Mounding Garden Beds For Succulents Out West

These linear plantings of agave and cacti are aligned perpendicular to the slope to check the speed of runoff.

Love succulents but hate your clay soil?  Solve it by creating a simple mound of quality soil that ensures your finicky succulents will be happy with perfect drainage.  Under these conditions, your plants won’t suffer waterlogged roots, and rotting will be a thing of the past if you irrigate with a slow drip system.

A mound with all the mistakes: Pointed top, steep sides and dry sandy soil with no cohesiveness that will melt in the first hard rain.
A mound with all the mistakes: Pointed top, steep sides and dry sandy soil that will melt in the first hard rain.

Incorrectly constructed mounds become failures for a variety of reasons.  Most importantly, the soil won’t stay put and soak in when you water, and improper irrigation can leave conditions way too dry, even for succulents.  These problems are due to the shape of the mound; if made too steep on the sides, the water runs off before it can penetrate. Effective succulent mounds need to rise up gradually, provide that flat place on top, and drop down just as gently on the other side.

Sizing Your Mound With Math

A mound is composed of three plains: The upslope, a level zone along the top, and the downslope on the other side.  How high you go is dictated by how much space is available.  The slope is governed by the angle of repose, which for traditional plants must be no more than 30% if the water is to penetrate.  That’s a one-foot “rise” in elevation for every three feet of length or “run”.

Example:  6′ upslope distance + 3′ top of mound + 6′ downslope distance = 15 linear feet

To create a standout succulent mound, use a wide range of colors, sizes, and textures to give it endless visual appeal.

This equation limits the width of your hypothetical mound to 2 feet in height. Once you calculate its dimensions, do the same for the length.  Then use length x width x height to find the overall volume of the proposed mound in cubic feet. The largest bag of Black Gold Moisture Supreme Container Mix contains 2 cubic feet of material, so either make the whole mound out of this mix or blend it 50-50 with natural soil. Aggregate can be added to increase drainage.

Grading Your Mound

Where soils aren’t heavy clay, the container mix is an extender and to better integrate local soil flora into the mound.  Mix very well with a tiller or fork, then gently grade the mound into a graceful shape without broken curves or undulations. When grading out your mound, keep the soil damp and lightly compact the surface so it holds together.  Use boulders where conditions are irregular or create fields of smaller attractive pebbles to hold ground or control runoff.

Succulent Planting Strategy

angle of repose
Planting on this slope features larger agaves and masses of small succulents to hold soil against erosion.

Water flows downhill picking up soil particles with speed. To keep this from happening plant against the direction of the water flow.  (This is what wheat farmers do to minimize erosion; they align their planting rows perpendicular to the natural flow of water.)  Use small, densely planted succulents for steeper spots then irrigate with micro-spray irrigation to allow roots to create a network better able to hold the slope.  Larger succulents, like aloes, further guide runoff away from more vulnerable locations with a single drip emitter.

A mound for succulents doesn’t need to be as high as those for plants with deeper root systems.  In most cases, they are fine with just one foot of elevation to keep plants and root crowns high and dry.  When the mound is in and fully planted, finish it off with a fine layer of stone or pebble that blends in with your cobbles and boulders for a perfectly designed display garden you’ll be proud of.

About Maureen Gilmer

Maureen Gilmer is celebrating her 40th year in California horticulture and photojournalism as the most widely published professional in the state. She is the author of 21 books on gardening, design and the environment, is a widely published photographer, and syndicated with Tribune Content Agency. She is the weekly horticultural columnist for the Desert Sun newspaper in Palm Springs and contributes to Desert Magazine, specializing on arid zone plants and practices for a changing climate. She works and lives in the remote high desert for firsthand observations of native species. Her latest book is The Colorful Dry Garden published by Sasquatch Books. When not writing or photographing she is out exploring the desert on her Arabian horse. She lives in Morongo Valley with her husband Jim and two rescue pit bulls. When not writing or photographing she is usually out riding her quarter horse.

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