Hypertufa Slabs in a Bag and The Construction of Mount Zvolanek
by Eva Gallagher

Zdenek Zvolanek, our February 2004 speaker, is an ardent proponent of the vertical crevice garden. Since vertical strata are nonexistent in the Canadian Shield, it’s not a geologic feature that I am familiar with. Frankly, the few that I had seen illustrated in various publications looked very artificial and, to put it bluntly, rather ugly. While at first I did not share his enthusiasm, when the possibility of adding a crevice demonstration to our meeting was suggested, I put my bias aside.

In previous demos Zdenek had used rocks or Styrofoam cutouts. The Styrofoam was distressed with a propane torch and painted to simulate rocks. The thought of hauling around heavy slabs was not very appealing, but I thought that I could improve on painted Styrofoam by making the slabs out of hypertufa.

In November when I made my frivolous promise to provide the slabs for the February meeting, it seemed like a wonderful idea. By December reality had set in. This was going to be a messy, time-consuming project! With the deep freeze settling in, I would have to do all the mixing in the basement. By mid-January there was an anxious query from Zdenek inquiring how the slabs were progressing. “Yes, yes - I was just going to start on them today.” In the end the project took less time and effort than I thought....

I used the traditional recipe for hypertufa and did the mixing in a double plastic bag just in case of a puncture. A 40 kg bag of Portland cement made about 6-7 batches of the recipe and this was enough for all the slabs making up Mount Zvolanek. For the larger slabs I also added one container of coarse straw to lighten up the mix. As I had run out of peat and perlite near the end, I substituted Promix and for the tiny slabs where weight was not critical I used old potting soil that was mostly sand.

All measurements are by volume and use a 2-litre ice cream container. Be sure to use rubber gloves when mixing, as the mix is very caustic. This recipe makes one large slab about 28” x 18” x 3 ½” thick.

3 containers of peat (not sieved - just broken up by hand)
3 containers of perlite
2 containers of Portland cement

Mix the peat and perlite first by closing the bag and shaking/rolling it and then add the Portland cement. This needs to be stirred in by hand as it is heavy and settles to the bottom. Then add about 1-½ containers of water. Probably a little more will be required, but add it little by little. Be sure to mix well after each addition, as it is easy to get the mix too wet. The mix should be a bit wetter than what we use when making troughs, but it should not be runny. Then add a small handful of fibremesh and again stir thoroughly. Using the plastic bag minimized dust exposure and made the mixing easy as the bag could be tilted and rolled.

Tip the mix out on a plastic sheet on the floor and shape. The main thing is to have vertical edges to the slabs, as this is what will be exposed. You could even leave the mix in the bag and shape it through the plastic. This would allow the crumpled plastic to texture the cement. To give your slabs a more authentic look, you can put some pebbles underneath the plastic or, even better, use a lawn or other slightly uneven surface.

Cover the slabs with plastic and leave for 24 hours to harden. Then they can be lifted without breaking, yet will be soft enough so that the concrete can be shaped to give it a natural look. The edges will need to be scraped square. I found a paint scraper worked very well. This was the messiest part of the process as bits flew all over when scraping. The surface facing the plastic was very smooth so needed to be roughened as well. Then the slabs are wrapped again for a few days - for up to a month - so the cement can cure. Now you are ready to put your crevice garden together.

I must admit this (i.e. when the slabs were finished) is when I started to have doubts. My slabs were very ugly - mostly squarish, of different thicknesses. Would Zdenek be able to use them? Would the resulting construction embarrass our speaker? After all you can only do so much with the material you are handed. However I had greatly underestimated the artistry and experience of our speaker.

When making the slabs it is a good idea to have a vision of what the outcrop will look like so that you can make the appropriate shapes and thicknesses. You can even make some paper patterns. However, Zdenek had no preconceived idea and after a few tries at various arrangements, including a vertical Japanese style, he settled on a more classic arrangement. By siting the outcrop diagonally in the box he ended up with a very artistic construction - much better than my vision. By overlapping the slabs, building a smooth vertical front face and then gradually sloping down on the back, a very natural looking outcrop resulted. I think that Zdenek was pleased with the result. Judging by the enthusiastic response at the meeting so was everyone else! Even without plants it was a masterful creation.

There are several “rules” which Zdenek said should be followed when designing your crevice garden.

1. If you are going to have a sheer face, then face it south and have the top and back slope gently to the north. This way the north slope will mitigate the hot rays of the sun.

2. Make sure that the top of the outcrop has a smooth outline - remember you are mimicking the effects of weathering so no fracture line should stick up from the top of the weathered profile.

3. Maintain the same thickness for each vertical fracture layer and be sure that different layers are parallel to each other.

4. Build your outcrop so that you get weathering at the edges - shorter slabs near the back end and sloping down to the sides.

5. If using small rocks as finishing touches then put them in parallel lines, in line with the main lines of the large slabs.

6. The edges of the vertical crevices can be closed with small thin slabs. If you are building your outcrop with real rocks you can hammer small pieces into the crevice ends and this will anchor them tightly. With hypertufa, clay will help hold in the smaller hypertufa end pieces.

7. Be sure not to make any of the crevices wider than 2.5 cm. A wider crevice will tend to destroy the cool root run that is the big advantage of the vertical crevice garden.

Zdenek's favorite plant for closing the vertical crevices is Sempervivum arachnoideum - and he stressed using only the smallest rosettes. He also recommends using a pure mineral soil and tamping it down tightly into the narrow crevices as the crevices are built. He strongly advises against a scree mix for this type of garden - which is what they recommend for alpines in England. Nor does he recommend adding peat or other organic matter. He finds that a coarse, loose mix destroys the capillary action that draws water deep from the soil. Even with his hot dry summers (very similar to what we have), he rarely has to water once plants are established.

He has had great success in directly planting out his seedlings into the crevices once they have developed several true leaves, and so he often bypasses that first transplant stage into pots or flats. However it is very important to shade the tiny seedlings for two weeks until they’re well established. It is also important that the planting be done in the spring before it gets too hot. Alpines stop their rapid root growth with the onset of hot weather. This way the roots naturally grow deep into the cool crevices, rather than having their long pot-bound roots forced into a crevice.

The large rock mass of the vertical crevice garden is also super for rapidly ripening the tissue of woody plants as the rock surfaces provide extra heat and the woody tissue ends up fully ripened before winter. He mentioned that several gardeners are also growing alpine rhododendrons in these crevices, but in that case they do add some peat to create a more acid mix.

From the numerous enthusiastic comments at the meeting I think that we are all converted to the vertical crevice. Now members have asked for slab workshops! I can see building a small outcrop directly into the garden as a focal point or using them in troughs to give height. It will be interesting to see how the slabs hold up over the winter, but I imagine that, like the troughs, they should be good for many years. However, the properties of the hypertufa slabs in terms of insulation, heat conduction and water absorption are very different from real rock, so whether your plants will do better or worse, only time will tell.

John and Linda Soper have real rock slabs on their property and John can’t wait for spring to start prying them up. The lucky b....... as Zdenek would say! But for the rest of us that only have round rocks I’m afraid that it’s back to mixing slabs in a bag.