Growing medium for a crevice garden

Hi, Janet,
I hope some people will respond to your question - until then, here is some advice and experiences from past threads:

http://nargs.org/smf/index.php?topic=240.msg17637#msg17637
http://nargs.org/smf/index.php?topic=240.msg11153#msg11153

Janet, I use sand (coarse) and 1/4" gravel in the crevices.  When I plant, I use a variety of tools (shish-ke-bob sticks, carving knife, horse hoof pick) to make the planting hole and I dribble in some subsoil "marbles" at the bottom.  The soil here is clay so I roll some in my palms and make tiny balls for a little waterholding capacity.  If plants look like they need fertilizer I'll take care of it.  In the humid northeast I just don't want to deal with any humus in a confined space.  So far it's worked well for me but every garden is different.  The crevice gardens here (but one) are sunny, open and windy.

Lori, thanks for the links. That thread on your tufa garden has already been an inspiration to me.  The most recent post is from last May... how has the tufa garden been doing since then?

My garden will be quite a bit smaller, since I have only 800 pounds of tufa. And I won't be able to use tufa gravel for topdressing. Having that much tufa is something gardeners in this area can only dream of.  :)

Anne, thanks for the information on your methods! I'm going to bug you for details. For example, what ratio of sand and gravel do you use? How many subsoil marbles per plant? And how deep? Are they just at the bottom of the rootball, or do the plants have to reach their roots down to get to them?

I saw your garden when the Delaware Valley chapter visited -- around 8 years ago and much earlier in my development as a rock gardener. (You probably don't remember me, but if you do, I was the one who pointed to the Edraianthus and asked which species of Campanula it was.) Your garden has given me something to aspire to.

Janet wrote:

 The most recent post is from last May... how has the tufa garden been doing ?

Overall, reasonably well, I suppose... ?  Some successes, some failures, of which I suppose some may be due to inappropriate plant choices for my laissez-faire style of gardening.  ;)

Janet - this is something I don't have experience of but would very much like to make too. If you can get hold of it, from the AGS, there is a super little booklet by Zdenek Zolanek on Crevice Gardens. He recommends using a richer soil based mix in the actual crevices, the roots then growing down to a sandier mix below. To stop the soil washing out of crevices you can use little slivers of rock and clay soil. Once you have made one bed you will almost certainly want to make another, so can try slightly different techniques. There is a lot of info. on the SRGC Forum too.

Hi Janet.  The ratio is a lot more sand than gravel, but almost all gravel and rock around the crown of the plant.  The sand is very coarse, not mason's sand. My clay marbles go at the bottom of the planting hole which is usually deeper than the roots of the new plant.  I want the roots to reach down for water.  It really depends on how dry or wet your climate is.  We can have wet winters and wet springs and usually hot and dry summers.  By then the roots have hit the clay marbles.  There's no science to the number of marbles - it depends on what kind of roots I think the plant will make and how extensive I think they may be.  Eriogonums, for example, can be like icebergs.  I once dug out a dead eriogonum and showed it to a visiting group.  The top growth was maybe 6-7" (it was Eriogonum douglasii), and the twisted, thick roots when stretched out were more than two feet long, quite amazing.  It was a plant that died from old age and the roots were quite woody.
All the crevices in the various crevice gardens are a minimum of 18" deep at the ends and up to 3' or morefarther in. 
Hope this helps.  In the end you try different things and do what works for you.  Rich mixes were a disaster here so I don't use them.  Of course, it also depends on the types of plants you grow.
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It's kind of an embarrassment but I love it any way. I made this little tufa thingy a couple years ago as experiment. There isn't much to see yet because it's March and because everything there big enough to see without a magnifying glass is growing from seed. It's about 3 x 4 feet. I did not break any of the stones whereby I might employ Harvey Wrightman's clay crevice technique or something similar but, instead, set them as close together as I could get them to fit. They are set on naturally sandy level soil (over the grass or weeds as was probably the case here). In the climate here they would probably shift excessively for happy rock plants if they were set on disturbed clay soil especially on a berm. Frost does that to things here. Likewise I considered the mix going in to the spaces remaining. I think it good advice for gritty/sandy mix for same frost-heave reasons. I put a little clay into mine, or used a clay rinse as I sometimes do, because that substance is naturally non existent here and many rock plants seem to like it. My goal was to minimize spaces between the tufas; it's to be a tufa garden, after all. The tufa is one stone thick as I plan to grow no deep rooted things on it (there's Silene acaulis and some little Androsaces on it now), but I did strive to lay them in a more or less vertical manner; it sits on sandy ground, wet in spring (as said by others; every situation different!) so is somewhat situated atop a giant plunge bed. I recently began careful addition of crushed (shale/limestone) to the surface of the thing. Careful because I've gone and put it inches away from supposed acid lovers. If you have good loam compost on your property you might use just a touch of that to give your grit a little oomph. You might avoid mix that's too well draining if the area is to be raised unless you plan to keep the hose on it. Be careful with a berm that's too high for its width 'cause that may slump too. I really think good crevice plants need good anchoring. One of my latest things here is using relatively pure (few additives in mix) stone dust from the local quarries. This is naturally graded from dust up to large sand grit in size and seems to weather in to a nice "hollow" type mix that, at least, resists heaving. You won't get it right the first time unless your really lucky so no need to worry; just go at it. Building a new rockery is always fun. Keep us posted Janet!

