Mulch vs Rock: Soil Heat Data and the 10-Year Cost by Bed Size

Most rock-or-mulch pages repeat two claims without a number: rock runs hot, and rock saves money. Both are narrower than they sound. At midday the bark surface is the hotter one; it is the soil under rock that heats up (Singer and Martin, 2008). And payback flips on stone size: 2 inches of pea gravel beats yearly mulch by year 1, while 3 inches of river rock takes until year 7.

Mulch vs rock cost: the year rock pays for itself

River rock costs 3.3 times as much as wood mulch on day one, then nothing to renew. Our inputs are the medians from our own price pages: $89.42 a ton for river rock, $64 a ton for pea gravel and $40 a yard for natural shredded mulch, all 2026 supply-yard listings. Top-ups run 1 inch a year per Virginia Cooperative Extension, which says permanent organic mulch “may need to be replenished annually.”

500 sq ft bed	Year 0	Each mulch top-up	10-season total	Rock pays back in
Wood mulch, 1 in. a year, you haul	$204	$68	$815 ($631 to $917)	Baseline
Wood mulch, 1 in. every 2 years, you haul	$204	$68	$475 ($368 to $535)	Baseline
Pea gravel, 2 in., you haul	$250	$0	$250 ($156 to $262)	Year 1
River rock, 3 in., you haul	$671	$0	$671 ($480 to $862)	Year 7 (yearly mulch); year 14 (every 2 years)
River rock, 3 in., woven fabric, you haul	$748	$0	$748	Year 9 (yearly mulch)
Wood mulch, 1 in. a year, contractor, delivered	$518	$242	$2,700	Baseline
Wood mulch, 1 in. every 2 years, contractor, delivered	$518	$242	$1,488	Baseline
Pea gravel, 2 in., contractor, delivered	$590	$0	$590	Year 1
River rock, 3 in., contractor, delivered	$1,226	$0	$1,226	Year 3 (yearly); year 6 (every 2 years)
River rock, 3 in., woven fabric, contractor, delivered	$1,538	$0	$1,538	Year 5 (yearly mulch)

• Wood mulch, contractor, yearly
• River rock, contractor
• Wood mulch, you haul, yearly
• River rock, you haul
• Pea gravel, you haul

Source: Green Industry Report estimate at median 2026 yard prices Same inputs as the table above. No inflation, no rock refresh. Contractor lines include delivery.

Show the numbers

	Wood mulch, contractor, yearly	River rock, contractor	Wood mulch, you haul, yearly	River rock, you haul	Pea gravel, you haul
Year 0	518	1,226	204	671	250
1	760	1,226	272	671	250
2	1,002	1,226	340	671	250
3	1,245	1,226	407	671	250
4	1,488	1,226	475	671	250
5	1,730	1,226	543	671	250
6	1,972	1,226	611	671	250
7	2,215	1,226	679	671	250
8	2,458	1,226	747	671	250
9	2,700	1,226	815	671	250

Trucks change the answer. A flat delivery fee ($60 to $150 for mulch, $75 to $200 for rock) hits mulch every year and rock once, so rock pays back fastest on small delivered beds.

Bed size	Rock / mulch at 3 in.	You haul both	Delivered, you spread	Contractor, 1 in. a year	Contractor, 1 in. every 2 years	Contractor, rock on fabric
100 sq ft	1.5 tons / 1.0 yd	Year 7	Year 2	Year 2	Year 4	Year 2
200 sq ft	3.0 tons / 2.0 yd	Year 7	Year 2	Year 2	Year 4	Year 3
500 sq ft	7.5 tons / 5.1 yd	Year 7	Year 3	Year 3	Year 6	Year 5
1,000 sq ft	15.0 tons / 10.2 yd	Year 7	Year 5	Year 4	Year 8	Year 6
2,000 sq ft	30.0 tons / 20.4 yd	Year 7	Year 6	Year 5	Year 10	Year 7

Bed area (sq ft)Material and path (10 seasons, median yard prices range)Wood mulch, 3 in. then 1 in. a year, you haulWood mulch, 3 in. then 1 in. every 2 years, you haulWood mulch, 3 in. then 1 in. a year, contractorWood mulch, 3 in. then 1 in. every 2 years, contractorPea gravel, 2 in., you haulPea gravel, 2 in., contractorRiver rock, 3 in., you haulRiver rock, 3 in., contractor

Add-ons

Rock only: delivery, one load (up to 17 tons) ($75 to $200) Mulch only, yearly: delivery, 10 trips ($600 to $1,500) Mulch only, every 2 years: delivery, 5 trips ($300 to $750) Rock only: woven fabric and pins, you lay it ($0 to $0 each) Rock only: woven fabric, contractor ($0 to $0 each)

River rock at 3 in. (tons)

7.5

Pea gravel at 2 in. (tons)

3.9

Mulch at 3 in. (cu yd)

5.1

Estimated 10-season cost$630 to $915

Default: 500 sq ft of wood mulch at 1 in. a year, you haul: $630 to $915, matching the table. Switch the material to compare, and tick only the add-ons for that material. Ranges use the middle half of 2026 yard prices; no inflation, weeding, removal or sales tax. Small jobs may hit a contractor minimum.

