Shredded Mulch: Choosing It Over Rock in a Colorado Bed

Nearly every bed in this state ends up covered with one of two things, and the decision usually gets made on looks when it should be made on what is planted. Shredded mulch is hardwood shredded into consistent medium-sized pieces, natural brown, spread 2 to 4 inches deep. It shades soil and breaks down into it. Rock does neither, and that difference matters more here than it does in a milder climate, because ground left bare between plants through an August of altitude sun and dry wind is ground that stops supporting anything.

Choose shredded mulch when the bed still has something growing in it

Pick it when the bed contains perennials, shrubs or young trees you intend to keep. The shreds shade the soil surface and slow evaporation between waterings, they moderate the swing between a hot afternoon and a cold night, and as the hardwood decomposes it returns organic matter to soils that are naturally low in it around here.

Pick it when the bed is new. A planting bed's first two summers are the ones that decide what survives, and a covered surface gives roots a steadier environment while they establish.

Pick it when the shreds can knit. Shredded material tangles into a connected mat that holds against wind far better than bark nuggets do, which is worth something on an exposed lot in a spring gale.

Skip it at a culvert outlet, and skip it in a rock garden

Three places it is the wrong material.

A culvert outlet or a ditch lining. Moving water floats mulch, and a storm will deposit your beds at the bottom of the drainage run. Anywhere flow concentrates wants stone sized to stay put: RipRap at an outlet, Large River Rock through a swale that runs seasonally.

A rock garden. Alpine and succulent plantings are grown dry on purpose, and organic cover holding moisture at the crown will see them off in a season. Those beds want a mineral surface, which is Pea Gravel or River Rock. A xeric bed of shrubs and grasses is a different thing and does well under hardwood.

A shed or garage pad. This one turns up more often than it should. Mulch carries no load and decomposes underneath whatever is standing on it, so a pad sinks unevenly within a year. Pads are built on compacted Class 6 Road Base over prepared ground.

Tree rings, and the gap that has to stay open at the trunk

A ring of mulch around a tree keeps mower decks and string trimmers away from the bark, which is the most common cause of damage to a young tree in a lawn. Make the ring as wide as you reasonably can, since a wide shallow ring does more for the roots than a narrow deep one.

Keep the mulch off the trunk. Leave a few inches of bare soil around the base rather than piling it against the bark, where held moisture does the tree no favors. The same applies along a foundation bed: hold the mulch back from siding and keep it below any weep holes or the sill.

Two inches or four, and what the layer does at each depth

Two inches is a refresh over an existing bed, enough to look finished and to shade the soil. Three to four is a full application over bare soil and the depth at which the layer actually holds moisture and slows weed seed at the surface.

Past four inches the layer works against you: water from a light rain or a short irrigation cycle never reaches soil, and the bottom of the layer can stay wet against stems. Depth is measured loose, because nothing about mulch wants compacting. Fluff it with a fork and leave it open.

Five yards for 420 square feet of bed and five tree rings

Hypothetical, worked in cubic yards since this material is sold by the yard, and checked twice.

Perennial beds totaling 420 square feet at 3 inches. 420 x 3 / 12 = 105 cubic feet. 105 / 27 = 3.89 cubic yards.

Five tree rings, 6 ft across, at 3 inches. One ring is 3.14 x 3 x 3 = 28.3 square feet, so five is 141 square feet. 141 x 3 / 12 = 35.3 cubic feet. 35.3 / 27 = 1.31 cubic yards.

Together that is 5.2 cubic yards, so 5 yards with a little hand-spreading of the last ring, or 5.5 if you would rather have some over. At the roughly 600 lb per cubic yard this site publishes, five yards weighs about 3,000 lb, and 3,000 / 2,000 is 1.5 tons, with the full 5.2 yards coming to about 1.6 tons. Those figures are approximate, since shredded hardwood picks up weight when it is wet, but even soaked it stays the lightest material on this site by a wide margin and the easiest to wheelbarrow. The mulch calculator takes depth in inches and up to five areas at once for beds that curve.

Ordering yards, and where a light material should land

Mulch blows. Pick a tipping spot with a windbreak or a wall if you can, and cover the pile if it will sit for more than a day or two. Delivery is arranged at checkout by ZIP code, the truck needs clear access to somewhere it can tip, and a light load spreads out wider than people expect when it comes off the bed, so allow more room than you would for gravel. Choose a quantity in yards and a date on this page, or ask Colorado Rock Supply about bed measurements through the contact page.