I should add that this will never be home for choice Eriogonums, Acantholimons or anything else deep rooted and preferring alkaline conditions.

Thanks to all of you for the advice. Tim, I've just ordered the booklet, and I'll check out the SRGC web site. Michael, your tufa garden looks realy nice. It looks like you've gotten the stones very close.

Spiegel wrote:

It really depends on how dry or wet your climate is.  We can have wet winters and wet springs and usually hot and dry summers. 

Anne, I'm in Philadelphia, so my climate is basically a warmer version of yours. We get mild winters and hot, humid summers, with rainfall evenly spread through the year. 

Janet, my site gets very dried out by wind, which also makes it considerably colder in the winter.  Eriogonums and astragalus, penstemon, convolvulus and oxytropis usually do well here as a result.

Janet wrote:

Michael, your tufa garden looks realy nice. It looks like you've gotten the stones very close.

That's one of the big problems with a lot of tufa. It is so roughly outlined that it does not lend itself to use in a crevice garden. My idea to use it was to set it on ground from which moisture would wick up into it. A mix that drains too well foils all of this. Tufa is often used set in a pan of water for this very reason. As for growing dryland plants, the tufa might better be used as root-run material, mostly buried in the construction. The mix I've used is fairly fine textured and moisture retentive and I have noticed that it is getting the unsettling habit of becoming unsettled and settling out of the crevices! Roots will anchor the raised soil in time but we are talking Androsaces here. They don't grow all that fast. A berm may not be all that necessary for technical reasons. As I recall Anne's beds are level though terraced on her ledges. I've personally seen no better example overall of dryland perennial cultivation than she displays, in the east but recall that Mike Slater once showed several fine dry sand beds -no tufa- he had made in Pennsylvania. Mike probably still does that. There's lots to aspire towards! Regarding this all; it might be best to aim at a certain type of micro habitat; alpine; dryland; etc. and mix mix accordingly. The chemistry is also very important; of this I become more and more certain as I dabble on into the unknown.

Thanks, Michael.  I don't, however, use tufa for drylanders.  The tufa here supports androsace and asperulas, daphnes etc.  The tufa is roughly a crevice garden but because of the irregularities you'd never know it from the surface.  Underneath, they're quite tight together and the irregularities leave small channels for delving roots.  When I've made a planting hole in tufa I frequently have drilled all the way through.  Most of the larger pieces are at least 2/3 buried and underneath it stays reasonably cool and moist.  I'm not able to water plants in the garden and the wind tends to dry things out too quickly for plants to establish in the tiny holes that are often recommended for planting in tufa.  I think if you do that you'd have to keep it misted or watered during periods of drought until well-established?  The great thing about gardening is that there are as many recipes for success as there are gardeners.  You always end up doing what works for you.  When I started rock gardening I tried to grow the traditional alpoine plants I read about.  It quickly morphed into something entirely different called "what does well here."

Great information here, folks! I have no tufa, but it's still interesting to read the technical musings...

An article by Zdenek Zvolanek has lots of good information on crevice gardening, with the advice that the medium in the crevices can be ordinary loam. It's not what I would have expected, but I can't argue with his success in creating crevice gardens. This is for crevices that are around a half inch wide and near vertical. In my tufa garden, the crevices are bound to be larger and at irregular slopes, so I'll be using a leaner mix, though I haven't yet decided exactly what to use.

