Use wood mulch in planted beds that need moisture and soil-building, and gravel around cactus, succulents and other dry-crown plants, on slopes, and next to the house in fire country. Most good xeriscapes use both. Whichever you choose, CSU Extension recommends most mulches, wood chips and pea gravel included, at 3 to 4 inches deep. Mulch is the cheapest water-saving tool in landscaping: it slows evaporation, suppresses weeds and buffers soil temperature. The real question is which mulch goes where, because gravel and wood behave very differently, and the wrong one in the wrong place quietly costs you plants.
Gravel vs. wood mulch at a glance
| Wood mulch (chips, shredded bark) | Gravel / rock (pea gravel, crushed rock) | |
|---|---|---|
| Recommended depth | 3–4 in | 3–4 in; about 4 in for weed control |
| Lifespan | Breaks down; top up every few years | Permanent |
| Soil | Adds organic matter as it decays | Adds none; soil stays lean |
| Water | Holds moisture; can crust and shed water in full sun | Lets rain soak in; CSU notes pea gravel improves infiltration |
| Heat | Stays cooler | Transfers more heat to soil and nearby walls |
| Weeds | Suppresses well when deep enough | Dust in the gaps grows weeds over time |
| Fire | Combustible; keep away from the house | Non-combustible |
| Wind and water | Blows and floats | Stays put |
| Best for | Perennial and shrub beds, under trees | Cactus, agave, yucca, lavender, penstemon, slopes, paths, drainage |
Wood mulch: the soil-builder
Shredded bark, wood chips, or arborist chips. Tree services often deliver chip loads free.
Strengths
- Feeds the soil. Wood breaks down into organic matter, which is what perennial and shrub beds want.
- Holds moisture over the root zones of thirstier plantings.
- Stays cool in sun, and it's comfortable to kneel and work in.
Weaknesses
- It decomposes. Plan on topping it up as it thins.
- It can crust. CSU Extension horticulturist John Murgel notes that in full Colorado sun, wood mulch can become hydrophobic and repel water. Rake it occasionally to break the crust.
- It travels. Wind and water move it, and it floats out of swales in a storm.
- Fire. CSU's fire-resistant landscaping guide recommends that the first 3–5 feet around a house be gravel, flagstone, pavers or another non-flammable material, and advises against pine bark and thick layers of pine needles.
Gravel and rock: the desert skin
Crushed rock, pea gravel or river rock over planting areas. (Decomposed granite is its own material, best for paths; see how to build a decomposed granite path.)
Strengths
- Permanent. Buy once; it never composts away. Murgel calls gravel mulch an excellent choice for xeriscaping because it lets water in while limiting evaporation.
- Keeps crowns dry. That's the difference between life and death for Havard agave, prickly pear, lavender, penstemon and the rest of the full-sun, sharp-drainage crowd.
- Stays put on slopes and in wind, and won't wash out of a dry creek bed.
- Won't burn, which makes it the right choice against the house in fire country.
Weaknesses
- Heat. CSU notes that gravel mulch transfers more heat to underlying soil than wood chips. In winter that can help; against a south wall in July it cooks nearby plants and the room behind the wall. Choose lighter colors in hot exposures and keep plants between the rock and the wall.
- Builds no soil. Beds under rock stay lean, which suits natives and is wrong for hungrier plantings.
- Weeds anyway. Dust settles between stones and weeds germinate in it. How to stop weeds in gravel covers the fix.
- Hard to undo. Removing rock mulch later is heavy, slow work.
Where each one belongs
| Zone | Use | Why |
|---|---|---|
| Perennial and shrub beds | Wood | Builds soil, holds moisture |
| Cactus, agave, yucca, Mediterranean herbs | Gravel | Dry crowns, sharp drainage |
| Rock gardens | Gravel | Look and drainage |
| Slopes and swales | Gravel or rock | Won't wash or blow |
| First 3–5 ft from the house (fire areas) | Gravel, flagstone or pavers | Non-combustible |
| Under trees | Wood chips | Mimics a forest floor |
| Hot south and west walls | Light gravel, plants set in front | Manages the heat load |
| Paths | Decomposed granite or gravel | Firm and permanent |
The two-mulch yard isn't a compromise; it's the correct answer for most sites.
How much to buy
One cubic yard of mulch covers about 81 square feet at 4 inches deep, or about 108 square feet at 3 inches. For gravel, the weight per yard depends on the stone, so suppliers usually quote in tons. The calculator below converts your area and depth into cubic yards, tons and bags.
Application rules for both
- 3–4 inches deep. CSU warns that more isn't better: mulch that's too deep can weaken trees and shrubs and keep water and air from reaching the soil.
- Keep it off crowns and trunks. Mulch against stems invites rot and rodents. CSU advises keeping rodent-harboring materials at least 6 inches from the base of woody plants.
- Skip the plastic and woven fabric. CSU says black plastic and woven weed barrier fabrics aren't recommended as mulch in landscape areas: plastic blocks water and air, and woven fabric clogs over time while weeds root into it. Correctly applied mulch doesn't need fabric.
- Top up wood as it thins, and rake and refresh gravel every few years.
Mulch goes down after planting and drip installation, and before the first deep watering. It's step 9 in the 10 Steps to Xeriscaping.
From our installs: on the Front Range we use gravel around nearly every penstemon, agave and cactus we plant, because wet crowns in a snowy winter kill more of them than drought does. Wood goes under shrubs and in the perennial borders near the house. The mistake we undo most often is fabric under rock: after a few years, a layer of dust and leaves builds up on top, weeds root through, and the fabric makes them impossible to pull cleanly. We lay gravel directly on the soil at a full 3–4 inches instead.
Frequently asked questions
Is gravel or mulch better for a xeriscape?
Both, in different places. Use wood mulch in perennial and shrub beds that benefit from moisture and organic matter, and gravel around cactus, succulents and dry-crown plants, on slopes, and near the house in wildfire areas.
How deep should gravel mulch be?
CSU Extension recommends 3 to 4 inches for most mulches, including pea gravel. CSU's John Murgel notes that about 4 inches of gravel is enough to hold back weeds, while a thinner layer still helps rain soak in.
Should I put landscape fabric under gravel?
CSU Extension doesn't recommend woven weed barrier fabric in landscape areas. It clogs over time, weeds germinate on top and root through it, and it disrupts soil life. A full 3–4 inches of gravel on bare soil works better long term.
Does rock mulch make a yard hotter?
It can. Gravel transfers more heat to the soil than wood mulch, and large areas of unshaded rock re-radiate heat against walls. Use lighter stone in hot exposures, keep planting coverage high, and add shade from shrubs or trees.
