results in lighter green leaf color, weak or slow growth, and even yellowing and loss of lower leaves.
     To counter this, spread a fertilizer high in nitrogen in recommended amounts on the soil surface before applying mulch. For established mulch, add more nitrogen fertilizer during the growing season as mulch decomposes.
     For tomatoes, avoid high nitrogen fertilizers since they will encourage foliage at the expense of tomatoes. Tomatoes do benefit from mulching during the heat of summer.
     While sour mulch is not a common problem, hardwood mulches tend to break down more rapidly, which makes them more likely to become sour than softwood mulches. Good mulch should have the aroma of freshly cut wood or good garden soil. Sour mulches smell of ammonia, sulfur, or vinegar.
    “The benefits of mulching greatly outweigh the concern of plant damage from sour mulch,” Trinklein said. “However, be aware of the problem and check mulches before applying.”
      Once mulch is established, it may not be necessary to add new mulch every year. This especially is true in areas where it initially is applied more thickly, such as around trees and shrubs. However, adding a thin layer of new mulch often improves the appearance of the landscape.
     Mulches help to make gardening more sustainable, Trinklein said. “Their ability to conserve water, retard weed growth and eventually add nutrients to the soil make their use a logical way to reduce the inputs needed to grow an attractive garden.”
     For more information, see the MU Extension publication “Mulches” (G6960), available for free download at extension.missouri.edu/p/G6960. This publication has an extensive list of the pros and cons of different types of mulch.
     Missouri Department of Conservation also has a good guide sheet on mulch recommendations for trees-
https://mdc.mo.gov/sites/default/files/2020-04/mulch.pdf
     Have more gardening questions? Please feel free to contact Cathy Bylinowski, MU Extension Horticulture Instructor, [email protected], for more information.
     You may also call or email the Extension Master Gardeners Hotline for assistance with common gardening problems or questions -
816-833-TREE (8733) – 24-hour voicemail
[email protected]
​