Key Takeaways
- A rockery on flat ground gets its height from a rubble core, with topsoil only in the planting pockets.
- Flat sites are drained by lifting the roots above the wet, because water on level ground has nowhere to run.
- The drainage layer on flat ground is 15 to 20cm of brick rubble or coarse gravel, capped with inverted turves.
- Turf under a flat-ground rockery is stripped off, never turned over and buried under a membrane.
- A rockery built on flat ground is made ground from top to bottom, so it needs about two weeks to settle before planting.
A few years ago, I was called to look at a rockery in a client’s back garden that had started life as a weekend project. The lawn was completely level, but they’d followed a guide intended for a slope: turned over the turf, laid membrane, added a layer of topsoil and piled the stone on top. By the time I saw it, the whole thing was a soggy mess replete with dead stumps where the alpines once briefly lived, and couch grass working its way through the planting pockets. I stripped it back and rebuilt the shape properly, using the stone to create the structure rather than relying on a thick layer of soil to hold everything up. A couple of years after it was raised, with adequate drainage and re-planted, it finally became a rockery feature that they loved.
All of the problems that plague rockeries on flat ground comes from the same place: the site gives you nothing. No bank to supply the height, no fall for the water, no solid ground under the mound. All three have to be built, and this article covers the four steps where that happens. There are five other steps that do not change on a slope or flat, which are summarised with links to the main build guide.
Here is the whole build in order, nine steps. Steps 2, 3, 4 and 6 are the ones that change on a level site, so those are the ones I have gone into properly. The other five are the same wherever you build, and they are covered at length in how to build a rockery, if you want the long version of any of them.
Step 1: Plan the shape and mark it out
Lay a hosepipe on the lawn for the outline rather than string, because a curve is easier to adjust than to redraw. Then go and look at it from the kitchen window and from wherever you sit outside, not from directly above, because that is the view nobody ever gets. Undersizing is the usual regret, and a rockery is far easier to shrink later than to extend once the drainage base is in.
Step 2: Strip the turf, do not turn it over
The turf under a rockery on flat ground comes off in slices and goes to one side, because grass does not stay dead just because you have flipped it. The classic mistake goes: turn the lawn over, lay a membrane on it, tip the rocks and a bag of topsoil on top, and by the second summer there is couch grass through every pocket and the mound is creeping about on a sheet of plastic. I have taken a few of those apart.
Slice the turf about 5cm deep with a flat spade so the roots and the seed bank go with it, and stack the pieces grass side down. Keep them, because they go back on in step 3 as the cap over the drainage layer. Dig out any bindweed or horsetail while the ground is open, because there is no getting at it under a tonne of stone.
Step 3: Dig out the drainage layer and cap it
The drainage layer under a flat-ground rockery is 15 to 20cm deep across the whole footprint, filled with brick rubble or a large gravel such as 40mm. Go to the full 20cm on clay, and 10cm is enough on a light sandy soil.
On level ground there is nowhere for water to go. A land drain needs somewhere lower to run to, and a soakaway needs ground that soaks, which heavy clay is not, so on a flat lawn the rubble is not carrying water away anywhere. It keeps the base of the mound out of the wet, and the mound lifts the roots higher still. The rockery drains by being above the water, not by sending it down a pit, so the height in the next two steps is a drainage decision too.
Cap the rubble before anything goes on it: the turves grass side down, overlapped like tiles, or a permeable geotextile (the felt-like filter fabric, not a woven weed membrane). Either stops soil washing into the gaps and clogging the layer.
Step 4: Build the core out of rubble, not topsoil
The bulk of a rockery on flat ground is rubble, and buying topsoil to make a hill is what makes people give up on the idea. The question I heard most on level plots was how to get a rockery up to waist height without ordering a lorry of soil. You build the hill out of hard waste instead. Old concrete fence posts, broken paving, half bricks and the hardcore from a shed base all work, and none of it is ever seen once the stone and the planting go on.
Only the planting pockets get soil, and 20 to 30cm of a gritty mix in each is enough for alpines; heathers manage on less. The mix itself is in step 8 below and does not change on a flat site. Someone dismantling an old rockery will often let weathered stone go for a song, moss and all.
Step 5: Buy one stone type in varied sizes
Mixed stone reads as a heap however carefully it is placed, so use one type in a range of sizes, from large feature pieces down to small infill. If your stone has visible grain lines, every piece goes in with those lines running the same way and parallel to the ground, and that one detail is what makes the finished thing look like a formation rather than a delivery. Local stone is cheaper and looks right for the area.