Local prices move those years. AZ Rock Express listed 1-inch river rock at $45 a ton on Nov. 5, 2026, which pays back in year 2 against median mulch. Raw wood chips at $20 a yard push DIY payback to year 17. Bagged on a 100 sq ft bed, at $270 to $377 a yard for rock and $67.50 for mulch, payback takes about 10 years. A two-person half-day minimum, about $483 at our labor rate, can swamp a 1.7-yard contractor top-up.

Rock has later costs too. One 1-inch refresh on 500 sq ft adds $224 and about 4 years to DIY payback, and UC IPM warns stone sinks into soil without fabric. Removal is worse: Portland Rock charges $75 a ton to accept clean rock, or $563 for this bed before labor. The model leaves out weeding and leaf-blowing time, because we found no published hours.

Is rock hotter than mulch? Only below the surface

The hottest surface in the bed is usually the bark. Montague and Kjelgren (2004) measured six surfaces over three summers, including gravel and lava rock, and pine bark had the highest surface temperature. CSU Extension says gravel beds stay cooler at the surface than wood chips. Bark traps heat at the top because it conducts poorly; rock passes it down.

Measurement, Phoenix, 2004 to 2005	Organic mulch (pine residue, tree trimmings)	Decomposed granite	What it means
Surface at 2 p.m.	143°F and 154°F (Aug. 2004, July 2005)	Cooler than the organic mulches	Bark surface is the hot one at midday
Surface at 10 p.m.	7°F and 14°F cooler than rock	Warmer	Rock holds heat into the night
Soil, 2 in. deep, afternoon	Never reached 104°F	Over 104°F for about 9 to 10 hours a day; organic ran 23 to 40°F cooler	The root-zone gap is largest here
Soil, 12 in. deep	About 82 to 90°F	14 to 18°F warmer	Deep roots feel it too
Heat conduction (W/m°C at 100°F)	0.05	0.18	Rock passes about 3.6 times as much
Depth lost over 2 seasons	27% and 48%	19%	Rock settles too, more slowly

Iowa shows the mild-climate version, and it names the rocks. Iles and Dosmann (1999) mulched red maples at Gilbert, Iowa (USDA zone 5a), and read soil 4 inches down: the 3 organic mulches averaged 74.1°F, the 5 mineral mulches 78.6°F.

Treatment (mineral mulches over woven fabric)	Stone or piece size	Soil temp, 1997	Soil moisture	Stem caliper	Tree height
Bare soil, no mulch	none	84.7°F	19%	4.1 cm	219 cm
Pea gravel, 2 in.	0.4 in.	81.7°F	31%	4.2 cm	220 cm
Crushed brick, 2 in.	0.75 in.	79.2°F	30%	4.2 cm	219 cm
Carmel rock, 2 in.	1 in.	79.2°F	29%	4.2 cm	218 cm
River rock, 3 in.	1.5 in.	77.4°F	29%	4.2 cm	214 cm
Lava rock, 2 in.	0.5 in.	76.1°F	30%	4.1 cm	222 cm
Shredded hardwood bark, 4 in.	1.6 to 2.2 in. long	74.5°F	31%	3.9 cm	210 cm
Pine wood chips, 4 in.	0.8 to 1.2 in.	73.9°F	32%	4.1 cm	222 cm
Screened pine bark, 4 in.	1.6 to 2.4 in. long	73.8°F	32%	4.0 cm	214 cm

1. Bare soil, no mulch84.70
2. Pea gravel, 2 in.81.70
3. Crushed brick, 2 in.79.20
4. Carmel rock, 2 in.79.20
5. River rock, 3 in.77.40
6. Lava rock, 2 in.76.10
7. Shredded hardwood bark, 4 in.74.50
8. Pine wood chips, 4 in.73.90
9. Screened pine bark, 4 in.73.80

Source: Iles and Dosmann 1999, Table 1 Converted from Celsius. Mineral mulches over woven fabric.

Show the numbers

	Soil temperature
Bare soil, no mulch	84.70
Pea gravel, 2 in.	81.70
Crushed brick, 2 in.	79.20
Carmel rock, 2 in.	79.20
River rock, 3 in.	77.40
Lava rock, 2 in.	76.10
Shredded hardwood bark, 4 in.	74.50
Pine wood chips, 4 in.	73.90
Screened pine bark, 4 in.	73.80

Those trees did fine: maples in river rock and pea gravel reached a 4.2 cm caliper against 3.9 cm in shredded bark, which the authors link to warmer spring soil. They call blanket statements about rock “unwise,” and note the site was fertile and the seasons cool. CSU Extension adds that gravel may help plants in cold-winter climates but can pass heat to buildings. Hot desert sites are the opposite case.

