The following is an email to Hamish Brown, a wonderful New Zealand plantsman whom I’m working with to build a crevice garden at the Christchurch Botanic. I realized that his prompts and my responses became a very useful and succinct formula to the secrets of creating a crevice garden. Stay tuned in March and see how the garden turns out! (All photos courtesy of Brown)
My gut says to pick the sunnier spot away from the trees (their shade and root competition and leaf drop,) unless in the context of the rockery’s current collections, it is most lacking plants or sites for cool saxatile plants from shadier/drier/forested places.
Tonnage/amt of stone:
I just came up with a formula for stone weight needed- it’s been needed forever and every project/stone is so different, everyone has always just eyeballed a guess! -Including me, for some 40 crevice gardens over the years… Anyway: I have figured 4-6 tons (american) for a 100 square foot unit/area. Which is 3.5-5.5 tonnes per 9.23 sq meters. The range is wide because granite can weigh almost double what a light sandstone does. Further complicating the range is that very flat stone buried deeply will use up so much more than more shallowly buried, chunky or less-plate like stone. Third complication is that a flat-topography garden (like a paved patio) will use much less stone than that of a high-mounded one which literally has much greater surface area. This is the reason I’ve been dragging my feet so badly that it’s just a blank page in the manuscript of our book on crevice gardens!
Anyway, I hope that is a little helpful and not overwhelming.
Thanks so much for organizing the timings and roles for this project.
On enlarging the planned crevice garden:
You talked about going larger- perhaps at our Friday morning meeting (I can have looked at the site and prepared some data/ideas for the group to work with) we can talk about the design of the site, deciding if that is the case, we can design it in such a way for it to be the first stage of a larger exhibit- I am more than happy to create drawings and notes for this and lend support for a future person executing it. There are also some tricks we can use to ensure that a new feature blends into the existing garden without being a sore grey thumb.
Staged build has worked well for other private/public gardens because it allows them to adjust soil mix/design informed by how things worked in early phases- for example, if the soil mix turns out to be no good for most plants, the rest of the garden can then use a better choice. Or, if you find that the plants you aimed to grow only truly prosper on just a north aspect, the future phases of garden design can change so as to maximize north aspects, and so on. This has been most helpful in areas with zero history of crevice gardening and when we had no idea how crevice/soil and local climate would interact. I guess, to make a long story short- a phased build lets one test out and dip a toe in the water before diving deep and committing to certain specifications.
I’m happy to build as large as we can in the time I’m there, too, should ya’ll need it.
Hamish, that quarry is delicious. Is this New Zealand’s greywacke hard sandstone? It has superb character. It looks somewhat basaltic/rhyolite in shape. Mmm, love those irregular but sharp shapes.
You are right in seeking out different sizes. I like to use a full spectrum from largest to smallest, with no discernable size being obviously dominant or scarce. The largest one could be whatever we as a group (or armed with a tractor!) can move- maybe 1 m at the longest measurement. If you can, make sure there is a gradient of sizes down from the biggest to smallest. Each size doesn’t have to be equally represented, just enough to be seen. In a perfect world, we’d have some little chunks (maybe 10% or less of the total rock) that are the size of the gravel topdressing to be used- a perfect gradient of size with no missing sizes will unify a garden in a magic and powerful “je ne sais quoi” sort of way. If the rock is hauled dirty and bulk in a dump-truck, the little bits appear are the result of rocks breaking when they are loaded/dumped. Convenient. Other times, little chunk are impossible- and that’s ok, too.
You are lucky to have a loader to pick them up! If there is a part of the pile with very rectangular, brick-ish stone, avoid those- let stonemasons have that. Irregular edges (which are the faces that show in a finished garden) and irregularly parallel cleavage make for the best gardens.
I see thick chunky guys and slab/plate-like shapes in that pile. Go for a mix and with intermediates.
Generally, gather more than you think we need.
Lastly- after all this thinking and planning, we’re going to organically wing it and it’s going to be great!
Very much looking forward to this!