Scoria/lavarock as a substrate
 

AI few people here grow their orchids in scoria / lavarock, particularily Catts it seems, and it seems like an interesting substrate to test. I have 3 Catts that are/were in need of repotting, and two are already repotted in bark, and for the other I'd like to give scoria a try, but the trouble is that I don't know very much about it.

A few questions:

Is the size of the rocks important? It doesn't seem super common in stores here, so it's good to know if I need to be picky about size or not.

How does it behave compared to bark in terms of watering?

It seems that it's often used in warm climates where people need to water frequently. Are there any issues with using it in a more temperate climate (mostly indoors)?

I have been using red lava rock for a number of years and here are some tips:

1) the rock is sharp so care must be taken when potting up orchids into it. If roots are damaged when removing the old medium, letting it sit a day or two before potting up will ensure that the roots are healed. To pot into the new medium, I hold the orchid at the height I want it to be and gently place the rock around and between roots, tapping the pot now and then on a hard surface to settle the rock. I usually put larger pieces of rock on the bottom. If using a clay or plastic pot, larger rock in the bottom and middle help with the air circulation.
2) the rock needs to be flushed very well now and then to remove unused fertilizer and any minerals that have begun to accumulate
3) rock doesn't significantly contribute any minerals so adding Calcium seems to help keep the orchids healthy
4) Using a basket pot means that you never need to mess with the roots again...you just place the entire orchid into a larger pot.
4) If you want to use a slow release fertilizer or oyster shell, there are little bonsai baskets available to put these substances into that will keep it from falling through the rock.
5) Watering more frequently will be necessary

I have never had any trouble using lava rock. Orchids can be kept in the same rock long-term as long as the medium is flushed well often enough.

I grow pretty much all of my orchids in scoria too. Works well in my tropical region here. All my catts are in scoria.

In my region, there is the local grey/gray coloured type, which is the kind I use. There is the red type available - which a hardware store calls it 'decorative scoria' (or something like that, maybe meaning for filling the sides of drive-ways etc for 'decoration'. But maybe ok for orchids too. I'm just sticking with the grey/gray type, as that's what I've always used - and also readily available here in various sizes. Inexpensive and various sizes to choose from.

Average size 'can' be important. But also the size of pot can be important too. And also size and thickness of roots.

But there's always exceptions. I have oncidium twinkles growing in 15 mm average diameter scoria (or maybe ranging from 10 mm to 15 mm pieces). But then I also have them growing in average of say 8 mm diameter scoria.

I have a big paph (Paph. Saint Swithin 'Jill') growing in 10 to 15 mm diameter scoria - no problems at all. And small paphs and phrags growing in roughly 8 mm average diameter scoria - also no problems.

The usual situation to watch out for (maybe with scoria and any media for that matter) is maybe too much water hanging in and around and on the roots for relatively long periods of time.

So decisions can sometimes be made about pot size (height and/or diameter) ----- drainage hole size and number etc.

But one useful method - assuming the pot has decent size and number drainage holes - is that of watering the media toward the sides of the pot. Focusing most of the water toward the rim region (or whatever band that the grower deems to be workable and beneficial for the orchid). This method isn't any rule. But is an option. The idea is to keep the bulk of the 'wetness' away from main masses of root that might be under the orchid. The moisture toward the sides can still support the orchid. And it is ok to put some water closer to the orchid, but maybe 'not as much'.

And those roots that eventually do grow out toward the sides (and reach the sides) of the pot will meet the very wet region on 'its own terms' ----- and maybe becomes adapted or used to it ------- sort of like the way that roots get used to those other growing methods (- like water culture etc).

I'm new to Cymbidium growing, and already have a big cymbidium (Cym. Regal Flames 'Queen of Hearts') growing in quite a large size pot (big diameter and relatively deep too). I focus the water mainly in a chosen band/region around the rim. This orchid is doing well!

I'm also growing four little Sedirea japonica, each in their own little pots ---- in 5 to 8 mm average diameter scoria. Also two juvenile Jkf. Appleblossom doing great in 5 to 8 mm average diameter scoria.

I have one psychopsis and one phal growing in scoria ----- doing really well too.

I think it doesn't behave like bark. If it dries out, then watering scoria recharges it right away, unlike dry bark. I think scoria probably dries significantly faster than bark maybe.

Also don't be afraid to try big rocks at the bottom of the pot, and smaller size toward the top layer.

I've been wondering about the why and why not of scoria and pumice too. Volcanic products are as cheap as dirt if not cheaper here, we spread it on the roads for traction in the winter. If I can get a 20lb box of 3/4 to 5/16 inch horticultural grade pumice from Hess for $4 more than a "mini" bag from repotme, what's stopping me from switching from bark/hydroton/blah mixes?

Herrania ------ probably just the moisture retaining factor maybe. I haven't read up about it or found out about it yet, but was always under the impression that bark can hold more water in it for longer ----- maybe.

