Grow Your Own Way

How Much Mulch Is Too Much Mulch?

Springtime is mulchtime—here’s what you need to know before you start.

April 22, 2022
Photo by Rocky Luten

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Pass through any residential street in the suburbs and you'll probably notice trees with mulch piled up high around their bases. In gardening lingo, this is called a “mulch volcano,” something not only unnecessary—and a waste of mulch—but also harmful to the tree.

Mulching, when done properly, is one of the most important gardening tasks, especially in the spring. Roger Swain, the legendary host of the PBS television show The Victory Garden, wrote, “If I did nothing more, I would mulch.” Similar to watering, fertilizing, and applying other products—organic or not—to your plants, more is usually not better; you need to strike the balance between being generous and overdoing it.

Choosing the right type of mulch is also important. Keep in mind that there is bad mulch, like one that contains the funky artillery fungus that messes up the siding of your home and other inanimate objects.

Here’s what you need to know about mulching.

No Gardening Without Mulching

Mulching does lots of good things. It keeps the soil cool and moist, which reduces the need to water so often. It also helps prevent weed growth. Don’t expect miracles though, weeds will still poke their heads out even through a thick layer of mulch, but much less than if you were to leave the soil bare. At the least, the mulch makes weeds easier to pull.

In the winter, mulch insulates plant roots from the cold, which is especially needed when there is no snow cover. With global warming, there are more frequent warm spells in the winter and the temperature difference causes the roots of landscape plants, especially perennials and young shrubs, to pop out of the soil—a phenomenon called frost heave. A layer of mulch protects the roots against the winter cold.

In the spring, mulch helps to warm up the soil for heat-loving crops like tomatoes and peppers so you can plant them earlier. Note that you should still watch the weather forecast for late spring frosts and not plant tender vegetables before the last frost.

And, finally, mulch helps to reduce soil erosion, and biodegradable mulches enrich the soil.

Not All Mulch Is Created Equal

The brown bark mulch that you can buy in bags at stores and garden centers is not the only type of mulch out there. There are plenty of mulches that come for free—if you have a yard, you can recycle organic materials such as leaves to make your own mulch.

But whatever mulch you use, the right mulch needs to meet a few specs:

Mulch should be lightweight so it’s easy to apply but not so light that it blows away. Hay and straw, unless you pack them around plants like potatoes and strawberries, are too light. Hay can also contain lots of weed seeds.

The texture should be dense enough to hold moisture, but also let rain and air pass through to the roots. Dead leaves are great (and free), but they need to be shredded with a rotary lawn mower before applying them as mulch. Unshredded leaves will form an impermeable layer that prevents moisture and oxygen from reaching the soil, which can lead to excessive heat, mold, and bad smells. The same applies to grass clippings, which don’t make for good mulch. Instead, leave them on the lawn to decompose rather than piling them up around plants.

Free of Contamination
If you're using going the DIY route, be careful of contamination. If you've sprayed a tree with a pesticide, the fallen leaves from that tree are not something you want to use as mulch around your vegetable garden and other edibles.

The mulch should not be contaminated with toxic substances—only get peanut or cocoa bean shells and other plant hulls from a reliable source.

Mulch should be organic and biodegradable. Black polyethylene film or geotextile weed barriers are the longest-lasting mulch, but they create more plastic waste. Also, black plastic heats up the soil, which might be a desired effect in the spring, but not in the summer because it can overheat and kill plants. The only time I use black plastic is to choke pesky weeds or invasive plants that I can't control with other methods.

Slowly Decomposing
Mulch should decompose, but not so fast that you need to reapply it frequently. The unprinted cardboard I use around my tomato plants lasts just about as long as the season.

Bark mulch can be hardwood mulch, cedar mulch, or pine park mulch. Pine bark mulch and pine needles (free if you can collect them from underneath a pine tree) were believed to make the soil more acidic but that is actually not the case. Bark mulch is still the preferred mulch because it decays slowly. Redwood mulch and cypress mulch are not sustainable mulch choices as native forests have been significantly depleted.

Watch out for Artillery Fungus

Shotgun or artillery fungus is an apt name for what this nasty fungus does: its tiny cream-colored or orange-brown cups shoot their spore masses high into the air, leaving small, tar-like spots wherever they land that are extremely difficult to remove.

The best way to avoid artillery fungus is to be inquisitive when you buy bark mulch. It should contain at least 85 percent bark and only a small percentage of wood chips because the large concentrations of cellulose in fresh wood chips feed the artillery fungus.

Once you have fungus in your mulch, there is no fungicide to remove it. The only way to get rid of it is to completely remove the contaminated mulch and start all over again with fresh, clean mulch.

Be Generous (Without Creating Mulch Volcanoes)

Before you add mulch, weed the area. Apply a mulch layer about 2 to 4 inches thick around the base of perennial or annual plants, or spread it in an even layer in your garden beds. Any thicker and the mulch won’t dry out after the rain. Excessive moisture encourages the growth of the artillery fungus in bark mulch, too.

Never cover trunks, stems, or leaves, and keep mulch 4 to 6 inches away from the trunks of trees. The notorious mulch volcano traps moisture and attracts rodents and other critters that nibble on the bark, which can kill your tree.

Have you begun your springtime mulching? Share your tips with us below.
Photo by Angelyn Cabrales

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1 Comment

Smaug April 22, 2022
Not all leaves are the same- at all. Some types of leaves- generally thinner ones- can form impenetrable mats, though I've never known them to do so when used as mulch. They sometimes do when piled up waiting to be worked into the compost pile. Other leaves- live oak leaves are a good example- have no problem at all. Bark mulches have their own problems- they can deplete nitrogen, feed new termite colonies, spread fires etc. In a properly planted garden, plants will shade their own roots and roots of other plants. Many of the synthetic "landscapes" that have, to a great extent, replaced gardens in America, are designed to be cared for (sort of) by untrained workers with a few machines and to be watered by inflexible watering systems; this results in large empty spaces between plants; a very unnatural sort of planting; in addition, these poor plants are often hacked into quasi- geometric forms with hedge trimmers- all of this results in wide open spaces between plants. In a more natural planting falling leaves are for the most part easily absorbed into the garden. If you wish to shred leaves, there are inexpensive machines made specially for the purpose that are quite fast (beware of twigs, though), and many corded leaf blowers have a reverse setting that will pick up and shred leaves- great for picking up leaves from corners and such but slow for large spaces. Rakes and brooms are still the best tools for general leaf pickup; a small cordless blower will round up stragglers efficiently without blowing clouds of dust and dis3ease spores all over the neighborhood.
Black plastic mulches will become brittle over time, making them both less functional and hard to remove, but landscape fabrics work well, they're permanent, and allow moisture and air to pass through. Cocoa hulls are nice, but some users report that raccoons will tear them up looking, no doubt, for Easter eggs.