Propagation of Porophyllum and Ligulate Saxifrages

Adrian Young, who is a noted authority on Saxifraga, as well as holding the position of Registrar for Saxifraga, International Cultivar Registration Authority, Waterperry Gardens, has given his authorization to post his article (below) on propagation of Porophyllum and Ligulate saxifrages. Enjoy!

(NB. This article may not be copied or reproduced in full or in part without the express permission of the author, Adrian Young.)

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THE PROPAGATION OF POROPHYLLUM AND LIGULATE SAXIFRAGES

Propagation is one of the great joys of gardening and I am sure more alpine gardeners would propagate from their Saxifraga plants if they realised how simple the process really is.

There are a few basic ground rules and techniques that, if followed, produce reliable results. Next year I will have been propagating saxifrages for 40yrs and it still gives me great pleasure to see the new plants progressing. In this short article, I will describe the system that I find works well for me at Waterperry - a system originally taught to me by Eve Young who worked with Valerie Finnis in the Alpine department. Eve was a highly respected Alpine plant propagator, Bill Archer once commented "that Eve Young can put roots on Billiard balls" and he was no mean propagator himself.

POROPHYLLUM SAXIFRAGES

Stem Cuttings

Most Saxifrage cultivars [cultivated varieties] are, of course, hybrids (some are clonal selections of wild species) between two or more species. For example, when pollen from S.lilacina was used to fertilize seed on S.burseriana, one of the seedlings from the cross was selected and named S. 'Walter Irving'. If seed were now taken from S. 'Walter Irving', germinated and grown on, the new plant could be very different from its seed parent.  So, to maintain genetically identical stock, propagation by stem cuttings is virtually essential. As Lawrence D Hills expressed it, "The plant of an outstanding hybrid, natural  or artificial, or a good form of a species, can be increased indefinitely by this method without ever introducing the random reshuffle of  inheritance,  which  may  never  reproduce  the  particular  fortunate combination of genes however many seedlings are raised."

It may be a little optimistic to say "increased indefinitely" because there is evidence of a loss of hybrid vigour after a considerable number of years, S.'Faldonside' for example has been reproduced from cuttings for more than a century.

I am constantly amazed that experienced Alpine plant growers seem not to understand the hybridization principle, many times people have told me that they have a plant of S. 'Noname' which they have grown from AGS seed or indeed from other Society sources, when I explain that their plant should not be labeled as S. 'Noname' because it could be very different from the original clone they are surprised, it is necessary to understand what a clone is.

The term clone is used in horticulture to refer to the offspring of an original plant which were produced by vegetative reproduction. Many horticultural plant cultivars are clones, having been derived from a single individual, multiplied vegatively, that is non sexual reproduction.

Cuttings can be taken at any time of the year, although I would recommend avoiding June, July, and August as the management of cuttings is difficult then due to hot weather. That great Alpine plant Nursery man Joe Eliot advised me to take Kabschia cuttings as soon as the evenings got cool, around early October, he would finish his main kabschia propagation around Christmas and have the new plants potted up in March which gave the plants time to establish before any hot weather could threaten them.

On reflection this makes sense because kabschia roots start to re-grow as soon as the cool evenings start, the rosettes grow during spring and early summer when maximum light is available.

Most Kabschia cushions split easily into single rosette cuttings although in some cases the stems naturally end in a tight cluster of rosettes. In these instances, treat the cluster as a single rosette cutting. When selecting suitable cuttings, avoid rosettes containing a flower bud as these are difficult to root. Remove all dead leaves with a pair of tweezers from the base of the rosette, plus any remains of roots from the stem, leaving about  7-10mm of clean stem below the rosette. Longer stems can result in poor plants. The cuttings are now inserted into a propagating mix. If you cannot do this straight away, put them in a polythene bag and store in a refrigerator [NOT a freezer!] where they will keep in good condition for days.

The cuttings mix I use has changed over the years due to many experiments and useful advice, it has simplified greatly, I now use 100% sharp sand only. The local sand is a ubiquitous Bedford sand, I would say medium grade, medium sharp, nothing special.

The pan or tray you use can be either plastic or clay. Experiment with both to find out which suits your environment best. If you use clay pans, fine zinc or plastic mesh is needed to cover the drainage holes. Cover this base with coarse grit or pea gravel, which helps to drain away excess moisture. The mix, which should be moist, not wet, can now be put into the pan/tray

