Roots & Radicles 5 (Guest Write-Up): Playing hard to propagate at Sir Harold Hillier Gardens

I’m Maggie and I recently went along to the fifth YPS Roots & Radicles event that took place at the Sir Harold Hillier Gardens in Hampshire. I work as a head gardener of a private garden called Bramdean House near here so I was very excited to hear that Barry Clarke the propagator would be giving a tour - a great chance to get a more of an insight of the garden there from an insiders point of view and to brush up on my propagation skills - rarely do I find one propagator the same, so it’s always interesting to hear and learn about different approaches. Barry has worked at Hillier for over 20 years, once a trained ballet dancer he has gone from being an entomologist to running his own nursery. He himself has 6 National Plant Collection including Lobelia (species one), Rubus (through his love of entomology) and Calycanthus. He was incredibly generous with his time and even having spent several hours with him I felt that it was still a fraction of his vast fount of knowledge, being the propagator of such a diverse plant collection he was well versed in propagating a huge range of unusual trees and shrubs. A great aspect of the workshop also is that it is attended by like-minded enthusiasts and through conversation and discussion the experience was enriched further.


Here are some of the highlights from the day:


Spit as a rooting substance

Propagation can seem like wizardry and mysticism, to add to this he told us that the success rate of Clematis montana internodal could be enhanced by using spit?! This was related to him by Clematis montana collection holder, dubious at first he himself has found this to be true. Male spit is apparently the best.

Horticulturists, Barry Clarke, Hillier, plant propagation
Barry telling us about using spit in the propagation of Clematis montana

Plant Heritage National Collection Holders - a great source of information for propagators

Plant Heritage National Collection holders are one of the best places to seek advice about growing or propagating a particular genus. They spend a degree of time specialising in a particular plant group and as a result of this often know and hold niche and unique knowledge.


Grafting for all levels

Some of us were surprised to hear him advocating for people to do more grafting. When I was a Wisley Diploma student and had to learn grafting I spent half the time cutting my own hands than my material, and although I managed to do it well enough to pass my exams (with a lot of blood stained plasters), I had always told myself that it was perhaps a specialist skill that was not for me and that someone else could be a master grafter. Turns out I was not alone in this thought, but because of this it had become a dying art. Barry’s confidence and enthusiasm that it is not as hard as everyone thinks and has many merits, has made me rethink again about adopting grafting as another way to propagate.


If one found the thought of grafting hard his suggestion was to start with a bud cutting as he felt that was the easiest and most accessible. The important thing was getting heat around the graft union. If one didn’t have fancy facilities like at Kew one could embrace the DIY spirit of rigging up a system with a warming cable and pipe insulators. A dark room for grafts is best so something like a shed/ garage could be ideal. It is the aftercare of bringing the plant on after the graft union has been made that is the trickiest (one has to be a bit more attentive to them at this stage especially in terms of watering).


This led us onto talking about rootstock and how there was validity in growing ones own root stock as there is not a ready supplier to buy these from, and how some trees grow well from seed e.g. Cercidiphyllum. Most trees did not work like apples and pears where the size and vigour could be controlled by the rootstock, it is more about finding the right host parent.


What’s the advantage, why graft then? One can get bigger and quicker to mature plants and that it can be the best method for slow to grow or hard to propagate trees. E.g. grafting a magnolia you can be having it to flower after one year rather than 8-9 years.


We observed different rootstock - silver birch Betula utilis var. jacquemontii are often grafted onto Betula pendula. Knowing ones plant group types is useful - for oaks one had to be careful that one uses like for like e.g. red oaks can only be grafted with red oaks and white oak with white oak etc. A stand of low growing Prunus incisa ‘Oshidori’ looked like it was apple wedge grafted onto a Colt or Prunus incisa rootstock. Taxodium and Metasequoia are interchangeable.


