Experiments With Gravel Mulch

Yucca filamentosa ‘Colorguard’ has appeared from under our newly installed gravel mulch.

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Most of the mulches we use are organic and improve the soil as they decay.  Shredded bark or leaves, pine straw, straw, newsprint or brown paper all have their uses.

When we consider inorganic mulches, there are definite benefits along with some obvious deficits.  Inorganic mulch won’t improve soil texture or fertility.  But neither will it harbor fungal disease, come pre-contaminated with weed seeds, provide a nesting site for ants or decay in just a few months.

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New terraces are planted to help control erosion, and mulched with pea gravel (spring 2017).

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I was first drawn to pea gravel mulch as we began to try to control erosion and cultivate the steep slopes of our back garden.    But I was also digging some gravel into the back-fill and planting hole when we installed new shrubs and perennials, to try to thwart the voles who would otherwise devour their tasty root balls.  Finishing the job with a nice mulch of gravel felt appropriate as a further deterrent to rodents.

Pea gravel definitely helps both with erosion control and rodent control.  But it also ‘disappeared’ into the soil on rainy days, after a while, and got covered with leaf litter and other organic matter over time.  I find myself renewing the pea gravel in spots after a while.

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“Soil security”

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Gravel mulch serves to help conserve soil moisture, just like every other sort of mulch.  It shades the soil, shelters root systems, absorbs the shock of falling rain and holds soil in place.

Additionally, gravel reflects sunlight and heat back up into the plants above it, helping to dry the plants more quickly after a rain and thereby deter fungal disease.  Gravel mulch also provides a dry barrier between moist soil and dry plant, preventing crown rot.

Soil doesn’t splash up onto lower leaves and branches, and the gravel perhaps makes it a little harder for invertebrates to travel up and back between soil and delicious plant above.

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Gravel mulch is used most commonly in rock gardens, where many drought tolerant and alpine plants are featured.  Some plants wouldn’t live long with a moist organic mulch, but manage just fine with gravel mulch that protects their crown.  Gravel is coming into vogue again as a fashionable and useful mulch for perennial gardens, too.  I have been reading about perennial and succulent  gardens grown under several inches of pea gravel in various garden magazines.

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Even a thin gravel mulch has helped conserve moisture around these newly planted perennials.

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I also recently enjoyed listening to a presentation by Joseph Tychonievich at the Williamsburg Botanical Garden where he presented his new book, Rock Gardening:  Reimagining a Classic Style.  Joseph inspired me to move ahead with my vision to incorporate more areas of gravel mulch in our sunny perennial beds in the upper garden.

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I use fine gravel as a mulch in potted arrangements, too.

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This area is gently sloping, and erosion isn’t as much of a pressing concern as in the lower gardens.  The entire area was left under several inches of freshly ground hardwood mulch in 2013, as the arborists who cleaned up our fallen trees ground up leaves and branches and simply left it all in place.

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Fresh compost piled on top of existing mulch allows me to plant in this area in 2013, right after the trees came down, without digging into the clay. A light covering of wood chips from the forest floor mulches the planting .

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As that has decomposed, I’ve renewed the mulch with bagged hardwood and Cypress mulch from the local hardware store.  It smells pleasant, and Cypress helps to repel insects.  It has an ecological downside, though as mature trees are cut for mulch.

The soil in much of this area still consists of thick, hard clay, despite my best efforts to dig in compost and improve its texture.  There may be a few inches of good compost on top of the clay, but the clay still holds heavy rainfall and keeps parts of the garden far too wet, especially in winter.

I am beginning to understand that a gravel mulch will promote better growth and vigor in most of the plants we are trying to establish, particularly the Iris and Mediterranean herbs.

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Recently,  I decided to experiment with a much larger gravel mulch in one of the beds that needed some TLC.  I lost several perennials here over winter, and so had quite a bit of bare ground.

On our shopping trip, my partner noticed this beautiful blue green rock quarried somewhere in Virginia.  We decided on the spot to give it a try, and I am very pleased with the results thus far.  Not only is this gravel not going to shift around on a rainy day, but I don’t believe it will sink down into the soil anytime soon, either.

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This three year old Siberian Iris bloomed for the first time this spring, and I hope the new gravel mulch will help it grow more vigorously in future.

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Now, please keep in mind that gravel is the heaviest mulch you can choose, and moving it around and spreading it takes both strength and commitment.  If I had the luxury of ordering up a truckload of it and hiring a crew to spread it for me, that would be a lovely thing.  But I don’t.

Rather, I’m buying it a couple of bags at a time and spreading it by hand.  It is going to take most of the summer to mulch this whole area working with just a few square feet each week.  But I am already seeing the benefit this mulch brings to our plants.

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This is a single bag of gravel spread around our new Monarda.  It will take a few more bags to finish this area….

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I bought three plugs of Yucca filamentosa ‘Color Guard’ about four years ago, as we were first planting this bed.  I wanted them to make a large focal point to anchor the area and planted them in a broad triangle.  Well, let’s just say that I expected them to grow much faster and they have largely gotten lost between larger and floppier perennials.  In fact, one of the three was struggling so much that I dug it up in late winter and planted it into a pot in full sun, hoping to give it a better chance to grow.

Never mind that I kept digging it up every month or so, checking to see if there was any visible growth, and replanting it again with the confirmation of a fresh root or tiny shoot.  That is a sad tale, and I ended up filling the pot with first one plant, and then another, simply to have something to look at besides the empty pot.  I ticked this off as a failed plant and moved on.

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Maybe if I put a fresh gravel mulch in this pot, the Yucca would finally grow?

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But not so fast!  Something of that Yucca was left alive in the original bed.  And finally, a month after I mulched over the area with the new Virginia gravel, look at what has emerged!  Plants really really want to live, and sometimes we just need to improve conditions for them and get out of the way to give them a chance!

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This newly planted Lavender was struggling with our weather extremes, but has improved under the gravel mulch this month.

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Our friends at our local garden center have a running joke that we always buy gravel or compost, if nothing else, and are their best customers for pea gravel. Gravel has made gardening in this difficult site possible.

If you happen to be in the neighborhood, and want to visit with me and bring a little gift, a fresh bag of gravel is always in style.   I’ll be so happy to see you, will show you around the garden and offer you a few divisions of something nice to take away with you.

And I might even let you help spread the gravel while we’re at it!

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Woodland Gnome 2018

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