Properly chosen and applied, mulch can be beneficial to gardens.

Mulch helps soil retain moisture, discourages weed growth and decreases variation in root temperature, keeping soil cooler in summer and warmer in winter. Properly mulched areas around trees prevent mower damage. Finally, mulch provides erosion control by stabilizing soil.

There are two main types of mulch: inorganic and organic. Organic mulch has the added benefit of supplying nutrients and improving soil quality, which does not occur with an inorganic mulch.

There are many types to choose from. Which is best for your yard?

Inorganic mulch

The most common inorganic mulch used in our region is gravel or larger stones. While attractive and relatively permanent, the stones can work into the soil and become a barrier to water absorption. If you notice that water is standing in areas with gravel or stones, use a metal rake to periodically scrape up the mulch, keeping it loose and near the soil surface.

These mulches are often employed over a layer of landscape fabric but be aware that the use of fabric can be a major headache, especially on sloped terrain. Landscape cloth, topped with mulch on a slope, often washes away during heavy rains, leaving unattractive, exposed fabric.

Shredded rubber — typically made from old tires — is another inorganic mulch. Tire rubber contains roadway contamination, which can harm plants and soil. As rubber mulch breaks down, it leaches chemicals, primarily heavy metals, into the soil. If you plan to use a rubber mulch, check its sourcing. There are rubber products that are safer than shredded tires, especially where children play.

Organic mulch

Mushroom compost, a spent mixture of peat moss, animal manure, ground corn cobs, hay or straw is commonly available here. Since it’s first used on commercial mushroom farms, this much’s nutrients are reduced by having supported the mushroom crop. But the compost is good for retaining soil moisture and does have some fertility value.

Pine needles are a good choice around fruits and vegetables as they help to keep fruit clean during heavy rains. Needles can also be used in shrub and perennial borders. Contrary to popular belief, research has shown that while pine needles are acidic, they do not significantly lower the soil pH.

Cocoa bean shells are a by-product of chocolate production. They have a deep brown color and smell for a few weeks after application. Since this mulch is more expensive than others, reserve it for highly visible areas. However, don’t use cocoa bean shells if you have pet dogs as ingestion can cause diarrhea and vomiting.

Grass is a good choice as a mulch in vegetable gardens. It is best to use grass from an untreated lawn or wait at least three to four weeks after applying pesticides to your lawn before using clippings in your garden.

Hardwood mulch, which may include shredded wood or bark from any hardwood tree, is probably the most commonly used mulch in our region and is a favorite of local landscapers and homeowners. Dyed hardwood mulch is also available and will retain an even color for a longer time. The dyes that are used are vegetable-based and are safe to use around plants. However, some inexpensive dyed wood mulch is made from pallets, which can contain chemicals that can leach into your soil.

Bark mulch is harvested from the outer tree layers and is nutrient-dense. It decays more slowly and lasts longer than shredded hardwood mulch. Products labeled as bark mulch contain at least 85% bark.

Arborist wood chips are an economical and environmentally friendly mulch for use around trees and shrubs. Whole trees are chipped and may be used fresh or allowed to age before using. The irregular sizes of the pieces knit loosely together allowing for air and water flow. Research shows that there is no need for concern about the wood chips tying up nitrogen and causing nutrient deficiencies in plants. Rather, the decaying wood gradually adds nutrients to the soil.

Leaf mold is partially composted shredded leaves. It is my favorite because it supports a healthy soil microbiome that boosts nutrients and water-holding capacity. It can be made at home, so it is free and natural. Compost your shredded leaves separately from your other compost bins because it takes a little longer to break down, depending on what variety of leaves you are using. Whole leaves mat down and can smother plants, so shred the leaves to allow air and water to accelerate the decomposition. I use it in both my vegetable and flower gardens.

Compost, whether commercially sourced or made in your garden, can be used as a mulch. Be sure to use well-aged compost free of weed seeds. Some commercially sourced compost is very light and almost powdery. It is a good choice for providing nutrients to soil but is best mixed with mushroom manure to improve the texture, making it easy to spread.

Perennial mulch

Leaf mold, compost and mushroom manure are the best mulches to use around perennials as they do not inhibit the spread of plants.

Bark suppresses not only weeds but also keeps groundcovers and perennials from knitting together and creating the benefits of a mass planting. By allowing perennials and groundcovers to fill out and touch each other, you are effectively using those plants as a mulch; the areas where the plants are dense and healthy discourage the growth of weeds.

No matter which mulch you choose, don’t overdo it. Two or 3 inches is plenty. Do not make mulch “volcanos” around tree trunks, a practice that encourages root growth into the mulch and increases the risk of infections and insects.

Leave a few inches open around the base of trees and plants. Assess how and when new mulch should be applied. The spring ritual of mulching every square inch of a property with bark mulch is a dubious expense, especially if your garden is thriving and the mulch from the previous year(s) is still doing its job.

Dianne Machesney is a Penn State Master Gardener. This volunteer program supports the outreach mission of Penn State Extension and provides research-based information on best practices in sustainable horticulture and environmental stewardship. Information: alleghenymg@psu.edu or 412-482-3476.

First Published: January 5, 2024, 10:30 a.m.
Updated: January 5, 2024, 5:14 p.m.