The short answer: Use mulch in beds where the plants matter: shrubs, perennials and new gardens. Use river rock where you want something permanent that you won't replant: along the foundation, under downspouts, in side yards and drainage strips. Most yards end up with some of each.
Every spring, tired garden beds raise the same question: top them up with mulch again, or switch to rock and be done with it? Mulch feeds and cools the soil but needs replacing every year or two. River rock stays put for years, but it heats up in summer and gives nothing back to the soil. Here's how the two compare on plant health, upkeep, cost and where each one belongs around a Waterloo Region home.
Key takeaways
- Mulch is better for plants. It holds moisture, evens out soil temperature and adds nutrients as it breaks down.
- River rock lasts far longer, but it reflects heat, adds nothing to the soil and is hard to weed once debris settles into it.
- Organic mulch usually needs replacing every one to two years. Rock is effectively permanent.
- Rock suits foundations, downspouts and side yards. Mulch suits planting beds, and around trees as long as it's kept off the trunk.
Mulch and river rock side by side
| Mulch (shredded bark) | River rock | |
|---|---|---|
| Best for | Planting beds, around trees and shrubs | Foundations, side yards, drainage areas |
| Plant health | Better: holds moisture and adds organic matter | Worse: reflects heat, adds nothing to the soil |
| Summer heat | Keeps soil cooler | Beds run hotter |
| Weeds | Easy to pull what gets through | Hard to pull once debris builds up |
| Lifespan | Replace every 1–2 years | Many years |
| Upfront cost | Lower | Higher |
| Changing your mind later | Easy: breaks down or rakes out | Hard: rock has to be dug out |
Plant health: mulch wins
Mulch is the better choice anywhere the plants are the priority. Washington State University Extension's Guide to Mulches lists the main benefits of most mulches as "conserving water, suppressing weed growth, and evening out soil temperature extremes," and adds that organic mulch "can also add nutrients." The Region of Waterloo gives the same advice in its water-wise gardening tips: "A layer of organic mulch helps keep moisture in the soil and provides nutrients too."
That moisture matters here. From May 31 to September 30, the Region's water by-law only allows gardens and shrubs to be watered on alternate days during set hours, so beds that hold water between waterings get through July and August in better shape.
River rock works against plants in summer. The same guide puts it plainly: "A drawback with most rocks is that they reflect solar radiation and can cause a very hot landscape environment during the summer months." Shrubs and perennials that like cool, moist roots struggle in a bed of hot stone.
For any bed where you want plants to thrive, mulch is the clear choice.
Rock lasts longer, mulch is easier to live with
River rock wins on lifespan. It doesn't break down, so you aren't buying it again next spring. Organic mulches, by WSU's estimate, are "usually replaced every one to two years."
Rock isn't maintenance-free, though. Leaves, seeds and soil settle between the stones, and once weeds take hold in rock they're hard to pull out by hand. Many people lay landscape fabric under rock to prevent this. The extension guide doesn't recommend it, and because the weeds start in the debris on top of the stone, fabric underneath doesn't stop them.
Mulch is easier day to day. Weeds that do come through pull out cleanly, and a top-up takes an afternoon.
Choose rock if you want to install once. Choose mulch if you want easy weeding.
The cheaper option depends on how many years you're counting
River rock costs more to buy and install. Mulch costs less up front but comes back every year or two. Over two or three seasons mulch is usually cheaper. Over many years, rock catches up, especially in beds you never plan to replant.
Working out how much you need is simple. One cubic yard is 27 cubic feet, so it covers:
| Depth | One cubic yard covers about |
|---|---|
| 2 inches (5 cm) | 162 sq ft |
| 3 inches (7.5 cm) | 108 sq ft |
| 4 inches (10 cm) | 81 sq ft |
For bark mulch, WSU suggests a layer 2 to 3 inches deep.
Along the foundation, rock is usually the better call
Right against the house, river rock often makes more sense. The WSU guide describes rock and gravel as "permanent installations" that "are best used around foundation plants." Rock also suits the places water runs: under downspouts, along drainage strips and in narrow side yards where little grows anyway. Mulch tends to float off in those spots during a heavy rain.
Whichever you choose, the ground under it should slope away from the house so water drains off rather than toward the basement.
Around trees, use mulch and keep it off the trunk
A ring of mulch around a tree holds moisture over the roots and keeps mowers and trimmers away from the bark. Keep it off the trunk itself. WSU's guide warns that mulch should never be "piled up like a volcano touching the trunk," a common habit that "invites insect and rodent damage." Leave a few inches of bare ground around the base.
Which to use where
| Spot | Our pick | Why |
|---|---|---|
| Front planting beds | Mulch | Healthier shrubs and perennials, easy to refresh |
| Foundation strip with few plants | River rock | Permanent and handles runoff |
| Under downspouts and drainage paths | River rock | Won't wash away |
| Side yards and narrow strips | River rock or gravel | Little upkeep where little grows |
| Around trees | Mulch ring, off the trunk | Protects the bark and holds moisture |
| New gardens you'll keep adding to | Mulch | Easy to dig through and replant |
Frequently asked questions
Should I put landscape fabric under river rock?
WSU Extension doesn't recommend it. Weeds in rock beds mostly start in debris that settles on top of the stone, so fabric underneath doesn't stop them, and it makes the bed harder to change later.
How often does mulch need to be replaced?
Most organic mulches need topping up every one to two years as they break down.
Does river rock make garden beds too hot?
It can. Rock reflects sunlight and holds heat, so rock beds run hotter in summer. That suits tough, sun-loving plants and bare foundation strips, but it's hard on plants that like cool roots.
Can I switch from rock to mulch later?
Yes, but it's real work. The rock has to be dug or scooped out before mulch goes down. Switching from mulch to rock is much easier.
How deep should mulch be?
For bark mulch, 2 to 3 inches. Keep it pulled back from stems and tree trunks.
Not sure which fits your beds?
We install mulch, river rock and gravel across Cambridge, Kitchener, Waterloo and Guelph, and we weed and edge the beds first so the new material stays put. See our mulch and river rock installation, or if the whole bed needs rethinking, our garden design service. Request a free quote.
Sources
- Washington State University Extension, Spokane County Master Gardener Program: C075: Guide to Mulches (checked November 8, 2026)
- Region of Waterloo: Outdoor Water Use and Restrictions (water-wise landscape tips) (checked November 8, 2026)
