Mulch once, and let it last.

Most mulch is bought to be replaced. Finely shredded hardwood looks tidy for a season, mats down, breaks down, and is back on the shopping list by next spring. That annual top-up is where most of the cost and nearly all of the overmulching comes from.

The alternative is to buy something that is still doing its job two and three years later. For permanent beds — shrubs, trees, established perennials — large pine bark nuggets are the best default most gardeners can buy. This guide explains why, where they are the wrong choice, and how to lay them so they last.

The short answer

• For shrub borders, tree rings, and established perennial beds, use large pine bark nuggets. Bark is built to resist decay, and big pieces have little surface for decomposers to work on, so a layer lasts several seasons instead of one.
• Spread them 3 to 4 inches deep, and keep them at least 3 inches away from trunks, stems, and crowns. No volcanoes.
• Do not use nuggets on slopes, in drainage paths, or where water runs during storms. They float. Use pine straw or arborist wood chips there instead.
• In vegetable and annual beds, use something finer that you can move and that breaks down within a season, such as straw or shredded leaves.
• Skip landscape fabric under any organic mulch.
• Top up only to restore depth. With nuggets, that is typically every two to three years, not every spring.

What mulch actually does

A layer of mulch does five useful jobs, and knowing them tells you what to look for in a material.

1. Keeps moisture in. Mulch slows evaporation from the soil surface, so the water you apply or the rain you get stays where roots can use it. The water-wise gardening guide treats it as the first way to reduce demand.
2. Evens out soil temperature. Soil under mulch warms more slowly in spring and stays cooler in summer heat. That is a benefit for established plants and a drawback for heat-loving vegetables in a cold spring.
3. Suppresses weeds. Mulch shades out weed seeds already in the soil and gives new seeds a poor place to germinate.
4. Protects the surface. Raindrops hitting bare soil break apart its structure and seal the surface into a crust. Mulch absorbs that impact and stops erosion.
5. Adds organic matter, slowly. Organic mulches decompose into the top of the soil, which is how undisturbed soils build naturally.

The last job is the one that trades against the others. A mulch that breaks down fast feeds the soil faster, but it also has to be replaced faster, and it spends much of its short life as a dense, fine layer that does the first four jobs less well. In a permanent planting, where you are not trying to build a vegetable bed's fertility each year, a mulch that decomposes slowly is usually the better trade.

Why large pine bark nuggets are the default

Pine bark nuggets are chunks of the outer bark of pine trees, a by-product of lumber milling, screened into sizes. Bags labeled "mini nuggets" hold pieces of roughly an inch or so; "large" or "jumbo" nuggets are mostly two inches and up. Sizes vary by supplier, so open a bag or look at a bulk pile before you buy. For the benefits below, choose the largest size you can find.

They decompose slowly

Bark is the tree's protective skin. It is rich in compounds that resist rot, and it has little of the easily digested material that decomposers go for first. Size compounds that advantage: a large nugget has far less surface area per pound than the same bark shredded or ground, so decay proceeds slowly from the outside in.

The difference shows up in the field. In a two-year University of Florida trial, mulches were laid at the same starting depth of about 3.5 inches. After two years, pine bark still measured about 2.4 inches — two-thirds of its original depth. Pine straw laid at the same depth was almost completely gone. Florida's heat and humidity break organic matter down quickly, so in cooler climates a bark layer usually lasts at least that well.

They stay loose

Fine mulches — shredded hardwood, ground bark, sawdust — settle into a dense layer. When that layer dries it can shed water instead of absorbing it, and when it stays wet it holds moisture at the surface. Coarse pieces leave large gaps. Water and air pass straight through to the soil, and the top of the layer dries quickly after rain, which gives weed seeds that land on it a poor place to germinate.

They resist artillery fungus

Artillery fungus lives in moist wood mulch and shoots sticky black spore masses up to several feet toward light-colored surfaces. The spots on siding, cars, and windows are small but nearly impossible to remove. A four-year Penn State study compared the fungus on 27 landscape mulches. Wood pieces and shredded bark-and-wood blends that absorbed moisture supported the most sporulation. Large pine bark nuggets were the most resistant, because big pieces of bark stay hard and dry. If your beds sit against light siding, that alone is a reason to switch.

Where nuggets are the wrong choice

Large nuggets have real limits, and every one of them follows from the same properties that make them last.

Slopes, drainage paths, and anything that floods. Bark is light, and nuggets float. On a slope, below a downspout, or in a low spot that carries runoff, a heavy storm will move them into the lawn, the street, or the storm drain. Penn State Extension specifically warns against using them on hills. On slopes, use pine straw — the long needles knit together once they settle — or arborist wood chips, whose mix of sizes and shapes locks together.

Vegetable beds and annual flower beds. These beds are dug, replanted, and sown every year. Nuggets get mixed into the soil, get in the way of seed rows, and take years to break down there. Use a finer mulch that you can rake aside and that decomposes within the season (see vegetable beds below).

Around small plants and fine groundcovers. A three-inch nugget layer can bury a newly planted plug, a low groundcover, or a small bulb's emerging shoot. Use mini nuggets, or keep the large ones well back until the planting fills in.

Paths you walk every day. Large nuggets roll underfoot and scatter. Arborist chips pack into a firmer, springy surface and are the better path material.

Next to buildings in wildfire country. Every organic mulch burns. A Nevada evaluation burned a range of mulches, including pine bark nuggets of several sizes, under hot, dry, windy conditions and found all of them combustible. Shredded rubber, pine needles, and shredded cedar behaved the worst; composted wood chips behaved best. Where wildfire is a risk, keep the first five feet around structures free of combustible mulch, whatever its size.

How the common mulches compare

Two materials in that table deserve a word of warning.

Dyed mulch. The colorants used on mulch are not the usual concern. The wood underneath can be: dyed products are often made from recycled wood, such as pallets or construction debris, whose history is hard to trace. If you want a dyed product, buy one certified by the Mulch and Soil Council, which excludes treated wood. Or buy pine bark, which is already a rich brown.

Rubber mulch. It never breaks down, so it never adds organic matter to the soil. It can release zinc as it weathers. And in the Nevada fire tests it burned hotter than any other mulch, with flames averaging more than three feet high.

How deep, and how far from the trunk

Two to four inches is the working range for mulch, with coarse materials at the deep end and fine materials at the shallow end. Fine mulch packs; three inches of it can seal the surface. Coarse mulch stays open, so it can go deeper without cutting off air. For large pine bark nuggets, aim for 3 to 4 inches. Less than that leaves gaps where light reaches the soil and weeds start. Much more than that and you are paying for mulch the plants do not need, and you start to restrict air and water to the roots.

Then keep it off the plant. Pull mulch back at least 3 inches from trunks, stems, and perennial crowns, so the base of every plant sits in a small bare ring.

The opposite is the "mulch volcano," a cone of mulch piled against a tree trunk. It is common, and it slowly harms trees:

• Bark is not built to stay wet. A trunk buried in damp mulch softens, which opens a path for decay and boring insects.
• Rodents tunnel in the mulch against the trunk and chew bark through the winter, unseen.
• Deep mulch against the trunk encourages roots to grow up into the mulch, and roots growing around the trunk can eventually girdle it.

For trees, spread the mulch in a wide, flat ring out toward the edge of the canopy if you can, rather than a tall pile in a small circle. The roots you want to help are out there, not at the trunk.

Laying it down

Weed first. Mulch smothers seedlings, but established perennial weeds will come through any mulch. Pull or dig them out before you spread.

Water dry soil first. Mulch keeps moisture in; it does not add any. Spreading it over bone-dry soil holds the dryness in until the next soaking rain.

Time it to the bed. Shrub and tree beds can be mulched whenever the ground is workable. In vegetable beds, wait until the soil has warmed (see below). Winter mulch over perennials and bulbs is a separate job: it goes on after the ground freezes, to keep roots uniformly cold rather than to keep them warm, as the fall tasks guide explains.

Work out how much you need before you buy. Coverage is simple arithmetic:

• One cubic yard covers about 108 square feet at 3 inches deep, or about 80 square feet at 4 inches.
• A 2-cubic-foot bag covers about 8 square feet at 3 inches, or 6 square feet at 4 inches.

For a 10-by-20-foot bed (200 square feet) at 3 inches, that is roughly two cubic yards, or about 25 bags. Measure your beds before you go; mulch bought by eye usually ends up piled too deep.

Edge the bed. A clean edge — a spade-cut trench, or a low border — keeps nuggets from spilling into the lawn, where they dull mower blades.

Skip the landscape fabric

Landscape fabric under mulch seems like a way to make weed control permanent. It is not.

• The mulch on top becomes the weed bed. Organic mulch breaks down into a thin layer of fine material on the fabric, weed seeds blow in and sprout in it, and their roots grow into and through the fabric. Weeds rooted into fabric are much harder to pull than weeds in mulch over soil.
• It clogs. Soil and fine particles fill the fabric's pores over time, cutting the water and air that reach the soil.
• It blocks what mulch is for. Organic matter from the mulch never reaches the soil, and soil life never reaches the mulch.
• It is miserable to remove. Taking out old fabric means pulling up torn, root-bound sheets buried under years of decayed mulch.

The coarse, slow-to-decompose mulch recommended here is already the low-maintenance option. It needs no fabric underneath. If you are converting lawn to a bed, kill the grass first, then mulch directly on the soil.

Top up, don't pile on

Check depth each spring before buying anything. Push a finger or a ruler through the layer to the soil. If there are still 2 or more inches of nuggets, rake the surface to loosen it and stop there. Add mulch only to bring the layer back up to 3 or 4 inches. With large pine bark nuggets, that usually means a light top-up every two to three years.

Annual top-ups regardless of depth cause more mulch problems than any other habit. Each year's layer goes on top of the last until beds are six, eight, or ten inches deep and every trunk is in a volcano. If the old mulch is still there, it is still working.

While you are checking, look for:

• Mulch creeping back against trunks and crowns. Pull it back again.
• Thin spots where weeds are getting through. Pull the weeds while they are small, then fill the gap.
• Mushrooms, slime molds, and bird's nest fungi. These are decomposers breaking down the mulch. They do not harm plants and do not need treatment. Rake them in if you dislike the look. Keep children and pets from eating mushrooms.
• Slugs around seedlings. Any moist mulch shelters slugs. Where they are chewing young plants, pull mulch back from those plants until they are bigger; the pest guide covers the rest.

Mulch in the vegetable garden

Vegetable beds want the opposite of what makes nuggets good: a mulch that you can move, that decomposes within the season, and that feeds the soil as it does.

Use straw, shredded leaves, or untreated grass clippings. Straw is the classic choice; make sure it is straw rather than hay, which is full of seed. Shredded fall leaves are free and excellent. Grass clippings work in thin layers that are allowed to dry between additions — and only from lawns that have not been treated with herbicide, for the reasons in the compost guide.

Wait for warm soil before mulching heat-loving crops. Mulch keeps cool soil cool. Tomatoes, peppers, squash, and beans mulched early into cold soil sit still. Let the soil warm to planting temperature — the seasonal planner explains how to check — plant, and mulch afterward. Cool-season crops can be mulched sooner.

Keep seed rows open until seedlings are up, then snug the mulch in around them without burying them.

Paths between permanent beds are the exception. A path is not replanted, so a long-lasting mulch belongs there. Arborist chips or bark, laid deep in paths, suppress weeds and keep your feet out of the mud, while the beds themselves get the finer mulch.

Mulch myths worth dropping

"Wood mulch steals nitrogen from plants." Only when it is mixed into the soil. Decomposers working on wood need nitrogen, and when chips are dug in they take it from the soil around them. On the surface, contact with the soil is limited to a thin boundary layer, and research reviewed by Washington State University found no nitrogen deficit in the plants below — woody mulches increase soil nutrients over time. The hügelkultur guide covers what changes when wood is buried instead.

"Pine mulch makes soil acidic." Pine needles and bark are acidic when they fall, but as surface mulch they do not change soil pH in any amount that matters to plants. Soil under pines is often acidic because it was acidic to begin with. If you need to change pH, test first and follow the soil health guide.

"More mulch is better." Beyond about 4 inches, extra depth adds cost and restricts air and water to roots. See depth.

"Landscape fabric is needed underneath." See above.

"The dye is the danger in colored mulch." The source wood is the usual concern, not the colorant. See the comparison.

"Rubber mulch is better because it lasts forever." It lasts forever because it never becomes soil. A large bark nugget gets you most of that durability and still feeds the ground in the end.

Sources and further reading

• University of Florida IFAS: Landscape mulches — how quickly do they settle? — the two-year depth trial of pine bark, pine straw, cypress, eucalyptus, and melaleuca.
• University of Florida IFAS Gardening Solutions: Choosing and installing mulches — coarse versus fine materials and application.
• Penn State Extension: Mulch — a survey of available options — bark, dyed, bulk and bagged mulches, and nuggets on slopes.
• Penn State Extension: Artillery fungus in mulch — the 27-mulch study and why large pine bark nuggets resist the fungus.
• University of Delaware Cooperative Extension: Artillery fungus and other things that grow in mulch.
• UMass Extension: What's growing on my mulch? — slime molds, bird's nest fungi, and mushrooms.
• Iowa State University Extension: Using mulch in the garden — depth by texture.
• Penn State Extension: Mulching landscape trees — rings, depth, and clearance from the root flare.
• University of Vermont Extension: Beware the mulch volcano.
• UNH Extension: Volcanoes kill trees!
• Washington State University: Wood chip mulch — landscape boon or bane? — Linda Chalker-Scott's review of nitrogen tie-up and coarse mulches.
• Washington State University Extension: Using arborist wood chips as a landscape mulch.
• UC Agriculture and Natural Resources: Myths of mulch.
• UNH Extension: Do pine trees and pine needles make soil more acidic?
• University of Illinois Extension: The disadvantages of landscape fabric.
• Penn State Extension: Putting an end to my landscape fabric nightmare.
• University of Nevada Cooperative Extension: The combustibility of landscape mulches — the fire tests of organic and rubber mulches.
• UC ANR Fire Network: Mulch — mulch near structures in wildfire-prone areas.