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Soil, Compost & Mulch

planting over drainage gravel?

10 years ago

Hi all, we've just bought our first house, and were hoping to do some planting in the front yard. The house is built along a hill, and the front yard is bisected by a retaining wall, making a sort of 15' by 15' plateau on the uphill side of that wall (see pic).

We were hoping to line this area with Burford Holly, but when we started preparing the soil we found that under the first 12-18 in. of (slightly clayey) topsoil is a good two feet of pea gravel--we're assuming for drainage, part of the drainage solution that the retaining wall was also presumably built for. So we don't think we should just dig all the gravel out, even if we could--someone clearly laid it there for a reason. But a foot of topsoil doesn't seem like much to work with. We're reluctant to build raised beds or lay a bunch of new topsoil above what we have, since this area behind the retaining wall is already raised a couple feet off the ground--we don't want the garden to tower over the street!

So--do we have any options? Are we stuck planting small, shallow-rooted plants? Could we selectively dig out areas and mix the gravel with compost under where we want to plant? What would you recommend?

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Comments (4)

gardengal48 (PNW Z8/9)
10 years ago
You don't ever want to dig the planting hole any deeper than the nursery container or rootball is anyway and it is a pretty darn big plant that totes around a rootball deeper than 12 inches!! So you should be good to go.

FWIW, the vast majority of plants - even large trees - have the bulk of their root mass within the top 12-16" of soil. And for the most part, roots grow laterally, not vertically so out, not down :-) 12-18 inches of workable soil is a very reasonable amount.......a lot of new construction efforts will grudgingly lay a couple inches!

Gravel is a common backfill for any type of retaining wall. And yes, it is there for drainage purposes. However I doubt that gravel is uniform throughout that upper tiered portion - usually it is concentrated immediately behind the wall.....right where you are digging :-)
TXEB
10 years ago
First thing you should know - the principle reason retaining walls fail structurally is because of hydrostatic pressure behind the wall. Really good drainage is a must - it was not built to be an above ground pool. So, keep the drainage.

As the gal above pointed out, plant high and you should be good. Leave the top level of the container soil 2-3" above the native soil level, then mound the soil from the hole around it tapering it away from the plant. Should be fine.
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CloudesleyB
Original Author
10 years ago
Thank you guys! It's so encouraging to hear that our gravel layer isn't a deal-breaker! We'll do a little mounding around the shrubs and we'll do some exploratory digging further back from the wall to see if there's an even less-gravelly spot deep enough to take a small tree. This is my first garden--thanks for the help and the good news!
TXEB
10 years ago
As you explore back from the wall, don't be surprised if at some point you run into a geotextile membrane extending down into the soil. If you do, leave it untouched. It's there to create a barrier between the soil and the drainage bed (gravel) , and keep the soil from migrating into the gravel, filling it up, and plugging things up.

I also wouldn't be surprised if down near the bottom of that wall on the inside that there would be a section of drain tile that exits somewhere on the outside. They work like an underground gutter at the bottom of the gravel bed. You might look for a drain exit around the outside near the lowest point.

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