Propagation by Layering | The Scottish Rhododendron Society

Layering Rhododendrons and Azaleas

Some thoughts and suggestions in regard to propagating old and/or difficult to root plants

John M. Hammond, Past-President, Scottish Rhododendron Society

Introduction and historical perspective:

In many walks of life there is a tendency for new materials, techniques and technology to displace the traditional materials and the time- tested ways of doing things. Gardening and horticulture are no exception to this trend and the “”new-fangled” ways of approach often got “short-shift” from the Head Gardeners of yesteryear. I recall the late John Basford, Head Gardener for many years at Brodick Castle. a friend and mentor with a most practical outlook on all manner of horticultural subjects, instructing me on a number of occasions to “do it the Basford way” And, viewed in retrospect, his techniques worked.

The Victorian era marked the zenith for many large estate gardens and nurseries where, other than raising plants from seed, layering was the traditional method of propagation for rhododendrons, azaleas and many other woody plants. Labour was cheap, but perhaps more importantly, it was readily available which meant that labour-intensive methods were not a problem. Commercial layering took a great deal of time, space and skill To propagate numerous small plants in the same timeframe a large stock plant needed to be “grown” on its side and this approach enabled most, if not all, the branches to be spread out like a cartwheel, pegged down and a raised as individual plants. Large hardy hybrids or large species laid on their side took up considerable space for the two or three years they needed to remain in this position whilst the layers grew roots large enough to form a viable plant.

The onset of hostilities in 1914 brought with it the winds of change and the years between WWI and WWIl saw a major reduction in the availability of trained gardening staff. Faster, less labour-intensive methods were needed and the early to mid-1920’s saw the beginning of “cutting-edge” techniques to propagate by the use of cuttings. To a large extent this was “driven” by the developing market for florist’s azaleas, particularly in the U.S.A. where propagation via cuttings and use of under-soil heating had been successfully introduced by a few nurseries in the early 1920’s. This in turn gave encouragement to specialist rhododendron growers to develop similar techniques on a commercial basis, although it had been known as early as 1902 that cultivars of Rhododendron caucasicum, such as R. Cunningham’s White’, could be propagated from cuttings. By the end of WWII the U.S.A. production of florist azaleas had reached almost 3.5 million plants per annum from a total of 1928 growers which, at an average of 1800 plants per nursery, suggests there was a shortage of capital investment due to the 1930’s depression and the war years. These figures take no account of the continued importation into the U.S.A. of vast quantities of florist azaleas from Europe; mainly from Belgium, Holland and England; but these tended to be grafts using Ghent azalea seedlings as rootstocks The cessation of hostilities in 1945 brought with it the eradication of cheap labour and the non-availability of able-bodied men for occupations such as gardening. Many nurseries were still family-owned concerns and these faced the option of introducing new methods of propagation or gradually going out of business. Also, by 1990 florist azalea production in the U.S.A had more than tripled whilst the number of growers had halved.

Many of the old labour-intensive horticultural techniques gradually fell by the wayside and are no longer in vogue; including propagation by layering. Nevertheless, even today, such techniques have their time and place. At the outset I suggest that you pay no attention to those commentators who, in recent years, have suggested that using layering techniques for propagation is a waste of time, or the method does not work. It is true that such techniques have little, if any, relevance in the specialised field of commercial propagation that serves a vast marketplace. However, layering still remains a viable option for the production of small quantities of difficult to root species and hybrids. Equally, commercial propagation has limited relevance to the average rhododendron enthusiast who is seeking to propagate small quantities of plants for a wide variety of purposes.

So where do we start? Perhaps the best place to begin is to discuss the timing of making layers. We can then outline the approach to simple layering, then build on this foundation and discuss some aspects relating to air layering.

Getting the Layering Time Right:

Plants can be layered at any time, except during freezing weather or when frosts are forecast. Many of the older nurseries carried out their layering of rhododendrons and azaleas in late summer or early autumn, however, this probably had more to do with the fact that labour was more readily available at this time of year. My discussions with head gardeners, together with my own experimentation, suggest that the best time to put down layers is in the spring. Not only has the plant been dormant all winter and is ready to put out new growth, there is also the advantage of the majority of the growing season ahead.

Layering techniques:

Layering is a relatively simple process and reference is made to the methodology in many gardening manuals, but often the explanations are less than adequate . So these notes should be helpful as to date I have found that the method is successful on a wide range of plant material. Very few tools are needed, a garden fork, a trowel, a pair of secateurs, a sharp knife that is clean and a good sized mallet. Very few materials are required, a few pegs made out of small branches [birch or hazel are said to be the best, but the key requirement is that the wood should not rot whilst buried in the ground and avoid using metal pegs that rust], a stake around 3 feet [90 cm] long, a label, a few bamboo canes around 2 feet [60 cm] long, a ball of twine or a supply of longish thin cable ties, half a bucket of sharp sand, half a bucket of chopped or composted bark and a scattering of bonemeal. Some authorities recommend the use of hormone rooting powder but avoid its use as it is difficult to gauge whether the correct amount has been applied. Too little powder has no effect at all, too much powder has an adverse effect on rooting, whilst an overdose can lead to rotting and dieback. I manage quite well without it. What you need plenty of is a commodity that is not for sale . . . . . . patience!

What we are looking for is to select a branch that can be easily pulled down and bent in to a “U” shape and, preferably, out of the ravages of full sun. Pull the branch down, carefully bend it and loosely push in the stake to mark the location where the branch is to be inserted in to the soil. Using the garden fork prepare the ground in the vicinity of the stake. Remove all weeds and rubbish, taking care not to damage the roots of the mother plant, and work the bark and a scattering of bonemeal into the soil as deep as is practicable to improve the drainage. The aim is to avoid water-logging but to retain sufficient moisture to keep the soil damp. Rooting will not take place if the soil is dry for long periods. If the soil is very heavy you may need to consider adding more bark and some grit to make it friable. Lightly firm the ground, levelling it off as you work.

