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Building a Rock Garden - Getting Down and Dirty
by Sue LeducYour 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.
Most 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.
You 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 article...).
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.
As 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 do.
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.