but first make sure that clay pans are moist, as bone-dry clay can draw the water out.  Firm the mix down thoroughly, using a brick or something heavy, until it feels very firm to the touch [a loose mix could fail completely]. The pan should now be filled to 15mm from the top, and watered lightly from a watering can fitted with a fine rose. Now fill with 15mm of sieved, washed sharp sand, I use a flour sieve. This now needs to be heavily firmed down and lightly watered. Although we have firmed down the mix far more than in other propagating systems, the use of sharp sand will ensure that there is still plenty of air throughout, and the mix will also drain evenly and quickly. You are now ready to insert the cuttings, for which a dibber is helpful. I use a 15cm length of 5mm garden cane, sharpened at one end. Make a hole with the sharp end, no deeper than the length of the cutting, insert the cutting, and firm the sand against the stem with the blunt end of the dibber. Holding the cutting with the fingers, you can feel the dibber pulling it firm into the sand. A small gap should be left between the cuttings. It maybe more effective for some people to use the blunt end of a pencil for firming the cutting as this has a larger surface area. Do not forget to label with the name, date and number. When the pan is full, lightly water the cuttings in. Ideally, the pan should be placed in a closed cold frame with a sand base. If the frame receives any direct sunlight, shading will be needed from May until September. If a cold frame is not available, use a plastic seed tray with a Perspex lid. Normal rooting time is between 8 and 12 weeks - when you can detect signs of growth, the cuttings are ready for potting up [although I prefer not to pot up rooted cuttings in midsummer as a hot spell can be disastrous], 15mm of root is enough for the young plant to establish itself - a large root system is not essential, once the root has started to grow it will continue. At Waterperry, we use a sterile compost for all kabschia potting up, 50/50% sharp sand and Moss peat (medium grade) I will not go into the reasons for this because that would be a major article in it's own right.

But I will say that the Humus in many types of compost kills kabschia Saxifraga roots. There is scientific evidence for this and much practical experience.

Seed

As I mentioned earlier, most cultivars are hybrids and plants resulting from their seed can vary significantly from the seed parent. Even seed from a species in cultivation could have been initiated by pollination from a nearby cultivar.  So propagation by seed is usually reserved for wild collected seed. Given that you have seed that you want to sow, it should be sown as soon as possible. Either plastic seed trays or thick-walled clay pans are suitable and the 50/50% compost that I mentioned earlier usually works well. The main issue is that Saxifraga seed is very fine and needs light to germinate and it is very easy for the seed to slip down into small gaps in the compost, so to avoid this I sieve the top 15mm of the compost with a flour sieve and firm down.

I put the seed on a sheet of white paper with a crease in the center, and then tap the paper while moving it across the surface of the pan to control the distribution. After covering the seed with a thin layer of 3-5mm horticultural grit, the pan or tray is then stood in a container of water which comes to just below the level of the compost. Watering from above is inadvisable as it is all too easy to wash the fine seed out of the tray or at least to spread the seed around the edges of the tray. The tray can then be placed either in a garden frame away from direct sunlight [but not in a dark shady place] or outside covered with a sheet of glass, and not allowed to dry out.  Some species and cultivars will germinate the same season in summer or autumn but many will wait until the following late spring. I used to leave the small plants undisturbed for one year, for, although they may form substantial-looking rosettes after a few months, the root systems are still  very  small,  just  fine hair-like  threads that  are difficult to transplant without terminal damage, however that great Saxifrage ambassador Sergio Bacci show me the results he got from early potting up, and I mean early, around eight weeks after germination the plant will have formed its first pair of true leaves, not the cotyledon leaves which appear initially. You need a steady hand and a good pair of eye's for this, knock the small seedlings out of the pot and gently tease out the plants from the compost, the juvenile plant has a single hair like root, this must be treated very gently and potted up with great care. If you have the nerve to do this better and earlier plants are the result.

Flowers can be expected in two years from a vigorous hybrid, but a slow growing species may need four or five years from seed.

Division

This method, which involves tearing rosettes from the plant with some roots attached, is not recommended. Porophyllum Saxifraga do not divide naturally, given the nature of their root system, and propagation by division usually results in poor plants. It is far better to convert the rosettes into cuttings and encourage them to grow new roots.

Of course some of the vigorous cultivars can be divided but just be aware that this method does not produce the best plants.

LIGULATAE (SILVER) SAXIFRAGES

Silver Saxifrages are surprisingly overlooked in most gardens, which is a shame for they are good garden plants, most of them that is. I have tried for many years to promote them with limited success, two recent cultivars, Ss.'Monarch' and 'Polar Drift' have appeared in some catalogues, which is a step in the right direction. Beryl Bland has done great work with this group and they should be more popular.

Propagation can follow the same lines as their Porophyllum cousins, rosette cuttings root very readily for most of the species, Ss. paniculata, cotyledon, hostii, cochlearis, callosa, cartilaginea, catalaunica, crustata and kolenatiana all root well. S.valdensis can be trickier and will rot off if the weather is damp and dull, best to wait for Spring with this species before striking the cuttings.

Some Silver saxifrages divide well as they have fresh roots forming close to the rosettes,  Ss. cochlearis, crustata and valdensis are best propagated from rosette cuttings. This group are generally very promiscuous and hybridise at the drop of a hat, so be very suspicious of any seed offered unless it is wild collected. If you are going to try seed the same rules apply as before.

In this article I have described methods which have worked well for me. But I have no wish to discourage experimentation.  Indeed, I would be very interested to learn of other growers' experience of different techniques or new materials.

One method I would like to try with Porophyllum cuttings is to use Gro-Lux Fluorescent Tubes in the cuttings frame, this would speed up the rooting process considerably, I would like to hear from anyone with experience of this technique.

Adrian Young August 2011