5 Mulching Mistakes That Harm Your Plants

When properly applied, mulch provides benefits ranging from water retention to weed suppression to erosion control. However, improper mulching can harm and even kill your plants. Join farmer Briana Yablonski to learn about mulching mistakes to avoid.

Written by Briana Yablonski Horticulture review by Sarah Jay Last updated: July 17, 2025 | 4 min read

A gardener wearing white gloves spreads bark chip mulch over a bed of flowering plants in a sunny garden, showing how to avoid common mistakes.

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Mulch is one of the easiest, science-backed ways to improve your garden. Adding a few inches of wood chips or straw helps suppress weeds, retain soil moisture, prevent erosion, and regulate soil temperature. But like with most garden tasks, adding mulch isn’t a foolproof activity.

Skimping on the amount of mulch you apply may allow weeds to grow and the soil to dry, but there are far worse mistakes you can make. Adding the wrong type of mulch or applying it the wrong way can harm or even kill your plants.

Here are five mulching mistakes to avoid.

Applying Mulch Too Close to Plants

Young lettuce plants with long, oval green leaves forming tidy rosettes grow in neat rows on a straw-mulched garden bed. — If you mulch too close, crispy leaves might be the result.

Since mulch prevents weed seeds from germinating and limits evaporation from the soil, many people are eager to mulch their entire garden. Sometimes, that means mulching right up the base of shrubs, trees, and vegetables. Unfortunately, this well-intentioned practice can severely harm plants.

In the landscaping world, mulch volcanoes are a dreaded sight. You may think that mulch around tree trunks looks tidy and professional, but it’s actually a serious mulching mistake. You won’t notice any issues in the weeks after mulching, but if you wait a few months, you’ll begin to notice some unwanted consequences.

Mulch limits oxygen and water flow to the roots. Most mulch initially traps water, but as it ages, it often repels water and causes dry roots. That’s why you may notice your plant’s leaves appear dry and crisp.

Mulching close to the trunk also causes plants to develop roots that circle around the trunk. After years of growth, the trunk becomes girdled by its own roots. Improper mulching can also cause the plants to produce a high layer of roots that become exposed to drying air as the mulch decays.

If you’re adding mulch around the base of a tree or other woody plant, avoid mounding the material against the trunk. Instead, aim for a donut of mulch 6-12 inches away from the plant. The raised layer will trap moisture while preventing harm to the plant.

You can apply mulch closer to the base of vegetative plants like flowers and vegetables. Leaving a small gap around the plants is always a safe option if you’re concerned about damage.

Using Contaminated Mulch

Close-up of hands in white gloves collecting leaves and tree bark for mulch into a white plastic bag. — Mulch from mystery sources can turn dreams into droopy leaves.

You can choose from many types of mulch, but most of them are made of some type of organic material. Straw, woodchips, hay, and pine straw are just a few mulching materials that are made from plants.

These materials may seem harmless, but it’s important to remember that people treat plants in many different ways. Some people spray herbicides on hayfields to prevent weed growth, and arborists may treat trees with pesticides. Therefore, it’s important to know where your mulch was sourced.

Hay and straw are particularly susceptible to contamination. These materials come from fields that are often sprayed with herbicides, some of which can exist in the environment for months. If you apply these persistent herbicides to the soil around plants, they will show signs of herbicide injury. Since these herbicides can quickly kill your plants, applying contaminated mulch is one of the most serious mulching mistakes.

If you’re worried that you’re working with contaminated mulch, you can conduct a simple test before applying it to your garden. Plant a bean seed in the mulch and look for signs of herbicide damage; curling and twisting leaves and/or stems are the most obvious signs.

A less serious but still unwelcome type of contamination involves trash. Sometimes you can get wood chips from local municipalities for a free or reduced rate. This is an option worth exploring, but be aware that this mulch may come mixed with small bits of trash.

Not Weeding Before Mulching

Close-up of a gardener's hands in grey gloves removing weeds using a long metal tool in a garden bed. — Save future weeding by pulling before mulching begins.

Adding a layer of mulch to the top of your soil can help prevent it from being overtaken by weeds. The mulch prevents the germination of weed seeds and shades out perennial weeds.

If you want to enjoy a weed-free garden, it’s important to remove any weeds before adding the mulch. A few inches of mulch may smother small weed seedlings, but larger weeds will grow right through it. Taking the time to pull weeds before you mulch will save you lots of time down the line.

Remember that some perennial weeds can still make their way through a thick layer of wood chips or straw. Bindweed, johnsongrass, spiny nightshade, and passionflower are just a few weeds that regularly pop up through mulch. Pull the plants as they emerge to weaken them and prevent them from producing seeds.

Mixing Mulch into the Soil

Close-up of female hands in white gloves holding a bunch of dry golden straw above a wooden raised bed with growing onion plants. — Straw’s slow breakdown is great—just not underground.

Most types of organic mulch are high in carbon. That’s one of the reasons why materials like wood chips and straw take months to break down. This high carbon content makes them excellent, long-lasting mulch, but it causes problems if you mix the materials into the soil.

When high-carbon materials are added to the soil, microbes work to break the large carbon molecules into smaller compounds. These microbes require nitrogen to complete this process, so they pull this nutrient from the surrounding soil. Nitrogen that would normally go towards plants ends up unavailable, and plants develop signs of nitrogen deficiency. Therefore, the mulch is said to immobilize or tie up the nitrogen.

If you make the mulching mistake of incorporating carbonaceous materials into the soil, you may notice plants that barely seem to grow, even after they’ve spent weeks in the ground. Their leaves may turn yellow and eventually fall off. Adding a high-nitrogen fertilizer like blood meal can help, but it’s best to avoid this problem.

Rather than mixing mulch into the top of the soil, leave it on the soil surface. Technically, the mulch will pull nitrogen from the very top portion of the soil it touches, but this won’t negatively affect plants.

Using Too Much Mulch

Gardener in white gloves mulching juniper plant with pine bark, close-up. — Roots can’t breathe under a heavy mulch blanket.

One of the most common questions about mulch is how much to apply. A thin layer of material will allow weeds to poke through and may even wash away in a heavy rain. That’s why most people think applying too little mulch is one of the most common mulching mistakes.

Applying too much mulch can also harm your plants. In most instances, two to three inches of mulch is ideal. While adding four inches of wood chips or straw may not cause issues, adding much more than this can lead to negative results.

A thick layer of mulch can eventually compact to create an impermeable surface that air and water can’t infiltrate. That means plants face difficulties obtaining the water they need, and have trouble with gas exchange.

Another result of this mulching mistake is an increased risk of disease. A heavy layer of mulch can sometimes trap too much moisture and increase the likelihood of root rot. It can also limit the amount of airflow around the roots and leave them susceptible to damage.