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Building a Rock Garden

April 27, 2020

Katharine Woods, aka the Tea Break Gardener, share her experience on siting and building a rock garden from scratch!

I’ve always wanted to build a rock garden. It was an ambition hidden within a long horticultural to-do list, crowded out by clamour for showier buxom blooms.

When I joined the Alpine Garden Society, I saw rock gardening at its best and this released a longstanding appreciation for daintier flowers. Quickly, my passion for alpines outstripped the troughs and pans in my possession.

I wanted to build a rock garden …but how do you do it?

Rock Garden Rules

When things have gone wrong in my gardening projects it’s usually because I rushed in without proper thought. So before starting my rock garden, I took to the books.

First port of call was the excellent Alpine Gardening for Beginners by John Good. John has his own garden diary, which you can read here. His book sets out the so-called “rules” of alpine garden construction:

  1. Choose an open site away from overhanging trees and buildings
  2. Pick a site with attractive natural background
  3. Make sure the soil is free from perennial weeds and tree roots
  4. Provide satisfactory drainage

You’re trying to recreate the conditions of being up a mountain, above the tree line, where air flows freely and high light levels prevail.

Rule 1: Open Site

The best rock garden sites are in the open, unshaded by trees or buildings. My garden is large, but there is no obvious place for a rock garden in a fully open site.

I chose as open a site as possible. In this area, the beds receive sunshine until at least 3pm, some even later.

Rule 2: Natural Background

Making the rock garden fit seamlessly into its surroundings by giving it a natural background is also important.

As you can see, my site has some “unnatural” background. The 1920s stone steps are an attractive feature when viewed from the patio and house. The brick retaining wall with railing is less attractive, but too expensive to replace.

My hope was to design the area to draw the eye to plant and rock, not the obviously man-made context of steps and wall.

Despite a couple of drawbacks, I picked this area for my rock garden. I’ve always been flummoxed as to how best to plant the beds either side of the steps for a year-round display and have found the slope problematic. A rock garden, I thought, would make the best use of this site.

Rule 3: Weed-free

Implementing rule three took a whole year. I tenaciously removed the ground elder that had crept into this bed, moved all existing plants and took out the stump of a long-since deceased tree.

Then, with a lot of effort and careful digging I removed the weeds.

Rule 4: Good Drainage

Rock garden plants need good drainage.

My soil is clay with large chunks of flint. The soil has been well cultivated over a century but dig down 3 or 4 feet and you’ll hit clay. Logic would argue against building a rockery here, where clay could impede drainage.

Optimism took over, however. Reasoning that the slope of my site would assist drainage, as would careful soil preparation, I planned to dig copious amounts of grit and sand into the soil.

Sourcing the Rocks

General advice is to source rocks local to you that will match your surroundings. My area has no obvious rocky outcrops, but I did want to find stones that would tone well with the existing stone walls and steps. These are greyish limestone.

I visited a stone merchant and chatted through the options. They sold bags of small quarry stones for rock garden construction but also large pieces of Purbeck limestone.

This limestone is slightly honey coloured compared to my steps, but the merchant assured me that this would grey with time. I bought 10 large stones and a bag of smaller stones.

The large rocks were chosen for their size and variety, but with no obvious design in mind.

I felt confident that I could arrange them artfully once on site as long as I had help to move them around… and as long as my helpers were willing to move things around again and again until they looked right!

Building the Rock Garden

I arranged delivery from the stone merchant. My landscape contractor agreed to be on site for the delivery with a small fork lift. The forklift transported the stones to my chosen site in the rear garden.

Placing the Keystones

Using the straps on the fork lift, the contractors helped me lift the large stones into place.

I’d read that placing the large stones first as “keystones” was a good approach and this definitely made sense to me.

We started at the bottom of the slope with two large stones either side of the steps to ground the rockery and give it weight at the base.

Next, I picked which stones went at the top, to draw your eye up the slope. Then I used the remaining stones to fill in the design.

While the stone wall had to stay in place, I hoped that removing some of the coping stones and overlapping rockery stones onto the top of the wall would integrate them into the design.

We dangled each of the larger stones in place from the forklift. This meant I could easily visualise what they would look like in situ.

Before they were placed, we dug soil out so they could be partially submerged. This helped ground the overall design and make it look more natural.

We rammed soil into nooks and crannies to make sure stones were solid and would not rock.

I was lucky that the keystone design looked right on the first go.

I stood back, looked at the rock garden from the ground floor of the house, from upstairs, from the bottom of the steps and from the top. It just worked.

Placing the Smaller Rocks

I then had the bag of smaller stones to play with. I used them to try to connect the larger stones visually and to provide interesting nooks and crannies for planting.

Having placed many stones I stood back and decided I’d gone too far and took a few out again!

I was happy with the balance of stone in my final design.

The fantastic thing about rock gardens is that by placing a rock you can create a north facing shady nook for a shade loving plant, create a planting hole for a trailing specimen or create a crevice for an attractive display.

Rock Garden Crevice

This turned out to be the hardest part of the rock garden to get right.

I needed a good finish to the top of the rockery so that it didn’t jar when approaching the top of the steps from the end of the garden.

To achieve this, I placed some old paving slabs on their ends to form a sort of informal wall and then sandwich more on their ends between this and the first large rock garden stone to form a crevice garden. Then, I rammed stones and gravel in to keep the slabs upright.

Preparing the Rock Garden Soil

At this point winter hit, a very wet one indeed. My clay soil got heavier and heavier and I knew soil preparation was impossible, let alone planting. I waited patiently until spring.

In early March a break in the weather mean that the soil was slowly drying out. I dug over the whole area, adding in 20 bags of sharp grit and 15 of sharp sand as I went.

The soil was still claggy at the point but over time started drying out and could be raked to a fine tilth.

Planting the Rock Garden

I’ve been collecting alpines for a couple of years for this rockery. I also grew some from the 2018 Alpine Garden Society Seed Exchange.

These, together with dwarf conifers from my local garden centre and mail-order purchases from alpine nurseries during the Covid-19 lockdown have been planted in my new rockery.

After planting, I covered the whole area with grit that toned well with the colour of the rocks.

My next diary will feature some of the plants and planting pockets for the design, but here are a few pictures of my young plants.

From start to finish, it took almost two years to build my rock garden.

As Reginald Farrer, who was the king of alpine gardening at the start of the 20th Century, says in his expansive book The English Rock Garden:

“No more advice can be given on design. Each site dictates its own, and each owner’s taste must do the rest”

My new rock garden is a function of me, my garden, and my own aesthetic taste.

Image of Katharine Woods Katharine Woods