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Are you adding the wrong amount of mulch aroundy our trees and in your garden beds?

Paul Cappiello Ph.D.

Paul Cappiello Ph.D.

Yew Dell Botanical Gardens Executive Director

Sometimes, a seemingly gentle nudge can hit the rawest of nerves. And it seems that last week’s gentle diatribe on the constantly perplexing practice of volcano mulching was just that kind of nudge. The inbox has been overflowing all week . . .

So let’s hit the basics of mulching.

What makes a good mulch?

This is a good and age-old question across the gardening world. Sometimes we think the only appropriate mulch is what’s available or what we’ve always used. But essentially anything can make a good mulch as long as it:

  • stays put
  • allows adequate water infiltration
  • does not appreciably restrict oxygen and CO2 exchange between the plant’s root zone and shoot zone
  • is not toxic to plants or other living things
  • doesn’t otherwise create a less than favorable growing environment for your plants. 

Cypress mulch is great for paths because it doesn’t wash away in a thunder storm as some others can. But cypress mulch is a horrible choice for planting beds because over time it forms a dense mat that is all but impenetrable to rain and irrigation water. Black mulch can lead to extreme heat gain in summer while white stone mulch can cause plant damage due to excessive reflection of the sun’s energy. And besides . . . what’s the deal with black and white mulch anyway? Unless you’re installing an art deco garden, I don’t really get the point!

When it gets right down to it, a box full of golf balls could make an effective garden mulch. Styrofoam packing peanuts would work as long as the earth’s gravitational field was 20 times stronger – so they wouldn’t fly around in a light breeze. Small stones can work, as can many of  the organic materials you’re familiar with.

The best organic mulches are those that are at least partially composted so the naturally occurring microbes in your soil don’t deplete all the soil nitrogen as they break down (compost in place) the organic materials. And of course no matter the material chosen, you never want more than 1 to 2 inches on top of the soil.

Finally, plants themselves can make a good mulch. If not overly aggressive in terms of root competition, groundcovers can help shade the soil surface through the heat of summer, they can create a favorable environment for our all-important soil microbes and they can keep wandering string trimmers and lawn mowers away from young and tender tree bark.

Why do we mulch our trees and garden beds?

Mulching, if done correctly, can reduce evaporative loss from the soil, keeping precious water in the soil longer to supply plant roots. Mulches can reduce erosion and suppress weed growth. They can cool the soil during hot summer months, protect tender roots in winter and protect tree trunks from lawn maintenance equipment. But there are limitations . . . and one size definitely does not fit all.

Take the bit about reducing evaporative water loss from the soil. If you’re growing a plant that needs a cool, moist root zone all through the summer, mulching to enhance that in your soil is a good thing. On the other hand, if you’re growing a plant that likes it high and dry, you’ll want to keep that in mind. Maybe it doesn’t need as much mulch. Maybe it needs a different mulch (stone/gravel) or maybe it would be better with no mulch at all.

And if you’re mulching to keep you plant’s root zone cool and moist, it’s generally a good idea to mulch where the roots actually grow! For young, newly planted trees, that means mulch is good right around the root ball and maybe extending a foot or so beyond. But if you have a 60 year-old sugar maple in the middle of the lawn, do you really need a 4-foot diameter mulch ring around the tree? Surely you’re not helping the roots, many of which extend way, way beyond the edge of the canopy. And once a tree develops its mature bark, it is generally much less susceptible to mower damage. Sometimes we just do what we’ve always done, or what everyone else does, and we forget to ask if it makes any sense. 

One other reason we mulch our beds is that it is sometimes the best way to improve the soil. If you have the luxury of starting a new bed from scratch – no plants to work around – it’s easy. You just dig up the whole thing, amend like crazy with some good compost and then set out to plant. But if you are working on a bed with existing plants, it’s sometimes impossible to make much improvement by digging into the space between mature plants. In this case you can improve that soil, albeit slowly and over time, by using well composted materials as a light mulch. Over time, as those materials continue to decay, they release nutrients and organic acids into the soil. And while that can’t improve your soils’ structure – the mineral make-up of your soil – it can improve what we call the macro structure of the existing soil. It takes time but it does work.

But no matter what mulch you choose or where you choose to use it, there’s still no reason to make mulch volcanoes . . . no matter how much hate mail I get!

This article was originally submitted to the Courier Journal on April 19, 2022.

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