Mulch: how much you need and when to put it down

What mulch does and how deep it should be

Mulch holds moisture in the soil, keeps soil temperature steadier, and blocks light so fewer weed seeds sprout. Organic mulch also breaks down over time and adds organic matter to the soil, which helps the clay soils common in Rockland and Bergen.

The right depth is 2 to 3 inches. Rutgers Cooperative Extension says total mulch depth should not exceed 3 inches, and that includes whatever is left from previous years. On compacted or poorly drained soil, the limit is 2 inches. Finer mulches, such as leaf compost, go down in thinner layers of 1 to 2 inches because they pack tighter and let less air through.

Fresh wood mulch can also tie up some nitrogen at the soil surface as it starts to break down, and a deep layer makes that worse. Keeping to 2 to 3 inches and topping up only as the old layer thins avoids most of these problems.

More is not better. Roots need oxygen, and a thick layer of mulch keeps the soil underneath wet and starved of air. Over time that weakens or kills shrubs and trees.

How to calculate cubic yards

Bulk mulch is sold by the cubic yard. To figure how much you need, measure the area of each bed in square feet, multiply by the depth in inches, and divide by 324. The 324 comes from 12 inches per foot times 27 cubic feet per cubic yard.

For example, a bed 40 feet long and 5 feet deep is 200 square feet. At 3 inches deep, that is 200 times 3, or 600, divided by 324, which is about 1.9 cubic yards. At 2 inches, the same bed needs about 1.2 cubic yards.

For curved or irregular beds, break them into rough rectangles and add them up. Round up slightly, because mulch settles and edges take more than you expect. If there is already 1 inch of old mulch in the bed, plan for only enough new mulch to bring the total to 2 or 3 inches.

Check the depth before you order. Push a trowel into the bed in a few spots and measure how much old mulch is left above the soil. That number, not the look of the surface, tells you how much new material the bed needs.

Types of mulch

Double ground or double shredded hardwood is the most common landscape mulch here. It is shredded twice, knits together well, stays in place on slopes, and breaks down at a moderate rate. Penn State Extension notes that hardwood bark, especially a blend of bark, wood, and leaves, makes a very good and inexpensive mulch.

Dyed mulch is shredded wood colored black, brown, or red. It holds its color longer than natural mulch, which is why many people choose it for front beds. The color fades over a season, and the wood underneath is often recycled wood rather than bark, so it adds less to the soil.

Whatever the type, buy from one supplier for the whole job if you can. Color and texture vary between suppliers and even between batches, and two shades of brown in one front bed look like a mistake. Screened, well-aged mulch is also less likely to carry weed seed than fresh, rough material.

Pine bark comes as nuggets or shredded bark. It is lighter, lasts longer, and is a good choice around acid-loving plants such as azaleas, rhododendrons, and blueberries. Nuggets float and wash out in heavy rain, so they are a poor choice on slopes. Leaf compost is dark, fine, and adds the most organic matter to the soil, but it breaks down fastest and should be applied thinner.

No volcano mulching around trees

Volcano mulching means piling mulch high against a tree trunk in a cone. It is common and it damages trees. The bark at the base of the trunk is not built to stay wet, and mulch piled against it keeps it damp, which kills the inner bark and lets disease in. Rodents also nest in the pile over winter and chew the bark.

The right way is a wide, flat ring. Rutgers recommends keeping mulch 3 to 5 inches away from the stems of young trees and shrubs and 8 to 12 inches away from the trunks of mature trees. The root flare, where the trunk widens at the soil line, should be visible.

If a tree already has a mulch volcano, pull the mulch back from the trunk by hand and spread it out. Penn State puts the rule as mulch out, not up.

Edging and bed prep

A clean edge between lawn and bed keeps mulch in the bed and grass out of it. We cut a spade edge with a sharp, vertical side about 3 to 4 inches deep along the lawn, and pull out the cut sod and roots. The mulch sits a little below the lawn line so it does not wash onto the grass.

Before new mulch goes down, we pull weeds, rake out leaves, and break up old matted mulch. Penn State notes that raking or breaking up matted old mulch refreshes its look, which often means less new mulch is needed. Matted mulch also sheds water instead of letting it soak in.

Edges usually need to be recut once a year, often at the spring mulching. Stone or paver borders are an option for beds that lose their edge quickly, and they are covered under our hardscape work.

When to mulch

Spring is the main mulching season. Penn State recommends mulching in the middle of spring, once the soil has warmed enough for root growth to begin. Mulching too early keeps cold, wet soil cold longer. In Rockland and Bergen that usually means late April into May.

Fall mulching also works, after the fall cleanup and before the ground freezes. A fall layer protects roots of new plantings from freeze and thaw and gets the beds ready so spring is only a light top-up. Do not add a full new layer in both seasons, or the depth builds past 3 inches.

Most beds need a top-up of about an inch a year, not a full 3 inches, because the lower layer is still there. See seasonal and specialty services for mulching and bed edging.

Bulk vs bagged mulch

Bagged mulch is usually sold in 2 cubic foot bags. A cubic yard is 27 cubic feet, so it takes 13.5 of those bags to equal one cubic yard. For a single small bed, bags are easier to handle. For a whole yard, bulk is less work and less plastic.

Bulk mulch is delivered by the yard and dumped on the driveway or a tarp. A 5 cubic yard job would take about 68 bags, which is a lot of cutting, emptying, and disposing. Bulk also tends to be a more consistent product across the whole property, since it comes from one pile.

If bulk is delivered, set it on a tarp on the driveway, away from storm drains, and spread it within a few days. A pile left for weeks can heat up and go sour inside.

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