Mastering mulch: Experts at Arboretum share what they've learned

When it comes to mulch, the folks at the Minnesota Landscape Arboretum have it covered.

Veteran groundsman Ted Pew had this to say about his three decades of experience with mulch: "Some have worked, and some have not."

After decades of trial and error, on countless plots throughout their vast acreage, these experts have winnowed their list of mulches to five. That doesn't mean that other options don't work, just that the Arb opts for the tried and true and readily available.

The five Arb toppings: two sizes of rock, Sudangrass, wood chips and a boatload (actually several 18-wheeler loads) of double-shredded hardwood.

The latter prevails for many reasons, said horticulture manager Tom Brinda. "For us it's aesthetic, it knits itself together and mats down in one dark coloring, it prevents weeds and retains moisture, and it breaks down into composted organic matter."

Double-shredded hardwood comes largely from bark that has been run through a hammer mill twice; it's sometimes called double-hammered or double-ground hardwood. The source material often is predominantly oak and never includes black walnut, which has a component called juglone that many plants find toxic.

Double-shredded hardwood is also preferred over more readily available wood chips, which can become incorporated into the soil before being completely rotted and, Brinda said, "made a mess of the fertility levels because it takes a lot of nitrogen to break down wood chips."

The small rocks — ¾ inch or less — come into play in (wait for it) the rock gardens. No matter the size, the Arboretum uses river rocks, Pew said, "because limestone will leach into the root system."

It is, as ever, all about the plants.

The Sudangrass is utilized as a hay mulch in autumn, when the workers actually bury 800-plus roses. That's one of two mulch-related complications wrought by Minnesota's colder climes, Brinda said. The other: "When you're planting 40,000 tulips really close together in the fall, they need rooting time, and we get early frosts. So we will mulch the tulips, give them time for roots to grow. We need a month."

Otherwise, the challenges have been of the live-and-learn variety.

Pew, who has been at the Arboretum for 27 years, recalled moving away from compost mulch supplied by the University of Minnesota farm campus because of concern about "nitrogen runoff going into our waterways, so then we went with mulch with less nitrogen."

A bigger headache unfolded almost 30 years ago, said U of M professor and extension horticulturist Mary Meyer, who focuses on the perennial grasses at the Arb. "We were very worried about weed control," she said. "So we chose to use a fiber material and put mulch on top of it. We're still digging it up. That fiber has never decomposed; you need the sharpest spade or a very sharp knife to cut through it. It's a running joke with the crew."

With all those cautionary tales behind them, it's understandable that these landscapers are loath to embrace the "hot new things" constantly hitting the market.

Two such offerings that have not been tried on the grounds: rubber mulch made from pulverized tires ("except maybe in the play area," Pew quipped) and "Woolch," a sheep's-wool concoction that the University of Minnesota, Morris has found works really well in strawberry patches.

So what visitors who check out the Arboretum's 1,137 acres will see is by and large the double-shredded hardwood. "It's just so all-purpose," Brinda said.

They get the stuff from a place called, fittingly enough, the Mulch Store in Minnetrista. The wood chips mostly come from the grounds, and the source material is culled meticulously.

Last week, Pew and his crew put double-shredded hardwood around the roses and the clematis vines. By contrast, the beds that until two weeks ago were a tulip-palooza — and now are packed with annuals — get no mulch except on the edges, Brinda said. "The tulips and then the annuals are so close together, they shade the weeds.

"Plus we do things differently because we know that every week we have [volunteer] gardeners on their hands and knees with weed buckets."

The rest of us, of course, don't have that luxury, but there's plenty we can glean (see guidelines above) from these experts' hard-earned knowledge about mulching.

Most important, "Avoid products that say things like 'You'll never have to mulch again' or 'Prevents weeds and fertilizes,' " noted Brinda. "Anything that sounds too good to be true probably is."