Extension pages seem to disagree. University of Illinois Extension says rock does not raise summer soil temperature; K-State Extension says it does and hurts plants. Against bare soil, Illinois is right: rock ran 6.1°F cooler in Iowa and 5 to 16°F cooler in Phoenix. Against organic mulch, K-State is right. UC IPM adds that white rock can reflect enough light to damage sensitive plants and dark rock retains heat.

Does rock dry out the soil?

Coarse rock does, slightly. Soil moisture under pea gravel (31 percent) matched shredded bark in Iowa, river rock trailed wood chips at 29 against 32 percent, and bare soil sat at 19 percent. CSU Extension sets the line at about 1/2 inch: smaller gravel cuts evaporation and weeds about as well as wood chips, while larger stone increases runoff and “can actually inhibit plant growth.”

Weed fabric and weeds: where each one fails

Fabric belongs under rock, if anywhere. UC IPM suggests it to keep stone from sinking and soil from rising, at $0.44 to $0.81 per sq ft installed by our river rock model, about 2 years of added payback. Under wood mulch it defeats the point: Iowa State Extension notes organic mulch over fabric cannot mix with soil, so fertility and structure do not improve.

• Weeds arrive from above. University of Arizona Extension says blown-in seeds establish in gravel “even if you use landscape fabric underneath.”
• Leaves speed it up. Virginia Cooperative Extension says stone may need regular blowing off; K-State says leaves left on gravel break down into silt that adds weed pressure.
• Control means hand pulling or multiple herbicide applications a season, K-State says; a hoe is useless in rock.
• Rock slides down even slight inclines and exposes fabric, K-State reports, and removal means hand picking or a front-end loader. Slopes belong in our erosion control guide.

Against the house: fire rules and foundation drainage

Rock wins the first 5 feet where wildfire is a risk. In a 2008 Carson City burn test of 8 mulch treatments, all 8 were combustible: shredded rubber exceeded 630°F at 4 inches above the bed, and shredded cedar spread at 47.9 feet a minute. The authors recommend noncombustible rock, gravel, concrete or pavers within 5 feet of the house.

California is writing that into rule. Draft Zone 0 text (14 CCR 1298.04 and 1299.03) bars mulches such as bark and wood chips within 5 feet of structures in State Responsibility Areas and locally designated Very High Fire Hazard Severity Zones, with existing homes phased in over 3 to 5 years. As of Nov. 5, 2026, it is not in effect: the Board approved it Aug. 19, withdrew it from state review for edits, and brought it back Nov. 5 for re-adoption. The September guidance allows weed cloth under rock mulch, though it calls cloth flammable.

Neither material fixes drainage. IRC R401.3 requires the grade to fall at least 6 inches in the first 10 feet from the foundation, and rock laid on a flat or back-sloped grade still drains toward the wall. Regrade first, or price a French drain if water collects there. University of Arizona Extension advises keeping organic mulch off structures to cut termite activity, and says rock raises cooling costs, but gives no figure and we found none.

Where water still collects after regrading, our guide to how to make a French drain sets out the trench, stone and pipe specs.

Which one fits this bed?

Run the questions in order; the first yes decides it.

1. 1Is the bed within 5 feet of a building in a wildfire-prone area?

   YesRock or gravel. All 8 organic mulches burned in the Carson City test, and California draft Zone 0 text bars them there.

   NoGo to the next question

2. 2Do you expect to dig or replant here within about 7 years?

   YesWood mulch. DIY river rock has not paid back by then, and removal means hand picking or a loader.

   NoGo to the next question

3. 3Are there trees or shrubs in a hot, dry climate like Phoenix?

   YesOrganic mulch. It kept 2-inch soil 23 to 40°F cooler than decomposed granite.

   NoGo to the next question

4. 4Do leaf-dropping trees hang over the bed, or does it sit on a slope?

   YesWood mulch. Leaves in gravel turn to silt and weeds, and rock slides on inclines.

   NoGo to the next question

5. 5Is it a planted bed where you want the soil to improve?

   YesWood mulch, 3 inches, then 1 inch every 1 to 2 years. Rock adds no organic matter.

   NoRock. Use gravel 1/2 inch or smaller at 2 inches for mulch-like benefits; river rock at 3 inches is the look, at about 2.7 times the material cost.

Source: K-State Extension; Singer and Martin 2008; Quarles and Smith 2011; CSU Extension

This page is general information, not legal advice. Zone 0 text was still in rulemaking when we checked; confirm the adopted rule and your parcel with your local fire agency or CAL FIRE.