Step 6: Set the rocks into the mound, and build it high
Every large stone on a flat-ground rockery is buried to a third to a half of its depth, and on made ground I lean towards the half. Dig a hollow and set the stone down into it rather than heaping soil up its sides. Anything over about 25kg goes on a broad-tyred trolley over boards, never a wheelbarrow, and large stones want three people: two moving, one directing.
Two tricks give you height you have not paid for. Build the mound as half a hill against a fence, so the eye assumes it carries on behind, and set one flat stone low and nearly flush at the front so it reads as the tip of something bigger underneath. That pair is most of how to make a rockery on flat ground look like it grew there, and my answer to how to build a small rockery on a level plot.
Then wait. On a flat site every centimetre of the mound is fill you put there, and all of it drops. Give it roughly two weeks, run a sprinkler for a few hours if it stays dry, then top up the dips before you plant. Settlement is what separates building a rockery on flat ground from building one on a bank.
Step 7: Fill in with smaller stones
Smaller pieces wedge the big rocks, stop them rocking and make the planting pockets, so scatter them unevenly rather than in rows. Where the mound meets the lawn, run a strip of gravel and tufty grasses so there is no hard line between rock and turf, which on a level site is the join that gives the game away.
Step 8: Mix the soil pocket by pocket
One part John Innes No.3 to one part clean horticultural grit is the base mix, with a part of ericaceous compost and half a part of vermiculite added for acid lovers such as heathers and gentians. Vary it as you go, grittier in the tight crevices and richer in the open pockets, instead of treating the whole rockery as one growing medium. Tamp it just enough to take the air out, not hard.
Step 9: Plant once it has settled
Water every pot an hour or two beforehand so the rootballs are properly wet, set each one level with the surrounding soil, firm it in and water again. Finish with a 1cm collar of washed grit around every neck, thicker around anything woolly or silver-leaved, because on a low flat-ground mound it is winter wet at the crown that kills alpines rather than cold.
Flat ground versus a slope: what actually differs
| What differs | Flat ground | Slope |
|---|---|---|
| Where the height comes from | You build it, from a rubble core | The bank supplies it |
| Drainage | 15 to 20cm rubble layer, roots lifted above the wet | The gradient does most of the work |
| Membrane | Filter fabric over the rubble only, none over the mound | None at all, it becomes a slide |
| Setting the stones | A third to a half buried, nearer the half | A third to a half buried, largest at the foot as anchors |
| Settlement | The whole mound is fill and drops | Only the added soil moves |
Next Steps
Once the stone has settled it is a planting job, and rockery plants covers which plant goes in which pocket. For styles and inherited rockeries the rockery garden guide is the wider piece, and slate rockery ideas shows what slate does wet and dry. Put the footprint and 15 to 20cm into the gravel calculator and it turns the drainage layer into bulk bags. Outdoor Aggregates delivers gravel and MOT Type 1 in bulk bags nationwide on a pallet, so check the lorry can get near the lawn before you order.
If you would rather talk the stone for your rockery through with a person, ring Outdoor Aggregates on 0330 043 7585. The office is open 7am to 7pm, seven days a week.
Have you ever built a rockery on a flat lawn? What did you use for the hidden bulk, and how much did it drop in the first year? Tell us in the comments.
FAQs
How high can a rockery on flat ground be?
A rockery on flat ground can be built to whatever height its rubble core will hold, and a low version inside 20 to 30cm walls works well on a small plot. Once you are stacking stone to retain fill above roughly waist height you are building a wall, and that wants footings and a professional opinion.
Do you need a membrane under a rockery on flat ground?
A rockery on flat ground needs a permeable filter fabric or a layer of inverted turves over the rubble drainage layer, and nothing else. A weed membrane across the mound itself does not help. It makes a slippery layer the stone and soil creep down, and it is a nightmare to get out once roots have grown through it.
How much stone and soil does a small flat-ground rockery take?
A small flat-ground rockery of about 3m by 2m needs roughly a cubic metre of rubble or coarse gravel for the drainage layer, which is two bulk bags of gravel. The core above it is rubble you may already have, and the soil only fills the pockets, so the topsoil order is small. Size the stone on the finished face, because a third to a half of every piece ends up buried.