Bark is obviously one of the medium of choice for lots of growers, as is scoria. Whichever is used will probably just depend on choice, as well as the growing conditions - environment etc.

Bark is organic, so eventually is expected to degrade. Scoria can apparently accumulate fertiliser salts etc.

But I've never needed to repot due to fertiliser or salt build up issue before. I'm on a once-a-month fertiliser schedule (and a mid-month cal-mag schedule). The rest of the times is just plain watering of the scoria (tap water).

A question SouthPark, since you use so much scoria... have you used LECA? And if so, have you any opinion about the difference in wicking between the two? Does scoria retain water like LECA? Or maybe I should just go get some scoria and check it out....

Hi WW! Good question about the LECA. I haven't used LECA before WW. I have just seen threads about LECA, which appears to be quite workable.

The only things about them is that they're round - so better not accidentally drop them on the floor - or they'll roll around like marbles - maybe --- or create a hazard for somebody walking on one hahaha.

The other thing I've read about - but uncertain as to whether it's an issue (about LECA that is) - is some people reported roots getting water or something sucked out of them (the roots) by the LECA - at the surface of the pot. So the growers had to think of ways to avoid or cut down on this effect.

Scoria probably doesn't retain water in exactly the same way as LECA ----- but the scoria definitely retains water --- the water goes into the little nooks and crannies in the scoria.

I don't think that scoria has that special wicking effect of LECA.

I've seen orchid growers very successfully grow catts in just pots of pebbles and regular rocks. But I'm guessing they might have to just water a bit more regularly when compared with using LECA or scoria etc.

I think my issue with LECA is --- I don't know if there's a local source for this product. I did see online somebody writing about 'hydroton' purchased from the Aussie chain 'Bunnings' (sort of like home depot equivalent), but their post is dated 2013, and I was searching for 'bunnings leca' and 'bunnings hydroton' ----- no luck!!! I'll keep a eye out for this product though!

I had just always used scoria here, as in my city, the scoria is abundant - relatively inexpensive, and comes in various sizes too. The sellers sell this very readily! I think for about 10 aussie dollars or so ----- can cart home a huge chicken pellet bag full of scoria (- got to really put some work into lifting).

When I was developing the S/H technique, I tried all sorts of inert materials, including pumice and lava rock/scoria.

The fact is that anything CAN be used as a potting medium - I even used glass marbles once (great drainage and aeration, no retention to speak of) - but you have to consider your conditions, watering desires/capabilities, and the availability of "good" material.

Pumice wasn't bad, but the supplies available to me at the time were quite irregular in size, which resulted in a lot of "bridging" water, and the whitish color led to a lot of algae growth.

The lava rock was pretty good, but accumulated a lot of "crud". It also made potting a chore as it didn't really "pour" into the pot well, and trying to position it to fill voids in the root system often led to damage. I did find that by making a mound, then "splaying" the roots over it and covering them was effective if you had a plant amenable to that.

You also need to consider that natural products can have variable mineralogy that can affect the plants - I recall folks killing a lot of plants with some grades of lava rock. I don't know the details.

Ray is right that pumice is not as readily available as lava rock or leca. That is probably the main reason it is not used more, once a grower finds a media that works they will usually stick with it.

All medias have their flaws and advantages, one disadvantage with leca is it dries out too much at the surface so needs to be covered by another media like moss or gravel.
This is a small price to pay though as long as you are aware of the correct way to use leca as it is reusable and one of the cheapest options to use.

Pebbles or gravel works but needs to be watered a lot - but still less than bareroot so that should be considered. But you can't just stick all orchids into gravel as it could suffocate the roots so you need to find the right ratio of air to water.

With bark you have the option of using fine bark which will hold more water, medium or large bark which will have lots of air pockets and hold less water. The biggest disadvantage with bark is that is degrades and so plants need to be repotted not only when they outgrow the pot but also when the media goes bad and that can be hard to judge.

Lava rock has been shown to hold very little water, a fraction more than gravel but it is one of the airiest substrates so air reaching the roots will never be an issue with lava rock.

Pumice is the best out of the lot in my opinion, it has the airiness of lava rock - it literally floats, unlike lava rock which is very heavy - and it holds approximataly the same amount of moisture as bark.

Scoria seems to be a particularly good medium for those in tropical, humid climates (depending, of course, on availability) . Since the goal for nearly all orchids is "humid air" around the roots, how one gets there is different depending on the related factors - watering frequency, and ambient humidity. For me, it would produce an effect closer to mounting - very rapid drying. Which could be great for orchids that want to be mounted but can't be managed that way. I use bark and sphagnum (not on the same plants) to get the conditions I need for potted/basket plants. If I lived in southern Florida or the Gulf coast in the US, or northern Australia, those would tend to rot, and would tend to stay 'way too wet. There, volcanic rock would be an excellent choice.