Inadvertently enabled erosion

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Kudos to highway landscape planners. Thank goodness we don't have their job! 

Steep slope no place for slick top dressing!

Let's avoid some pain and advance as a group by pooling our blunders. Here's a mistake we've learned to avoid -- the hard way! --playing out on a much bigger scale and more visible place. Wonder how many motorists are noticing and learning?



Steep highway embankments along below-grade freeways are a major concern to transportation engineers. Plantings there are vital to stabilize the slope, muffle noise, and provide visual relief.


Right: The presence of shrubs and groundcovers also eliminates mowing, an inherently dangerous job on such steep terrain.


Slope isn't the only challenge, either. Heat is a major issue. The slope at top right on this page faces due south. It warms and dries earlier in spring, heats more in summer than this north face. We've seen spontaneous combustion of the mulch layer on that type of hill, during a summer heat wave.


Give a nod to those road crews

Highway landscape crews do amazing things on steep slopes amid breakneck traffic and gut-churning noise. Much of their planting takes, successfully stabilizing the soil, eliminating mowing, buffering the noise and beautifying the ride. But when these big "gardens" slip, the fix is even harder than the initial work.

EmbnkSoilSlips9566as.jpg  EMbnkSlipClos9565s.jpg

We've watched some slips go through repair and renewed erosion for decades, noting one after another surface treatment applied, only to fail again. Lesson? When the base is flawed, the veneer can't hold.


Watch and learn, or sigh in relief

We who have smaller areas and slopes less steep should watch, count our blessings and learn.

Mistake #1: Loose soil over hard

Many factors combine to cause or allow erosion. One major contributor is this week's featured mistake: Loose soil added over hard packed soil. Those embankments were graded with heavy machinery before being blanketed with a soil-compost mix, and mulch. Water that permeates the new layer slows and puddles when it reaches the underlying hard pan. It forms a slick a few inches down that can move everything above it like water floats a boat.

Whether you're on level ground or a hill, it's important to loosen the original soil before adding new -- even if all you can do is to scrape across a hill to create long, transverse furrows. That rough contour provides a transition layer where water's absorbed a little better and the disparate soil types and roots have a chance to knit.

Mistake #2: Aiming for ultra-smooth

Another thing to avoid is an unbroken slope. Smooth can look better, and we all know that looks are often be-all, end-all in landscaping. However, terracing is ancient, simple and proven effective -- even without structure to reinforce it.


Related information

In What's Coming Up Issue #43.
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