My first hardy cacti will be arriving soon for planting in troughs, and I'm unclear about the planting medium. I think I'm OK with the base of 1/3 each loam, coarse sand, and crushed gravel 1/4"-3/4", but John Spain's booklet calls for 1-1/2" of 1/2" gravel as top dressing on all substrates in all situations. He says "Cactus plants that are not seedlings or weak cuttings will send their roots through a layer of this kind that is up to 4 inches deep."
Maybe it will be obvious to me when the shipment arrives and the plants will have thick 6" roots, but I thought I should try to find out beforehand.
Am I right in thinking that he is calling for me to perch the plants up on top of 1-1/2" of pure gravel, far away from any moisture-retentive medium, and that will be OK? Perhaps they're like Weebles and they won't fall down? Or they just root wherever you set them?
I guess I need to go crush more stone. I wonder why 1/2"?
The exact mix used depends a lot on your precipitation, in fact I am not aware of an exact mix. I use a mix 50/50 mix of 1/8 inch> DG and our native mineral clay. I top dress this with a few inches of 1/8 inch> DG, for the more commonly grown Echinocereus, Escobaria, Cylindropuntia, Pediocactus and Opuntia . When I grow species with moisture sensitivity such as Coryphantha, Sclerocactus, Echinocactus, and Grusonia, I add half inch pumice through out the mix.
I am only dealing with an annual perception average of six inches a year. I get by using this finer mix on eighty percent of the cacti I grow.
Plants grown from the Coryphantha vivipara seed from me will not be so moisture sensitive. To make a long story short, one winter when I had a January/Feb thaw, the melted snow water backed up and completely submerged my 40 pots of this same C. vivipara. They refroze, and when I discovered what had happened in the spring, I thought they would be goners. But I didn't lose a single one.
I have seen in a nursery here that what was labeled as C. vivipara, and from a different source (assume the Rockies). It was much more heavily clothed with spines than my South Dakota form, so it would surprise me if it was less moisture tolerant.
I have only grown three cactus species for a long period (C. vivipara, Echinocereus coccineus, Pediocactus simpsonii), but these have done fine in troughs with a mix of half sand, one quarter chick grit (#2 and #4) and one quarter standard quality potting mix.
I receive about 24 inches of precip a year.
"Am I right in thinking that he is calling for me to perch the plants up on top of 1-1/2" of pure gravel, far away from any moisture-retentive medium, and that will be OK? Perhaps they're like Weebles and they won't fall down? Or they just root wherever you set them?"
First are the plants coming in bare root or potted? If bare rooted you should snip off any damaged roots before you sink them in, to help alleviate rotting problems. If the damaged portions are on sizable segments of root, I always set the plant aside in a dry shaded place for a week. To allow the root to callose. Now if they are potted what is the potting medium. If it is peat based I will bare root it myself and plant it. If the medium is well drained with little peat or organic matter I just plant it as is. You can plant in course soil mixes as described with little if any detrimental effects to the plant. They will seek out moisture well below the surface dressing.
As far as rooting where ever you set them that depends on the cactus in question. Opuntias and Cylindropuntias will do just that. I have seen a shriveled Opuntia pad lay unrooted for a year and then decide to send roots into hard sunbaked clay soil. With the cylindrical cactus such as Echiocereus a freshly severed stem should be set aside to dry for up to two weeks or more. I would then place the cut surface in contact with the planting medium. I just set them down and scuff a little of the medium around the base. It can take up to a year for it to establish, as long as it is not disturbed. It will look awfully sad during that period. Lossing well over half it's volume.
When you talk about perching your cacti plants on top of pure grave. Yes you could do it if you get a few summer showers. But if you are ucomfortable doing it as descibled just lace the medium where you are placing the plant with a hand full fines. They will wash down eventually.
<<First are the plants coming in bare root or potted?>>
They are coming from High Country Gardens, and I just watched their video on how to plant cacti, which I hadn't noticed before. It appears that they send potted plants, and now I have an idea of what the root mass will look like.
Thanks for the advice about callousing. Certainly counter-intuitive for someone who has had experience only with perennials.
I'm getting Echinocereus, Pediocactus and Coryphantha, and I think I'll reduce the top-dressing to 3/4" which should allow the roots to easily reach the growing medium. I get about 20" more precip yearly than Rick does, but these will spend the winter indoors (cold) and at least part of the rest under glass, so I don't think I need the full 1-1/2" of gravel mulch that was recommended.