The article was originally published in 2003 in the Rock Garden Journal, but it's available for downloaded on the SRGC site (http://www.srgc.net/forum/index.php?topic=4656.msg125155#msg125155).

Baldasarre Mineo give the same advice for very narrow crevices.

Janet wrote:

The article was originally published in 2003 in the Rock Garden Journal, but it's available for downloaded on the SRGC site (http://www.srgc.net/forum/index.php?topic=4656.msg125155#msg125155).

What a fantastic art form! Thanks Janet for refreshing the link to this SRGC thread.

A little "advert" for the resource available on the SRGC site      ;)

http://www.srgc.org.uk/monthfeature/feb2004/content.html
describes a revamp of some raised beds after inspiration from ZZ. These beds can regularly be seen in Ian Young's Bulb Log Diary .

All SRGC Journals, from the first up to July 2010, freely available online. There is a full  Index as well - all on this page :
http://www.srgc.net/site/index.php/extensions/journal

A tremendous resource - and not just for crevice gardens!  I commend it to you. 

Cheers,
Maggi

Using tufa for crevice gardens is completely different to using much less porous rocks. Tufa will allow an exchange of moisture from itself to the compost surrounding it. The exposed surfaces of tufa will have higher evaporation than a non-porous rock so the moisture dynamics of a tufa rock garden are very different to a crevice garden built from something much less porous.

To go all the way along this route - image a crevice garden built with peat blocks. The peat will hold and give up moisture. Using loam in the cracks is one way round the problem but it does not encourage deep root growth down the crevice in the same way that a better drained compost and less porous rock will.

RickR wrote:

Baldasarre Mineo give the same advice for very narrow crevices.

Good thought. I think if you use too much native soil and rocks are not close together you may run the risk of creating a tufa glorified perennial bed. You must ask yourself while building your new garden; "what am I doing differently". If you wish to grow plants other than those you are already growing in your other rockeries or raised beds or wish to grow plants that are always dying in those same conditions, you must ask yourself this. A narrow crevice suggests more rock, more raw mineral surface as well as less native soil and that IS different. I've questioned the common usage of sand with the same thought in mind; if the crevices become spaced too far apart is not simply a glorified sand bed being created? There may be good consideration in the SHAPE of the planting space that mitigates these concerns, people who grow plants in pots may think more about this than we who grow in gardens do. The tight, quickly drained/dried planting pockets created by the crevice garden may, indeed, have quite a bit to do with its success regardless of the fill mix, and crevices/pockets over an inch wide may actually be just fine. These larger pockets might essentially approach the growing conditions in a rock walled raised bed, which is, indeed, a tried and true method for growing rock plants.

Maggi, thanks for the links! I've been spending a lot of time this week on the SRGC forum, learning a lot and getting a lot of inspiration. I'm thrilled to know that all those issues of the journal are available.

McGregor wrote:

Using tufa for crevice gardens is completely different to using much less porous rocks. Tufa will allow an exchange of moisture from itself to the compost surrounding it. The exposed surfaces of tufa will have higher evaporation than a non-porous rock so the moisture dynamics of a tufa rock garden are very different to a crevice garden built from something much less porous.... Using loam in the cracks is one way round the problem but it does not encourage deep root growth down the crevice in the same way that a better drained compost and less porous rock will.

Malcolm, this is a good point. Do you have a recommendation on the sort of mix to use with tufa?

At the moment, I'm leaning towards a mixture of 50% sand, 25% quarter-inch gravel, 25% loamy soil (dug from 4-12 inches below a former lawn, to avoid too much organic material). But I'm still open to advice.

Most of my crevice gardens are built without any tufa - only one has tufa.  The mix used is lean and fast draining but there seems to be moisture on the rock surface below even when the top is very dry.  Since I grow a lot of drylanders this seems to work quite well.  The crevice sizes range from less than 1/2 inch to as much as 11/2 inches with stone chocks. 

Janet, I use sand/gravel mix with no loam. It's what Peter Korn does and if it's good enough for him it's good enough for me. Then I scout round and look for the drier and damper places, more sunny and more shady to decide which plants go where.

I have some pieces of tufa but not a tufa crevice garden - tufa is so scarce over here that I can't imagine anyone doing that.

I have seen peat blocks used to create crevice gardens. Again with sand/gravel as the fill.

