Forward mulch! Choosing between bark and rock

By Marianne C. Ophardt, Special to the Herald

I have long been a strong advocate for the use of bark mulch in the landscape.

Looking at the Herald archives, I found a column that I wrote in 1996 about mulches. Back then, lava rock was the predominant rock mulch gardeners were using.

I thought it was extremely ugly and wasn't reticent about sharing my opinion. I also thought the less common river rock wasn't aesthetically pleasing either. Thank goodness that the "abominable" lava rock is not as popular today.

If you look around newer area neighborhoods, there are a variety of different mulches used from bark to river rock to crushed basalt, and even fancier crushed rock of a variety of tones and colors.

While I still don't like lava rock, I've become accustomed to these other types of rock mulches and don't find most unattractive. (I suppose that's fainthearted praise.)

However, I still urge gardeners to use bark mulches over rock mulches in the everyday landscape.

Why not rock?

We live in a very hot climate. Our recent record-breaking temperatures point that out very clearly. Most of the trees, shrubs and other plants we typically plant in our landscapes are not well adapted to the hot summer temperatures, intense sun and low humidity experienced here. Rock mulches work against our plants by raising the temperatures around them because of their reflectivity, especially light-colored stones. Rock mulches also serve as a heat sink because of their heat retention properties. Both of these factors lead to increased water demands and stress on landscape plants not native to a hot summer climate.

It's not just the temperatures around the plants that are raised. Soil temperatures under rock mulches can rise to root-damaging extremes during hot, sunny weather. Not all university research substantiates this argument as results seem to vary, but much of the research has not been done in regions with extremely hot, dry summer conditions. I don't believe rock mulches are a problem in areas with more moderate climates than ours. Because of their reflectivity and heat sink property, they're probably a plus in areas with long, cool springs where spring soils take a long time to warm up, like on the west side of the Cascades.

However, there are a few other problems associated with rock mulches, especially in our wind-prone area. Windblown soil and plant debris accumulates over time in rock mulches, decreasing the attractiveness of the darker types of rock and allowing weeds to germinate and grow in this new layer of soil deposited between the rocks and the landscape fabric mulch often placed below the rocks to prevent weeds from growing. (I know of one dedicated gardener who removed all his old lava rock, washed it, then replaced it.)

If you mulch your home with a rock mulch, you also should realize that not only does it keep temperatures warmer around your plants, but it also keeps your house and outdoor living areas warmer than bark mulch, turf or ground covers. Of course, the same is true that it will provide you with some warming during the winter.

Why bark?

So why am I such a fan of shredded bark mulch? Shredded bark gradually decomposes, adding organic matter to the soil. This helps retain soil moisture and nutrients and also feeds beneficial soil microorganisms. Because it decays slowly, periodical renewal applications will be needed.

Bark is not highly reflective and minimizes the amount of sunlight that's reflected back on tree and shrub trunks and crowns. Because of its ability to hold moisture, it also tends to cool the area around plants rather than acting as a heat sink and stressing plants. As I already pointed out, bark mulch keeps the soil and roots cooler than rock mulches in hot weather.

Over time, bark does a better job of controlling weeds. Really! To accomplish this goal, bark should be applied to weed-free soil surfaces, not over the top of landscape fabric or over the top of persistent perennial weeds, such as Bermuda grass or bindweed. Bark mulch can be very effective in controlling weeds if a 4-inch (6 inches if bark nuggets or extremely coarse bark is used) layer is applied. This takes some advance planning to make sure your landscape beds are graded to make a 4-inch-thick layer possible. It's also helpful to use some sort of edging around the beds, whether applying bark or rock, to contain the mulch. Special note: It's important to keep both bark (because of moisture) and rock (because of heat) about six inches away from the base of trees and shrubs.

Finally, I think bark has a more natural look, providing the most attractive setting for the landscape plants. I'm not surprised that I still prefer bark to rock, but I wish I could convince more gardeners that it's the best mulch to use in this hot climate of ours.

The only exception is for homeowners who are vulnerable to wildfire. In these areas, rock mulch makes sense, hot or not.

* Marianne C. Ophardt is a horticulturist for the Washington State University Extension Office in Benton County.

Similar stories:

Dogwoods surprising in Tri-Cities
Dogwoods surprising in Tri-Cities

KENNEWICK -- The flowering dogwood (Cornus florida) isn't a tree that should thrive in the Tri-Cities.
It's a tree that's native to eastern and central U.S. forests. It's considered an understory tree which means that in its native habitat it grows beneath the canopy of bigger forest trees, such as oak, beech, and maple trees.
Have yellow trees? Low iron often the culprit
Have yellow trees? Low iron often the culprit

KENNEWICK -- Earlier in the season I noted that, once a hater of shrubs, perennials and ornamental grasses with naturally yellow to yellow-green leaves, I have started to like them for the color contrast they provide.
However, I still don't like yellow trees.
Gusty wind may be spreading herbicides
Gusty wind may be spreading herbicides

KENNEWICK -- It certainly has been windy over the past several months.
The tattered and torn leaves on garden plants are obvious consequences of the worst winds.
The small joys of rock gardens
The small joys of rock gardens

PHILADELPHIA -- Fashionistas in the garden like those great big blooms with petals thick as petticoats, but there's another way to go: rock-garden plants.
Their flowers are small, subtle, and every bit as beautiful as their hefty peers, as Ann Rosenberg of Bryn Mawr, Pa., discovered around 1985. On a trip to England that year, she delighted in some small penstemons, which sparked an interest in other plants commonly used in rock gardens.
"They're so cute!" she says.
WSU extension Q&A: May 29, 2010
WSU extension Q&A: May 29, 2010

Q. The Master Gardeners identified the weed that I have in my landscape as equisetum. They said it was an invasive weed and all I can do to control it is to keep pulling it up, but I saw it sold as a pond plant at a local nursery. Is it a weed or a desirable plant?
A. There are about 30 species of equisetum. None should be planted intentionally or accidentally since it's invasive and hard to control. This primitive, 1- to 2-foot plant has green jointed stems that bear a tan spore producing a cone on top. Some types also have sterile stems that develop whorls of branches at each joint. Equisetum spreads by underground rhizomes and also by spores. It's poisonous to livestock, especially horses. Control is achieved by removal of all the tops for about two weeks after emergence in the spring -- for the next three to four years. Equisetum is somewhat impeded by landscape fabric mulch, but it will grow right through a layer of bark mulch. Home garden glyphosate products do not provide control.
Q. Can asparagus

tool name

Saturday, Jul. 05, 2008

Forward mulch! Choosing between bark and rock

By Marianne C. Ophardt, Special to the Herald

Similar stories:

Dogwoods surprising in Tri-Cities

Have yellow trees? Low iron often the culprit

Gusty wind may be spreading herbicides

The small joys of rock gardens

WSU extension Q&A: May 29, 2010