Dig out a slight indentation with the trowel, about 2 or 3 inches [5 to S cm] deep and about 12 inches [30 cm] long, bend down the main branch to be layered and fan-out the smaller branches so the ends can be bent later to form the individual layers. Wind protection is a sensible precaution in a changing climate that in recent years has resulted in significantly more windy and stormy days. Drive the stake into place with the mallet then firmly secure the main branch in position with the ties, or twine, so it will not move around in the wind. Remove any leaves in the vicinity of where the smaller branches will be buried in the soil. Each of the smaller branches needs to be wounded on the underside by removing a 2 inch [5 cm] length of bark prior to being bent in to a “‘U and secured in place with a peg [see below for some comments on whether wounding is necessary and use your own judgement]. The end of each branch is then secured with a tie, in as vertical position as is practicable, to a cane in an upright position that has been driven in with the mallet. The vertical positioning results in a better- shaped plant when the layer is eventually severed from its parent. When all layers have been placed in position and secured, then fill most of the indentation with sand, covering the branches at the base of the “U” to encourage rooting. Level off with the surplus soil. Carefully firm the soil and water in thoroughly, repeating the process to ensure the area is level. Finally, label the layered branch with the plant name and date.

Rhododendrons are usually easy to layer when the young branches are thin, pliant and the tendency of the plant’s lower branches to grow horizontal, or droop, means that it is not difficult to peg them down. Nurseries tend to use the supple young branches as these are more easily dealt with and do not need cutting or wounding to induce root growth. Sufficient constriction of the branch occurs at the sharp “U” bend to encourage rooting. However, this often leads to straggly or leggy plants that require additional time planted out in a nursery bed, pruning after the first year to encourage bushy growth, then left for another year or so to mature in to a saleable sized plant.

I prefer to use branches that are two or three years old that start out as a prospective layer that is 15 to 18 inches [38 to 46 cm] tall if suitably long branches can be found. These need to be wounded and may take another year to produce an adequate root system, but in the long run the end result is a much more robust plant that needs little if any pruning and attention once it is severed from the parent.

There are many variations to this methodology; some produce good results, others fail. Many enthusiasts tend to cut corners, the most common practice being to roughly clear away the weeds with the side of a boot, scrape a thin layer of soil over the main branch and hold it down with a house brick or a heavy piece of stone. The end of the branch[es] is then tied in an upright position to a stake or cane and left to nature to do its work. Depending on the rootability of the species or cultivar the enthusiast may get lucky, but often this approach will fail. One of the main problems with this approach is that the house brick, or heavy stone, can get very cold in the autumn and winter and this retards any growth. They become cold storage devices and retain a significant measure of the cold night temperatures and maintain these well into the daytime in the spring and summer growth seasons, particularly when the brick or stone is damp. Warmth and stable temperatures are two essential components that are required to encourage rooting, so house-bricks and heavy stones are not helpful devices to use.

Next comes the difficult bit ! Leave the layers undisturbed for at least two full growing seasons. There are no exceptions to this rule. Some old head gardeners recommended leaving layers in-situ for three seasons. This is where many enthusiasts tend to fail as their curiosity wins out, they have a look to see whether any progress is being made and they break-off the fragile root system before it has had time to mature.

A good indicator to look for in many instances, particularly with deciduous azaleas, is once the layer has rooted it begins to send out new branches from rooted area. Once these new branches are evident I sever the new plant from the parent in the early-autumn and leave the plant in-situ to establish itself for another growing season. Alternatively, if it becomes necessary to move the new plant at this stage I get a very wide container, position the roots of the new plant against one side of the container, making sure the growth is vertical, then get a piece of cane and insert this in the soil so it runs diagonally across the container and secure the main branch of the new plant to it with some twine; much in the same way as you would secure a newly planted young tree with a cross-stake. I then put the container in the shaded area of a cool greenhouse for a year. After this additional year’s growing season is over I then lift the plant, or take the container out of the greenhouse, and find a home for the plant in a dappled-shade area of the garden, firmly securing the main branch in position with a diagonal cross stake to prevent wind damage to the relatively young rootball. By this stage, and bearing in mind that I tend to root branches that are two or three years old, the new plant is now four years old and the main branch is usually 30 to 36 inches [75 to 90 cm] tall.

Why use Layering?:

Layering replicates what readily happens in nature with species such as R ponticum or cultivars such as R. ‘Beauty of Littleworth’. The end products have cost you virtually nothing other than a minor investment of your time and a major investment of your patience. In practice layering is a most useful tool. Many old hybrids are notoriously difficult to root from cuttings, as are Ghent azaleas, and layering presents an easy alternative if there are branches that can be brought down to the ground. One word of warning: difficult to root plants are often sought after by knowledgeable individuals who have little, if any, respect for other people’s property. Layers not only grow roots, some have a tendency to grow legs as well, so make sure your layers don’t get the opportunity to walk.

Over the years many of us have had the unfortunate experience of having large plants blown over on to their side by the wind. Sometimes the root-ball is lifted out of the ground, other times the roots are torn out. Either way, this damage presents a problem, particularly if there appears to be little hope of the plant being viable even if a means could be found to return it to the upright position. Providing that the fallen plant does not present a major hazard and that at least some of the roots are still in the ground, or the root-ball can be back-filled with soil, then it is worth suggesting “Why loose an old friend without attempting to propagate it?” In the introduction to this article I explained that many years ago it was common practice for a nursery to “grow” a plant on its side and put down layers. A fallen plant provides the same opportunity.