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Washington State University Extension

Garden Tips

BARK VS. ROCK MULCHES

written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

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 back 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, shrub, and other plants that 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 the plants 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 very 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 that 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 (dirt and dust) and plant debris accumulates over time in rock mulches, decreasing the attractiveness of the darker types of rock and allowing weed seeds 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, and then replaced it. There’s a very hard worker. )

If you mulch your home with a rock mulch, you should also 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 slowly decays, 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 it’s moisture holding ability, it also tends to cool the area around plants rather than acting as a heat sink and stressing plants. As already pointed out, bark mulch keeps the soil and roots cooler than rock mulches in hot weather.

Overtime, 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 controlling weeds if a four-inch (six inches if bark nuggets or extremely course bark is used) layer is applied. This takes some advance planning to make sure your landscape beds are graded to make a four-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.

Published: 7/5/2008 1:49 PM

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