Some great ideas - I like the revamped crevice beds in Maggi and Ian's garden, and the arrangement of troughs together. Those deep troughs based on vertical paving slabs must be ideal to provide a deep root run. I had never thought of using peat blocks in the way Malcolm suggests (I don't know how easy they are to get hold of?). In our dryish climate in the south they could be effective in holding a bank of moisture (Adrian Cooper succeeded in growing choice primulas, shortias etc. in a London garden using these, which surprised me). Very stimulating discussion!

Janet wrote:

Maggi, thanks for the links! I've been spending a lot of time this week on the SRGC forum, learning a lot and getting a lot of inspiration. I'm thrilled to know that all those issues of the journal are available.

Happy to help, Janet

You'll see that for quite a few years the editor of the SRGC's  "The Rock Garden" was none other than current NARGS Editor, Malcolm McGregor  ;) 8)

McGregor wrote:

Using tufa for crevice gardens is completely different to using much less porous rocks. Tufa will allow an exchange of moisture from itself to the compost surrounding it. The exposed surfaces of tufa will have higher evaporation than a non-porous rock so the moisture dynamics of a tufa rock garden are very different to a crevice garden built from something much less porous.

To go all the way along this route - image a crevice garden built with peat blocks. The peat will hold and give up moisture. Using loam in the cracks is one way round the problem but it does not encourage deep root growth down the crevice in the same way that a better drained compost and less porous rock will.

How fortuitous is the timing of this statement.  An acquaintance in another group recently posted some very interesting work he is involved in.  He is conducting tests that show how a root grows in a droplet of water on a dry surface, if and how it ventures outside the droplet edge, etc.  A very controlled environment. 

The gist of it is this:
He says the findings are only preliminary, as they have only done about 100 replicate tests, "But, so far, the data suggests that when the plant is happy (it has all the nutrients it needs) it will be less adventurous, it will be less prone to leave the droplet. On the other hand, if the plant is starved for nutrients, it seems more willing to push through the edge of the droplet."

Rick, that's really interesting.  That seems to bear out my observations in the crevice gardens here.  My "clay marbles" are always well below the plant roots' immediate reach and the mix is very lean.  When plants have had to be removed the roots were quite deep and also clinging to the side of the rock, so that they almost had to be scraped away.  When I built the crevice gardens I had Zdenek's book with me at all times. similar to having Linc Foster's book with me when I built the rock gardens.  Always so much to learn!

Spiegel wrote:

When plants have had to be removed the roots were quite deep and also clinging to the side of the rock, so that they almost had to be scraped away. 

Yet another practical observation upheld by his project's preliminary findings:
When the roots ventured beyond the droplet edge on the dry surface, they pulled a film of water around themselves as they grew.  This would be analogous to the side of the rock.
"When we saw this, we thought that this has to do with the surface tension of water, which the root uses to carry water along on a dry surface. So we thought, why don’t we prevent the root to do so, by changing the surface tension of water. We changed the droplet into a gel (this is the kind of thing that can happen in soil in the presence of some organic matter or particulates).  What happens is the following...

"The root escapes and finds itself on a barebone dry surface and it switches completely growth behaviour. It starts forming a completely insane amount of root hairs. This is quite interesting because it shows an interesting experimental system that can display two radically different root developments in the same plant. I believe the plant is forming this large number of root hairs to collect water from the very humid atmosphere (a common strategy used, for example, by insects in the Namibia’s desert)."

Thank you these reports, Rick - full of interesting points. 8)

RickR wrote:

"The root escapes and finds itself on a barebone dry surface and it switches completely growth behaviour. It starts forming a completely insane amount of root hairs. This is quite interesting because it shows an interesting experimental system that can display two radically different root developments in the same plant. I believe the plant is forming this large number of root hairs to collect water from the very humid atmosphere (a common strategy used, for example, by insects in the Namibia’s desert)."

Interesting, Rick! I remember that I looked at the roots of Streptocarpus plants growing on the wooded slopes of Kilimanjaro. They never seemed to penetrate the growing medium (which was a kind of very firm volcanic silt-like deposit) but made a "lattice" on the surface. I couldn't see roothairs though but assume they were there as it was a lot of fresh root growth. It was a very humid atmosphere in the wood of course.

Fascinating observations on root behavior. This is what I love about NARGS... a practical question might lead to insects in Namibia!