EXPLAINER

How much mulch is too much mulch?

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

By Nadia Hassani

Published May 2, 2022 4:15PM (EDT)

 (Rocky Luten / Food52)
(Rocky Luten / Food52)

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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:

Lightweight

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.

Permeable

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.

Biodegradable

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.

Sustainable

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% 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.


Nadia Hassani

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