Propagation (seed, cuttings, etc): Pumice for rooting cuttings
Started by: John GoodGo to latest contribution by John Dixon, 18 July 2014, 17:23. Go to bottom of this page.
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I am experimenting for the first time with using pure pumice granules for rooting cuttings, starting with daphnes, and wonder if any of you have used this material and if so how you rate it. The material I obtained was from a supplier of orchid composts and is, I think, just a bit too coarse (c. 2-4 mm) for rooting cuttings, but it 'sits' better and holds moisture better than Perlite.
We have never used pumice but at the moment use an underlying mix of fine propagation grade bark mixed with vermiculite or perlite, topped with fine sharp sand. The latter looks to be derived from the fines from cornish grit but is very uniform and good for the smallest of cuttings that need close contact with the medium - I suspect it has the same or similar benefits as pumice which many growers who propagate dionysias etc. swear by. Robin White and others who grow a lot of daphnes use plenty of bark in potting mixes for these, which makes it very open and also reduces fungal problems, so this would probably help once the cuttings begin to root and grow away.
I and others do use a finer pumice for Dionysias and similar, I always liken it to the texture of granulated sugar, around 1mm nominal. The only place I've ever been able to get it is Viresco in Thirsk but they don't always have it; I think theirs is Italian. Frustratingly we have just driven several miles through the caldera of the Eifel supervolcano in Germany (still technically active, last devastating eruption less than 13000 years ago!) but despite being able to see lots of lava and pumice quarries we were on a tight schedule. I'm sure however that for cuttings with a greater stem length yours will be fine. Tübingen Botanic Garden use even finer pumice for their cuttings, probably less than 0.5mm. I first saw theirs when they took cuttings at our house a couple of years ago and I thought it might be too fine but they get fantastic results and I found success with the sample they left with me.
Thanks Tim and Paul, I will report back in due course my success (or otherwise!) with the daphnes in the grade I have mentioned. In the meantime I will try to get hold of some of the finer grade.
I've been using pumice, mainly for Dionysia cuttings, for over 20 years now with reliable results. In fact I'm still using the original pumice I bought from Viresco's predecessor at Thirsk - a 10kg bag and mostly unused, (I think it was somewhere in the greenhouse when I last saw it but in an 8 x 6 there shouldn't be much room to hide...)
I have usually prepared one or two 'standard' seed trays (without drainage holes) each year, I sterilise the pumice by pouring boiling water over it in old lasagne foil trays and collect it after cooling using a fine stainless steel sieve, packing it down with anything smooth and flat. Over-compaction isn't a problem, the grains are angular and stable enough to ensure plenty of air spaces remain.
I normally take cuttings from say mid-May through to October, (whenever the cutting material's available!), and pumice makes it extremely easy to manage the water content of the tray. Years ago I remember not needing to add any water after initial preparation for 6 to 8 weeks, but with continued sieving over the years I think some of the finer grains have gone, which makes the medium slightly drier. This is not a problem since pumice is easy to re-wet, with care! - just watch out for a mini pumice slump if you overdo it. It's a good idea to have a play with some pumice without cuttings to find out just how much water you can add, you may be surprised.
Naturally this also depends on grain size, I think mine came as a nominal 1-2mm range, checking the tray now reveals an actual range of say 0.7 to 2.2 with a few larger grains, so a little coarser than Paul's. Where I think the smaller grain sizes might have an advantage is in keeping closer contact around small cuttings, I have to be careful to gently pack around the smaller cuttings, so I can see why Tubingen have good results with finer grains.
I've used pumice successfully for Alkanna aucheriana and similarly woolly subjects which dislike the wetness of sand, I've only tried Daphne cuttings in sand, but I would expect them to work in pumice so long as moisture is closely monitored. Pumice has many advantages, it's easy to sterilise, easy to re-wet, re-usable, (important if supplies are patchy), and it's very easy to dig out rooted cuttings without too much root damage. Late cuttings are not a problem either, just leave the cuttings over winter and pot in spring. Finding some is the main problem, but since it's fairly commonly used in such things as cosmetics, water filtration, etc., there must be some sources, (especially if you can buy by the tonne...)
Hope this helps, good luck!
Thanks for those very useful contributions. I contacted Viresco UK and the proprietor very kindly sent me some sachet samples of the different grades. He does not have any pumice in stock at present but will re-order in the autumn if there is sufficient demand, so if you would like some I suggest you contact him via their web site now and express an interest.
It would be very interesting to try pumice for some of the cuttings of really choice alpines in particular - I wonder what Aberconwy use for their dionysias, or Mike Smith who used to grow such a fine range of these plants? On a nursery scale, with a great variety of plants there is often more need to compromise and find a regime that fits a wider range of plants. These two pictures show the fine cornish grit we have been using to top dress pots for cuttings; it is really used as a way of anchoring such small cuttings which have only very short stems, and below is the more open bark/vermiculite mix into which the roots will grow, with better aeration. The grit is crushed granite and presumably not good at holding moisture except by capillarity, whereas pumice must have the great benefit of retaining both moisture and air. The smallest particles are no more than 0.5mm and not as angular as some sharp sands; this must mean they pack together more closely and capillarity will be enhanced. (There is something very agreeable about pots of small cuttings like these, especially when most actually root successfully!).
A quick update.
Judging from a brief trawl through the web there are few UK suppliers of pumice, and I suspect fewer still who'd be happy to supply the small quantities an alpine grower might need.
I registered onto one supplier's website to gain access to their library - all sorts of useful stuff about the chemical composition of pumice, perlite, etc. - and part of the registration process was a questionnaire asking why I was interested. A few days later I received an email telling me they were happy to supply in 25kg bags, at a price of £130. I didn't bother to ask if that included VAT, or how much they charge for delivery.
Now, where did I put the rest of that 10kg bag...