WHF held a propagation skills workshop on a cold weekday in early March 2023. Talks were given by Chris Lane on grafting, Peter Shotter on seed germination, Maurice Foster on cuttings, and Nadeesha Bandara shared details of the research project she’s part of at Nottingham University about the influence of hormones on rooting. We had eighteen students from Kew and Great Dixter plus Rod White from the RHS Woody plant Committee and Jack Aldridge from Wisley, so were crammed cosily in the conservatory on a snowy day with a continuous supply of tea and coffee, followed by a muddy tour of the arboretum when the skies cleared.
Maurice introduced the day by saying that the next generation of horticulturalists carry a great responsibility precisely as propagators, because without ‘looking after, growing on, sharing all the stuff that’s been collected’ the wonderful plants we’ve got now that are not being produced commercially will disappear. He said ‘botanists used to be sniffy about what they call ex situ conservation but have come to recognise this is increasingly important as large areas of wild populations are eradicated…so skilled propagators now and in the future will bear a huge responsibility in conservation’ – not just in enlarging the range of good garden plants available to designers and gardeners.
Chris Lane has done hundreds of thousands of grafts in the past five decades, the prerequisite of being able to create and maintain five national collections at his Witch Hazel nursery in Kent – Prunus (flowering cherries), Hamamelis, Wisteria, Parrotia and Amelanchier. After stressing the importance of maintaining your own razor sharp knives from his talk I learned more about the logic behind the differences between the two main times of grafting, winter and summer. In winter (usually whip grafts – a diagonal cut that binds the end cut of the rootstock with end cut of the scion, aligning and matching the circling edges of the cambium – there will be minimal activity in the scion and rootstock, so pot conditions need to be dry and very well-aired, and low temperatures can be taken advantage of to that end – what Chris Lane calls ‘cold callousing’. With summer grafts, usually side veneer grafts – where only one side of the root stock is cut, peeling back to a short flap at the base where sharply-tapered young scion material is inserted – both scion and rootstock are in the midst of vigorous growth, so moisture is less of a problem, but leaves need clipping by at least half of their area to minimise transpiration. So summer grafts will do well in a fogging unit, or under polythene; whereas winter grafts need to be kept dry in the pot while dormant.
Hot pipe grafting over the winter enables warmth to applied confined to just the union to encourage callousing while avoiding rot. Chris showed students how to make a home-made version of this technique. For more details of this and other grafting tips Chris mentioned, there’s a good comprehensive print introduction to grafting by Brian Humphrey, The Bench Grafter’s Handbook, by CRC Press (2019).
Peter Shotter said the key to sowing seeds of woody plants was the composition of the compost – it should be low in nutrient and very free draining. He mixes his own sterile medium with as much as 50% grit, and 50% peat or peat substitute. He also makes his own bespoke leaf moulds. Perlite is a commonly-used alternative to grit, but Peter has found it associated with rot in lilies and other bulb seeds, or other seeds that need a long time to germinate. A heated propagator isn’t necessary if more patience with a cold frame is an option – and artificial warmth can backfire for slow-to-germinate species by encouraging rot and disease, especially in the case of large woody plant seeds such as camellia, peony, oak and magnolia etc. Chris Lane pointed out the need now to be able to grow your own understocks from seed because of reduced commercial availability, and Maurice said he’s seen the same sorbus seeds take from 1-7 years to germinate, which is an evolutionary survival mechanism – if the same seed treated in the same way germinates at different times, if the first to grow dies, more are on their way. For this reason he sometimes uses petri dishes in the fridge to be able to lift seeds as they germinate without disturbing the others. Peter discussed growing-on, and his potting on compost mixture for the seed of difficult bulbs, such as lilies – a third leaf mould, a third grit, and a third commercial ericaceous compost.
Moving to cuttings, Maurice emphasised three things – the importance of juvenility, of the timing when the cutting is taken (a function of the state of the wood, not the calendar), and aftercare. Cuttings have to be taken early enough for the rooted plant to retain enough carbohydrate to survive their first dormant winter. With woody plants such as birches and maples it is essential to keep them dry and not to transplant them until they start growing the following spring. The idea is to keep them in a state of suspension or stasis. During this period if moisture is not controlled they will rot off, so the sterile compost needs good aerating grit to ensure a supply of oxygen at the root: dryness while dormant is key. He added that 80% of losses in planting out woody plants from pots in the garden is due to planting too deep – and it’s the same with cuttings when you first pot them on: allow the root shoulders to be just visible, perhaps adding some grit on top – and again, avoid nutrient-rich commercial composts, as well as potting on into too big a pot. Keeping constant temperature during sunny periods is important, so misting and shading will be necessary, but without restricting too much light for photosynthesis – it’s a balance.
Then Nadeesha Bandara from the University of Nottingham shared details of an ongoing research project on the effects of hormones on rooting.
Our next Study Day will be on Magnolias, on Saturday April 8th, with a few places remaining. The early Asiatics – campbelliis, sargentianas and dawsonianas will be mostly over, but there will be plenty of others to see – and the soulangeanas, loebneris etc should be at their peak. To apply, or for more information about our 250+ collection of Magnolia cultivars and species, contact Clare Foster at whitehousefarmarb@gmail.com.
by Clare Foster, Chair of the WHF Arboretum Foundation
White House Farm Garden and Arboretum 