Wood Chips vs Mulch: What 14 Published Trials Actually Measured

The wood chips vs mulch question is mostly argued from opinion: none of the supplier and arborist pages ranking in the top 10 as of November 2026 cites a trial. We logged 14 published ones below. Six put chips directly against bark or shredded mulch, and the gap they found is far smaller than the gap between either one and bare soil. In an Iowa trial, soil 4 inches down ran 73.9°F under pine chips, 74.5°F under shredded bark and 84.7°F with no mulch at all.

Are wood chips and mulch the same thing?

Partly. Oregon State University Extension defines mulch as any material spread over the soil that influences soil or plant growth, so every chip layer is mulch. But OSU then lists wood chips, arborist mulch and bark as 3 separate products, and says most people hear “mulch” and think of bark. On a supply yard price list, the name is the first clue to what is in the pile:

Name on the pile	What it is	Texture and look	Source
Arborist chips, arbor mulch	Chipped limbs and trunks: wood, bark and leaves from one or more species	Very coarse, with sticks; not uniform, resists packing	Oregon State EC 1629
Wood chips (manufactured)	Chips of heartwood, or of chipped lumber and pallets	Pale, grays with age. Iowa State calls the lumber and pallet version inferior: it breaks down quickly and may be dyed to hide its origin	Oregon State; Iowa State
Shredded or double-shredded mulch	Shredded bark, or blends of bark and wood	Dark, thin strands; blows and washes away less than chips, per Iowa State	Penn State; Iowa State
Bark dust and nuggets	Mostly bark: Douglas-fir, hemlock, pine. pH near 4, high lignin	Uniform. Grades have no standard definition; fine grades seal over	Oregon State
Dyed mulch	Ground wood plus colorant. Amerimulch estimates 60 percent of the fiber is land-clearing, green-waste and tree-company chips	Even color; can begin as the same chips	Our mulch color guide
Playground chips, engineered wood fiber	Wood product built as a fall surface: ASTM F2075 spec, tested to ASTM F1292	May look like landscape mulch, but sold with critical-height test data	CPSC Pub. 325

What 14 trials measured, chips against mulch

Swipe the table sideways to see every column.

Trial	What was compared	Result	Direct chips vs mulch test?
Greenly and Rakow, 1995 (Cornell authors, silt loam)	Chipped pine vs shredded hardwood at 3, 6 and 10 in.; white pine and pin oak; 2 seasons	No differences between mulch types. Depth drove moisture, temperature and weeds; shoot growth peaked at 3 in.	Yes
Iles and Dosmann, 1999 (Gilbert, Iowa)	Pine chips vs shredded hardwood bark vs screened pine bark, plus 5 rocks; red maple; 2 years	Soil at 4 in.: 73.9, 74.5 and 73.8°F (bare 84.7°F). Moisture: 32, 31 and 32 percent (bare 19). Caliper: 4.1, 3.9 and 4.0 cm.	Yes
Duryea et al., 1999 (Gainesville, Fla.)	Utility-pruning chips vs pine bark, cypress, eucalyptus, melaleuca, pine straw at 3.5 in. (9 cm); 1 to 2 years	Mass lost in year 1: utility chips 32 percent, eucalyptus 21, the other four 3 to 7. Depth left at 2 years (UF/IFAS): utility chips about 5 cm, pine bark 6. Utility chips highest in N, P, K, Ca and Mg. Weed counts did not differ.	Yes
Bartley et al., 2017 (Auburn, Ala., nursery containers)	Chipped red cedar, pine, privet, sweetgum vs pine bark mini-nuggets at 1, 2 and 4 in.; 168 days	Depth was the only factor still significant: spotted spurge down 90, 99.5 and 100 percent.	Yes
Quarles and Smith, 2011 (Carson City, Nev.)	8 mulches burned in plots, including composted chips, fuel-reduction chips, bark nuggets, shredded cedar	All combusted. Composted chips and a single layer of chips were least hazardous; shredded rubber, pine needles and shredded cedar most.	Yes
Duryea, Huffman and Osbrink, 1999 (Lab, forced feeding)	6 mulches as the only food; 2 three-week experiments	Termites ate utility chips most in experiment 1; intake tracked N and P (r = 0.91, 0.96). Melaleuca was eaten least.	Yes
Cregg and Schutzki, 2009 (East Lansing, Mich.)	Pine bark, hardwood fines, cypress, color-enhanced ground pallets at 8 cm; 8 shrub taxa	All four raised soil moisture; all but cypress raised growth of most taxa. No foliar nitrogen difference.	Partly: pallet fiber, no raw chips
Steward et al., 2003 (outdoor plots)	13 mulches, discarded cigarettes, 8 tries each	Ground recycled pallets ignited 4 of 8, shredded pine bark 3, shredded hardwood bark 0.	Partly: pallet fiber, no raw chips
Fraedrich and Ham, 1982 (South Carolina)	Fresh pruning chips at 10 cm vs none; maples; 1 year	Silver maple on sandy loam grew 0.66 m vs 0.16 m. On clay loam only current-year shoots gained.	No: chips vs nothing
Green and Watson, 1989 (Illinois)	2 in. compost plus 2 in. chips vs lawn; bare-root sugar maple; 4 seasons	Diameter growth about 3 times lawn, crowns almost twice as large. 5 of 40 trees failed, not tied to treatment.	No: mulch vs lawn
Watson and Kupkowski, 1991 (10 Chicago playgrounds)	18 in. of chips vs adjacent soil; 2 years	Oxygen diffusion 0.38 vs 0.46, above the 0.20 root limit. Nitrate 2.8 vs 4.2 ppm. Root density no different.	No: chips vs nothing
Tahboub et al., 2007 (Las Cruces, N.M.)	Pecan chips mixed into silty clay, up to 17,936 kg/ha (about 8 tons/acre); 3 years	No significant drop in inorganic nitrogen at any rate or number of applications.	No: mixed in, not on top
Long et al., 2001 (Maryland, plus lab)	Eucalyptus, hardwood, pine bark vs pea gravel vs bare; cardboard monitors	Groundcover did not change monitors found. Feeding ran higher under gravel. Mulch-fed termites survived less.	No: no raw chips
Koski and Jacobi, 2004 (Fort Collins, Colo.)	Cankered honeylocust pieces in 4 in. of cottonwood chips; up to 143 weeks	Canker fungus survived 143 weeks; isolates recovered at week 98 made new cankers.	No: disease survival only

The cleanest test is the Cornell one: chipped pine against shredded hardwood, the same 3 depths around the same 2 tree species for 2 seasons. Greenly and Rakow found no difference by mulch type in soil oxygen, moisture, temperature, weeds or growth.

Iowa says the same with numbers. Pine chips and shredded hardwood bark sat 0.6°F and 1 moisture point apart; bare soil ran 10 to 11°F hotter and 12 to 13 points drier. Trees in chips calipered 4.1 cm against 3.9 cm in shredded bark, a gap Iles and Dosmann did not separate statistically. Bare-soil trees also reached 4.1 cm there, on fertile, well-drained ground, so soil gains do not always become growth.

1. Bare soil, no mulch84.70
2. Shredded hardwood bark74.50
3. Pine wood chips73.90
4. Screened pine bark73.80

Source: Iles and Dosmann 1999, Table 1 Converted from Celsius. Means of 12 dates, 2 to 4 p.m. The three mulches are not statistically different.

Show the numbers

	Soil temperature
Bare soil, no mulch	84.70
Shredded hardwood bark	74.50
Pine wood chips	73.90
Screened pine bark	73.80

Two limits. Greenly and Iles used pine chips, not the mixed leaves-and-wood load a tree crew drops; only the Florida trials used true utility prunings. And we found no published trial of arborist chips against dyed bagged mulch on tree growth or survival, so a claim that chips grow better trees than bark is unproven. The stronger evidence is mulch against lawn or nothing: silver maples in fresh pruning chips grew 0.66 m against 0.16 m in South Carolina, and sugar maples in a compost-and-chip circle added about 3 times the diameter growth of lawn trees, though survival did not change.

Depth moved the numbers more than material did

Going from bare soil to 3 inches raised soil moisture 58 percent in the Cornell plots, measured after a rain-free week. Going from 3 inches to 10 added under 2 more points, and pine and oak shoots grew best at 3 inches. In Auburn nursery containers, mulch depth outlasted the herbicide and every wood species: 1 inch cut spotted spurge 90 percent by day 168, 2 inches 99.5 percent.

1. No mulch9.80
2. 3 in. (7.5 cm)15.50
3. 6 in. (15 cm)16.80
4. 10 in. (25 cm)17.40

Source: Greenly and Rakow 1995, Table 1 One sampling date, June 1992. Pine and hardwood pooled, because mulch type did not differ.

Show the numbers

	Soil moisture
No mulch	9.80
3 in. (7.5 cm)	15.50
6 in. (15 cm)	16.80
10 in. (25 cm)	17.40

Deep layers did not smother roots in these trials. Soil oxygen fell from 19.8 percent bare to 18.9 percent under 10 inches, and 18 inches of playground chips left oxygen diffusion well above the root-growth limit after 2 years. Wet sites are the exception: OSU says to leave poorly drained spots bare or lightly mulched, and CSU warns chips can float away where water stands.

Published depth advice splits by goal. OSU says 3 to 4 inches of chips and no more than 3 of bark; Iowa State says 2 to 4; WSU says 4 to 6 for weed control. Use 3 to 4 inches, and our mulch calculator converts that to yards. A thick layer will not fix runner weeds: yellow nutsedge pushed through 3 inches in the Cornell plots, and OSU lists bindweed and horsetail as uncontrolled by mulch.

The claim that shredded mulch mats and sheds water has real support but no head-to-head number. OSU says the more fine particles, the more surface sealing, and caps fine bark at 2 to 3 inches. We found no trial measuring infiltration through arborist chips against triple-shred, so treat that advantage as likely, not measured.

Chips rot faster, and that cuts both ways

Utility chips lost 32 percent of their mass in a year in Gainesville, against 3 to 7 percent for pine bark, cypress, melaleuca and pine straw (Duryea et al.). Green leaves are the likely reason: the utility mix carried the most nitrogen, phosphorus, potassium, calcium and magnesium and a low lignin-to-nitrogen ratio, the best predictor of decay (r = -0.88). So the same load feeds soil faster and thins faster. OSU ties bark’s staying power to high lignin but also calls coarse arborist mulch slow to decompose, so Florida’s rate may not hold for coarser loads or cooler yards.

The same Gainesville plots measured the lowest soil pH under pine straw, set beside burn and termite data in our pine straw vs mulch comparison.

Depth tells a milder story than mass. After 2 years in full Florida sun, 3.5 inches of utility chips had settled to about 5 cm and pine bark to 6 cm, roughly 0.8 inch against 0.6 inch a year. That is the only 2-year chip figure we found.

Do wood chips tie up nitrogen, and how shallow is it?

On top of the soil, no trial here found a nitrogen problem in established woody plants, though soil tests can dip. How shallow the effect runs is less settled than either side claims, because sources name different depths:

Source	Finding	Depth named
Chalker-Scott, WSU, 2007	A nitrogen-poor zone that inhibits weed seeds but not established roots; she labels it “my hypothesis”	The mulch and soil boundary
Oregon State EC 1629	Most nitrogen is drawn from the top layer; small plants and recent transplants suffer, established ones generally do not	Top 3 to 6 in.
Greenly and Rakow, 1995	Nitrate 9.8 to 10.3 ppm with no trend from 0 to 10 in. of mulch	Samples from the top 6 cm (2.4 in.)
Watson and Kupkowski, 1991	2.8 ppm under 18 in. of chips vs 4.2 ppm beside it: significant, but both far under the 30 to 39 ppm range, and no deficiency symptoms	Top 30 cm (12 in.)
Cregg and Schutzki, 2009	No foliar nitrogen difference among treatments on shrubs	Leaf tissue
Broschat, 2007, citing Sonsteby et al., 2004	Spruce bark lowered soil nitrate and ammonium after 3 years on unfertilized loam; Broschat saw no ammonium change at 6 weeks	Not stated
CSU GardenNotes 245	Surface chips do not tie up nitrogen; chips mixed in can, and so can fine chips or sawdust on top	Not stated

Even mixing chips in is not automatically a problem: New Mexico pecan chips tilled in at up to about 8 tons an acre did not significantly lower soil nitrogen over 3 years. For an established shrub bed, the trials give no reason to add nitrogen because of the mulch. For a new planting, OSU suggests 2 to 3 inches of low-salt compost mixed in before planting; if you suspect a shortage, fertilize a few plants and compare. Feeding a lawn is a separate job, and stump grindings where grass will go are the case where Iowa State does want wood removed.

For when compost belongs in the soil and wood belongs on top, with rates by job, see our compost vs mulch comparison.

Disease, termites and fire: the “no” answers that need conditions

An archived WSU fact sheet answers “No” to all 3 fears. The trials support something narrower.

Can chips from a sick tree infect yours?

Sometimes. A honeylocust canker fungus survived 143 weeks in chips and still caused cankers at week 98, so Koski and Jacobi say not to spread chips from infected honeylocust around honeylocust. That paper also cites Verticillium infecting about 90 percent of young eggplants through chips mixed into their potting mix. OSU says most pathogens fare poorly once chipped and spread from arborist mulch is little documented, but names Port-Orford-cedar root rot and sudden oak death as carried by uncomposted material. Ask the crew what they cut, and turn down chips from a diseased tree of a species in your yard.

Do wood chips attract termites?

Termites will eat chips, but Iowa State finds no evidence that chips attract termites not already present. Given no other food for 3 weeks, termites ate utility chips more than any other mulch in one of two experiments. In the Maryland field test, though, hardwood, pine bark and eucalyptus mulch did not change how many monitors termites found, and feeding ran higher under pea gravel; raw chips were not in that test. UF/IFAS gives the usable rule: under 2 inches of mulch within 12 inches of a foundation, with a 6-inch inspection gap below the siding.

Are chips a fire risk?

Yes, like every wood mulch tested, but texture matters. All 8 mulches burned in the Nevada plots; a single layer of chips and composted chips burned least, shredded cedar among the worst. WSU rates coarse chips least flammable of the organics, while Steward found 6 months outdoors made shredded pine bark and ground pallets easier to light with a cigarette. For wildfire zones, our mulch vs rock report covers California’s proposed 5-foot rule.

One cosmetic risk is shared: artillery fungus, which fires black specks onto siding and cars, seems to grow on either wood chips or double-shredded mulch. Penn State found that blending in 40 percent used mushroom compost greatly suppresses it.

Wood chips vs mulch cost

Raw chips run about half the price of shredded mulch at the yard. At Conestoga Stone in Pennsylvania they were $15 a yard, at Kirk Allen in New Jersey $18 and as Arbor Mulch at Sequoia in California $25 when we rechecked on Nov. 5, 2026.

Option	Price per cubic yard, middle half (median)	Sellers	500 sq ft at 3.5 in. (5.4 yd), at median
ChipDrop arborist load	$0	n/a	$0 (offering $20 is optional), but you take the whole truck
Raw wood chips	$18 to $25 ($20)	9	$108
Natural shredded mulch	$31 to $45 ($40)	23	$216
Dyed mulch	$40 to $49 ($42.50)	24	$230
Pine bark mulch or nuggets	$43 to $55 ($47.50)	6	$257
Bagged mulch, 2 cu ft at about $5	$53 to $77 ($67.50)	3	$365 (73 bags)

Faster decay does not erase the saving. Using the Florida settling rates above, a 500 sq ft bed needs about 1.2 yards of chips or 0.9 yard of pine bark a year. Five seasons, starting bed plus 4 top-ups, comes to about $206 for chips and $429 for pine bark at median prices. That is our arithmetic on one trial, before any delivery fee.

Free has its own costs. ChipDrop says a load is 4 to 20 cubic yards, usually 7 to 12, with no partial drops and 1 to 5 weeks of waiting. At 3.5 inches, 7 to 12 yards covers 650 to 1,110 sq ft. Crews may include under 5 percent unchipped brush and as much trash as you could hold in 2 hands. Spreading labor runs $25 to $57 a yard if you hire it, the same for chips or mulch, and a local arborist may also give away loads.

Where chips are the wrong answer

• Seedbeds, annuals and vegetable rows. CSU does not recommend chips where soil is routinely cultivated, and Chalker-Scott calls high-carbon mulch inadvisable where roots are shallow. OSU rates yard-waste compost as the better summer mulch there.
• Slopes, standing water and open wind. CSU says chips float and do not work in open windy areas; Iowa State says shredded mulch stays put better. See erosion control methods for grades.
• Formal beds. OSU notes arborist mulch is less uniform and harder to spread. If that matters, pay for shredded, or read our cedar and cypress trade-offs.

Playgrounds are where the label flips. CPSC Publication 325 rates 9 inches of compressed wood chips as protecting to a 10-foot fall, against 7 feet for non-CCA wood mulch, and says to fill 12 inches to hold 9. That is 18.5 yards on 500 sq ft, or about $889 at the $48 median for certified playground chips. CPSC also says to avoid mulch of unknown CCA content, which argues against a mixed truckload whose source you cannot confirm.

This page is general information, not safety certification. For a public play area, have surfacing checked against ASTM F1292 and the CPSC handbook.

1. 1Is it under play equipment?

   YesWood chips or certified wood fiber, 12 in. filled to hold 9. CPSC rates 9 in. of chips to a 10-ft fall, wood mulch to 7 ft. Confirm the source is free of CCA.

   NoGo to the next question

2. 2Are you seeding, or growing annuals or vegetables here?

   YesNeither on the seedbed. OSU points to yard-waste compost as the summer mulch there; keep chips on the paths.

   NoGo to the next question

3. 3Is it a slope, a spot with standing water, or an open windy site?

   YesShredded mulch, or a different erosion method. CSU says chips float and blow; Iowa State says shredded stays put better.

   NoGo to the next question

4. 4Is it within 12 in. of the foundation?

   YesEither one, under 2 in., with a 6-in. inspection gap (UF/IFAS).

   NoGo to the next question

5. 5Could the load come from a tree species you are mulching, with a known canker or root rot?

   YesTurn the load down, or use composted material from a known source (OSU).

   NoGo to the next question

6. 6Do you need a uniform look, or are you unable to take 4 to 20 yards at once?

   YesBulk shredded mulch, $31 to $45 a yard at 3 in.

   NoRaw or arborist wood chips, 3 to 4 in. deep, tapered to nearly nothing at the trunk.

Source: CPSC Pub. 325; OSU EC 1629; CSU GardenNotes 245; UF/IFAS ENY-832; Koski and Jacobi 2004