Brian – you can really put a guy on the spot – I guess I have to get this out before June is over. You already have three (scratch that) six more posts since the “Lime-Lovers Trough”. Since you asked for instructions, my science educator background has taken over and this is a bit lengthy.
Just as with cooking recipes, where a good cook understands a substitution or alteration may produce a new and exciting result, these “recipes” are just variations on the basic. The first rocks were from pieces of broken troughs and/or hardened leftover mix.
A piece of a trough that broke before curing. It can be used to make a very attractive ‘rock’ with some chipping and carving. At least one half will be buried in the trough mix.
Basic hypertufa mix - three parts perlite and peat, two parts Portland cement. Bang and hack it with a mason’s hammer ‘til it looks like a nicely shaped rock. (I am reminded of a story about a sculptor. When asked by an admirer how he carved his magnificent images the sculptor replied, "Take a block of stone and remove anything that doesn’t look like the subject.") That’s what I do. I take a piece of hypertufa and remove everything that doesn’t look like an interesting rock, with a couple of fissures or niches to plant in or allow plants to scramble through. Voila, serviceable ‘limestone rock’. As they age, the peat chunks decay or flake off to form pits and voids for soil to fill and where plant roots can take hold.
Natural coral stone (left) and aged Hypertufa 'rock' (center) with Rosularia sedoides (right)
It's a great way to use material that would have been thrown out, but.... I always cringe slightly at the white beads of perlite exposed on the surfaces. Yes, they too will eventually weather and disintegrate, but much more slowly than the peat and in the meantime....distracting.
Hypertufa 'rock' showing white perlite granules
So my second variation had no perlite. The perlite is mainly to make a light trough and the rocks are small enough that the weight is no issue. So I mixed cement and peat with enough water to make a cottage cheesy mix slapped together to form a ‘proto-rock’. (Note: Be sure to wear gloves when hand shaping wet cement. It’s not nearly as caustic as lye but it does react with the outer layer of your skin.) As it sets up I fine tune the shape, try to make interesting bumps, peaks, fissures and niches and form a good portion to be buried in the trough planting media. Rocks, natural or fabricated, aren’t just decorations. They provide coolness and shade in summer, warmth and shelter in winter, channel moisture and encourage deep root runs. Like icebergs, the important part is below the surface. A little carving and roughing up of the above ground portion before it gets fully hard and a very convincing and attractive ‘rock’ is ready to age for the trough planting. (Like the troughs the rocks must age to modify the high lime content of the cement surface. Calcicoles like lime but not that much lime.)
Hypertufa 'rock' made with only cement and peat
I was satisfied until I got a good look at the tufa stones at the Frelinghuysen Arboretum in Morristown, NJ. The fissures and niches are like miniature caves with wormhole-like tunnels and tubes and minute stalactites and stalagmites scattered over the surface. Little seedlings have sprouted and matured in these pockets. The tufa is home to choice alpine and rock garden plants, their roots running deep into the rock via the channels. I covet these rocks immensely but the rarity and expense of such magnificent specimens are beyond even MY penchant for indulgence and budget busting. But could something vaguely similar be crafted? I had an idea: Variation three.
The raised scree bed of alpines at the Frelinghuysen Arboretum
Close up of natural tufa stones.
The particular ‘rocks’ in your trough are probably not something too many people will be able to duplicate. They have a very unusual ‘ingredient’ in the mix. Although I indulge in troughs and their exquisite inhabitants, and have been training a collection of trees as bonsai, and dabble in all sorts of horticultural pursuits, my great passion is and always has been my orchids. And all orchids have very unique roots. In most the center of each root is a thin, almost wirey core. It is surrounded by a thick, almost spongy layer of tissue that absorbs water and nutrients in live roots and continues to absorb and hold moisture after the root dies. I decided to use the roots trimmed from repotted plants instead of peat to make the artificial rocks. My hope is that as the roots break down in the cement they will leave a network of long, branching tubes for plant roots, just like the tufa, with the added bonus of a compost filling. I have also thrown in some used osmunda, a thin wiry fern root sometimes used to grow orchids and other epiphytes.
Orchid roots from different varieties come in various sizes. The dark roots above are from osmunda fern.
New hypertufa 'rock' made with orchid and fern roots
We’ll have to see how it works out as it ages.