There has been a huge problem with Rhododendron and the fungal disease Phytophora as many gardeners know of which Rhododendron ponticum the invasive one is the most susceptible and has traditionally been used as a rootstock for many Rhododendron cultivars. Rhododendron ‘Cunningham’s White’ has been a saviour as an alternative rootstock as it has been found to be Phytophora resistant. Amazingly Hillier has managed to phase this disease out of the garden and is mindful of the use of certain plants that can help carry and spread the disease like Viburnum tinus.


Bud grafting or chip budding is best done July - September and tape around grafts should be left on for at least 8 weeks, but best to check plants on an individual basis. He demonstrates this on a Cercidiphyllum japonicum ‘Rotfuchs’ and we marvelled at just how small the bud was and how this technique could still be conducted with such a tiny specimen.

Bud grafting, chip budding, Cercidiphyllum, Plant propagation
Taking a bud graft from Cercidiphyllum japonicum 'Rotfuchs'.

Bud grafting, chip budding, plant propagation, propagation knife, Cercidiphyllum
Close up of taking a bud graft from Cercidiphyllum japonicum 'Rotfuchs'.

Plant propagation, bud grafting, chip budding, Hillier, Cercidiphyllum
The tiny bud of a Cercidiphyllum for bud grafting/ chip budding.

The book he recommends on grafting is The Bench Grafter’s Handbook: Principles and Practice by Brian Humphrey


The trials, errors and pitfalls of grafting:

Taping the union - although there are lots of biodegradable tape on the market, he did not find them so good as the tape can’t tell when the union is actually ready and may not give enough time before it disintegrates. Often he is wary of what these tapes biodegrade down to and if it’s just breaking down into more micro-plastics.


He doesn’t think rooting powder is good for people but because of the nature of the woody material he propagates he does use one called Rhizopon (only usable by professionals who have a spraying certificate) and they come in different hormone percentages from 0.5% to 2%. There does seem to be a lot more organic and alternative rooting powders on the market now that is probably worth exploring.


Tips on a range of easy to difficult to propagate plants


Blue Bamboo

Hillier is cultivating a blue bamboo collection - these tell you themselves how easy it is just to come from division, after dividing and lifting some up he put them in pure sand with barely any watering and they still grew. They are planted in place with 3ft of plastic barrier around them to stop them from overly spreading.

Blue bamboo, plant propagation, division, Hillier
Barry talking about Hillier's blue bamboo collection.

Cornus (ones grown for colourful winter stems)

Do not grow as well from hardwood cutting as one would think but Barry found that softwood cuttings from them worked better.


Camellia take from hardwood cuttings better.


Syringa - hard to propagate, the blacking out technique

First tape or black velcro out a section of the stem from where you want to cut and would do a standard semi-ripe cutting. Leave for a few weeks before taking that cutting. This will help to activate it to root more easily.


Softwood cuttings

He does these on winter deciduous shrubs and trees and finds that tearing off the cutting material better than cutting it off as then more hormonal material comes with it. As well as reducing the leaves he takes off the tip. The lengths of material are no less than an inch - with plants from Hydrangeaceae as an exception for getting away with being shorter.


We talked about the effectiveness of ‘Nearing Frames’ - essentially a North facing coldframe filled with sand and has a cover for when conditions are cooler. Here is where he puts some cuttings he takes end of August - September. With the use of rooting hormones he finds that the success rate of his cuttings put there after 8-12 months were good. These have been found to be effective for Rhododendron cuttings which are known for not taking so easily in the conventional mist and bottom heat units.


Self layering

Some plants like Edgeworthia, Daphne and Kalmia has a tendency to self-layer. When this happens first get a spade in and sever it from the mother plant and wait some time before digging it up - this helps encourage them to generate their own roots, instead of just staying attached to the mother plant. One can try midsummer semi-ripe cuttings on these too.


Playing hard to propagate

He does air layering on some hard to propagate plants and again if he wants bigger, more mature specimens sooner. Usually these are done end of March, April, May. He demonstrated this for us on a big leaved Rhododendron.

Air layering, Rhododendron, plant propagation
Barry air layering a large leaved Rhododendron

First he cut two rings around a stem slightly thicker than a standard pencil approximately a couple of inches apart: