How much mulch is too much mulch?

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Cross any suburban residential street and you’ll likely notice trees with mulch piled around their bases. In gardening jargon this is called a “mulch volcano”, something not only unnecessary – and a waste of mulch – but also harmful to the tree.

Mulching, when done correctly, 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 straw.” Similar to watering, fertilizing and applying other products – organic or not – to your plants, more is generally not better; you have to find the balance between being generous and overdoing it.

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

Here’s what you need to know about mulching.

No gardening without mulching

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

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

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

Finally, mulch helps reduce soil erosion and biodegradable mulches enrich the soil.

Not All Mulches Are Created Equal

The brown bark mulch that you can buy in bags at stores and garden centers isn’t the only type of mulch out there. Many mulches are free. If you have a garden, you can recycle organic materials such as leaves to make your own mulch.

But whatever mulch you use, the right mulch should meet a few specifications:


The mulch should be light so that it is easy to apply, but not so light that it blows away. Hay and straw, unless you wrap it around plants like potatoes and strawberries, are too light. Hay can also contain a lot of weed seeds.


The texture should be dense enough to retain moisture, but also allow rain and air to pass through to the roots. Fallen leaves are great (and free), but they should be mulched with a rotary lawnmower before applying as mulch. Unshredded leaves will form an impermeable layer that will prevent moisture and oxygen from reaching the ground, which can lead to excessive heat, mold and bad odors. The same goes for grass clippings, which don’t make good mulch. Instead, let them decompose on the lawn rather than piling them around plants.


If you go the DIY route, watch out for contamination. If you’ve sprayed a tree with a pesticide, the dead leaves from that tree aren’t something you want to use as mulch around your vegetable patch and other edibles.

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


Mulch should be organic and biodegradable. Black polythene film or geotextile weed barriers are the most durable mulches, but they create more plastic waste. Also, the black plastic heats up the soil, which may be a desired effect in spring, but not in summer as it can overheat and kill plants. The only time I use black plastic is to smother weeds or invasive plants that I can’t control with other methods.

Decomposes slowly

The mulch should break down, but not so quickly that you have to reapply it frequently. The unprinted cardboard I use around my tomato plants lasts about as long as the season.


Bark mulch can be hardwood mulch, cedar mulch, or pine mulch. Pine bark mulch and pine needles (free if you can pick them up under a pine tree) were thought to make the soil more acidic, but that’s actually not the case. Bark mulch is still the preferred mulch because it breaks down slowly. Redwood mulch and cypress mulch are not sustainable mulch choices because native forests have been significantly depleted.

Watch out for artillery mushrooms

Shotgun or artillery mushroom is an apt name for what this nasty fungus does: its tiny cream-colored or orange-brown cups shoot their masses of spores high into the air, leaving little tar-like specks wherever they go. they land which are extremely difficult to remove. .

The best way to avoid artillery mushrooms is to be curious when buying 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 fungi in your mulch, there is no fungicide to kill them. The only way to get rid of it is to completely remove the contaminated mulch and start over with fresh, clean mulch.

Be generous (without creating mulch volcanoes)

Before adding mulch, weed the area. Apply a layer of mulch about 2 to 4 inches thick around the base of perennials or annuals, or spread it in an even layer across your garden beds. Thicker and the mulch will not dry out after rain. Excessive moisture also promotes the growth of artillery fungus in bark mulch.

Never cover trunks, stems or leaves and keep mulch 4 to 6 inches away from tree trunks. The famous mulch volcano traps moisture and attracts rodents and other bark-munching creatures, which can kill your tree.