It has become a gardening reflex: As soon as we finish planting a bed, we tuck it in with a blanket of ground bark mulch. Personally, I don’t care for the dyed versions, the bright red or sooty black. I prefer the natural bark color. That’s just a matter of taste though, and if you prefer those more assertive hues in your landscape, you should please yourself.
Aside from providing a colorful background, however, what is that mulch contributing to the garden? We all know that a couple of inches of some organic mulch helps to suppress weeds. I learned when I was researching gardens designed to conserve water that mulch can also play a crucial role in that. Because it keeps water from evaporating off the surface of the soil, mulch can reduce the need for irrigation during hot, dry spells by as much as a half. That can add up to a lot of water if your garden is of any size, and can save you lots of time dragging hoses and moving sprinklers.
A blanket of a loose, fluffy mulch also serves to insulate the soil underneath. By blocking solar radiation, it keeps the soil cooler, which is better for root growth. That also enhances water conservation, as deeper-rooted plants are more effective at searching out and absorbing soil moisture. This, incidentally, is one function that ground bark mulch performs poorly, as it quickly packs down into a dense, relatively airless, layer on top of the soil, losing most of its insulation value.
Likewise, an organic mulch, as it decomposes, adds humus to the soil surface, although there are also claims that it robs the soil of nutrients by absorbing nitrogen in its decomposition process.
Does mulch really steal nutrients from the soil? For the facts I went to the most meticulous garden researcher I know, Robert Kourik of Santa Rosa, Calif. Kourik has been gardening hard for decades and when he has a question, he goes to the library or online to find scientific research on the subject.
Kourik doesn’t accept the kind of anecdotal experiences, the kind of common “wisdom” (which is often anything but) on which most of us base our gardening practices. Instead, he looks for relevant, scientifically designed experiments that have been published in peer-reviewed journals, and extracts from those information that tells him how to handle his garden.
Kourik told me that the warning about mulch extracting nitrogen from the soil with its decomposition is largely a myth. If you use some carbon-rich mulch such as wood chips or ground bark, it will indeed absorb nitrogen as it decomposes, but it’s effect is only on the very top layer of soil, unless you dig it in. Anyway, the humus that is the product of the decomposition over time re-releases that nitrogen.
The research Kourik has uncovered mostly applies to organic materials that are available for free or at low cost, such as “arbor mulch,” the mixture of leaves and wood chips produced by tree trimming. That has a loose texture that makes it effective as insulation, and the leaf shreds help to supply the wood chips’ need for nitrogen as they decompose, helping the mulch turn into a beneficial compost. Indeed, Kourik says that using this mulch on his own garden, he weathered 30 years of dry northern California summers without needing to irrigate, and the decomposing mulch supplied all the nutrients needed for lush growth.
According to one study Kourik uncovered, four inches of mulch provides maximum water conservation. Any further benefit from additional mulch is so minor that it does not justify the additional expense and labor.
Another use that Kourik has made of mulch is to erase lawn areas; turf is a water-waster that Californians cannot afford in their state of chronic water shortage. I have also used mulch to convert lawn areas into planting beds. I let the mulch smother the turf, leave it in place until the grass dies and decomposes, and then plant right through it. Kourik has found that sheets of cardboard from re-cycled cartons work well for this. He strips off any tape left on the cardboard, lays the sheets down on the soil, and then covers them with a couple of inches of some more attractive mulch to hide the cardboard.
If you want to hear the rest of my conversation about mulches with Kourik Kourik, you’ll find it available online, courtesy of Berkshire Botanical Garden, at https://www.thomaschristophergardens.com/podcasts/robert-kourik-analyzes-the-american-love-affair-with-mulch.