Building a Rock Garden - Getting Down and Dirty
by Sue Leduc
Your anchor rocks are in position, with their best faces
exposed, but there are gaps between them big enough to drive a truck
through... Perfect! You're ready to start filling your rock garden.
But first, let's take a quick look at our Ottawa Valley
weather conditions. Our 46 inches of rain are unevenly spread throughout
the year. In the spring, we get snowmelt and plenty of rain. In the
summer, we frequently have extended periods of relative drought broken
by torrential rain. Then, in the fall, we get more rain. The ground
starts to freeze in October or November but the snow may not arrive
until January. And then to cap it off, we frequently get a major thaw
in January or February followed by a week or two of -35° C before
the snow cover returns to protect the plants.
For alpine and rock garden plants, these weather conditions
can spell death. Too much water or ice in the wrong place at the wrong
time will seriously damage or kill many of the really desirable plants.
The first line of defence is the soil.
Soil is made up of 4 components: sand, silt, clay and
organic material. The biggest challenge for rock gardeners is to get
the mix of these components correct for the plants and the climatic
conditions. Your soil mix will either make or break your rock garden.
For more information about soil and how it works, refer to articles
in previous issues of OVRGHS newsletters - one by Eva Gallagher from
April 2002 and another by Marilyn Light in May 2004.
alpines and rock garden plants have enormous root systems compared
to their aboveground vegetative parts. They need a loose, airy and
gritty upper soil area with a cooler, moister and richer lower layer.
The roots want to go deep for food and water.
Everyone who has ever had even a minor success with
alpines will tell a beginner that their own recipe for soil mix is
the only one to use. We’re no different. But we have only gardened
in eastern Ontario so we have had to work with our local climate and
the materials that are readily (and inexpensively) available here.
It's time for another field trip to your favourite sand/gravel/topsoil
place. You want to find the sandiest topsoil that you can. This is
no easy feat because so much of our local soil has a very high clay
content. Clay is bad for rock plants.
will also need 'grit'. We have found two variations at our local quarry.
They call them both 'stone dust', but one is derived from limestone,
the other is from sandstone. Both are a nice mix of small shards of
rock, sand and smaller particles. We usually use the sandstone-based
grit for general rock garden building and the limestone-based grit
for paths and for soil mixes for very specific lime-loving plants
(like encrusted Saxifrages). You will need more grit than soil for
two reasons. The first is that you will reduce the amount of soil
in your mix as you get closer to the final planting grade. The second
is that you may be using sifted grit for topdressing (but that's another
The easiest way to mix your soil is right in the wheelbarrow
- so many shovels of soil, so many shovels of grit, and give it a
quick stir. Then you’re ready to dump it in the spaces between
your anchor rocks, starting in the middle and working your way up
and out. The lowest level should be about half soil, half grit (a
1 to 1 ratio). As the fill gets deeper, gradually reduce the proportion
of soil to about a third of the grit (a 1 to 3 ratio).
Generally, we don't add supplemental organic material
to our basic mix. If we have a plant that needs a richer planting
medium, we beef up that plant's specific area a bit as we plant it.
And now the heresy - we don't use peat, ever, in the rock gardens.
Peat can make the soil pH more acidic, while a majority of rock plants
do best in neutral or alkaline soil pH. When peat gets dry, it is
very hard to moisten. And when peat gets very wet, it stays wet. We
prefer good old-fashioned homemade compost.
the level of your soil rises, you can position the internal rocks.
Keep in mind that you are 'sculpting' a miniature landscape. It shouldn't
look like a tiered wedding cake or a pile of dirt with rocks sitting
on it or sticking out of it. Be sure to build lots of little crevices
and nooks as you go, as well as places to stand or kneel for tending
the garden. Try to avoid perching the rocks on the surface of the
soil; they should be at least partially buried. If there is a grain
in your rocks, be sure to match its direction with the other rocks.
Above all though, remember, this is your garden. If you love it, then
you did it right. If you don't love it, move things around until you
Now, take a well-deserved break. Let the soil settle
for at least a week, then give it a serious soaking with the hose.
There may be hidden air pockets in the soil that will result in sink
holes after the first really hard rain. You may also find places where
the water runs off too quickly so you will have to change the slope
or build a dam to slow the water down. It's much easier to correct
these problems now.
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