How to plant a tree: long version

Almost twelve years ago, as a student, I spent one year at Longwood Gardens in Pennsylvania, half way between New York City and Washington D.C. Longwood, which is one of the largest and best endowed gardens in America, has an enormous staff of gardeners, technicians and teachers, and over a million visitors annually.

One of the stipulations of my tenure there was that I submit a relevant piece of written work. I submitted it when the time came, and thought that that would be the end of it. But, unknown to me, my report did the rounds of the hierarchy of department heads, until it landed on the desk of the Director of the Gardens, Fred Roberts. He had taken umbrage at many of my comments, and one of them was what I had said about tree planting depth.

My interest in the rigmarole of successful tree planting is something I can thank Longwood for. In fact, much of the research into the science of tree development is carried out in the United States, influenced by the work of Dr Alex Shigo, the godfather of modern arboriculture. My diary records the process of planting a tree at Longwood, as well as the opening of my eyes to the craft of gardening:

'The topic which was very much flavour of the month was planting depth. Before digging any tree, we would scrape back the soil to the point where the trunk flared out, below which roots began. Even this job had its difficult side, as mulch would occasionally accumulate up around the stem, causing roots to flare out above the official zone of the ‘trunk flare’ or ‘root flare’. This was particularly evident in a Taxodium distichum, to be planted near the Brick Walk. This tree, with a three inch girth stem, had what appeared to be a healthy flare in the trunk just at soil level. But when it came to planting the tree, it appeared that the trunk continued uninterrupted below this point, and that this was perhaps a root which had developed because of tilling between rows in the nursery. The discovery of this led to endless discussion, with the heads of various sections coming in to lend their voice. In the end, it was agreed that the tree was suitable, and that this was an adventitious root that was by this stage worthy of being considered the root flare in itself. Thus, the point where this root emerged from the trunk would be treated as ground level.'

Later that week we planted two Yulan Magnolias (Magnolia denudata). Again, we were careful to ascertain where the trunk flare began, and to keep this area slightly above ground level. A tree is far more likely to die from lack of oxygen than from lack of water, I learned, and the greatest threat to its survival is being planted too low, where the root flare is buried. Science aside, I can’t help thinking how visually unattractive it is to have a tree or shrub growing out of a little mound of soil and mulch, and I wonder if trees planted in previous generations, which are now so healthy and huge, found this treatment necessary. Working on these projects, and on many others, opened my eyes to the way in which trees around Longwood, and elsewhere, were planted, and I could not help myself wondering if this obsession - for that is what it seemed to amount to - smacked of faddism, and that in twenty years time research will prove something quite else. Reading through Creating the Urban Forest: The Bare Root Method, a leaflet published by the Urban Horticulture Institute at Cornell University, I notice that their only comments on the subject are as follows:

'Plant the tree so that the beginning of the root flare is visible at soil level. It is critical not to plant the tree too deep. Lay your shovel a cross the hole to see where the shovel meets the root flare and adjust the planting depth accordingly. If you anticipate settling of the soil, plant a little high. It is better to plant too high than too deep.'

Fred Roberts gave me a photocopy from a book published in America in 1842, A Treatise on Landscape Gardening, by Andrew Jackson Downing. This book, which was hugely influential in its day, put me in my place:

'Another very erroneous practice, of frequent occurrence with planters of little experience in the United States, consists of planting the tree too deep. This is not only visibly contrary to nature, and in violation therefore of correct taste, but it is destructive to the health of the tree, by placing the mass of young roots below the genial influence of the atmosphere. Treated in this manner, trees will frequently struggle against the adverse situation for years, without ever attaining any considerable degree of luxuriance. If we observe a tree growing in a natural state after it has attained some size, we must at once remark that the base of the trunk, or that part nearest the ground, is much larger than the trunk a few inches above, and that in consequence of the development of roots just below this point, the tree appears to stand on a base a little elevated above the level of the ground around it. This gives it an appearance of strength and dignity, and connects it, by a natural transition, with the surface around it. Now a tree, however large, which has been planted too deep presents no appearance of this kind, but rises out of the level ground without any base, in a manner precisely similar to a post. In order to prevent this appearance it is advisable, in planting, to set out the trees on a hillock, a few inches raised above the surface, in order that they may, when the ground settles about them, have a natural appearance to the eye, and that the roots may also be placed in the most favourable condition.'

Few pieces of writing about plants have opened my eyes the way this brief passage did, and it’s as current now as it was then. Walking along O’Connell Street a couple of years ago, I witnessed the root-ball of a lime tree being covered by layer upon layer of soil, sand and gravel, so that the root flare was a foot or eighteen inches below ground level – and this was happening all along the street.

Conversely, a trip out to a place like Powerscourt is a real reminder that somebody, a long time ago, knew their craft; yet we benefit from this now, decades or centuries later. The grace with which a tree hits the ground can be what makes it, and the more so the older the tree becomes.

Recent research into tree planting depth adds scientific and statistical fact to anecdotal evidence. A University of Rhode Island survey found that over 75% of nursery-grown trees had 10 to 30 cm of growing medium above the root flare, and in a separate study at Clemson University in North Carolina, over 50% of the trees [Prunus x yedoensis] planted with the flare at more than 15 cm below soil level had died within two years of transplanting. It should be pointed out that some species are more sensitive to root-flare depth than others.

Girdling and circling roots are also exacerbated by too-deep planting, the effects of which are often only evident many years after planting.

The current trend for planting larger and more mature specimens does not make it easier to plant a tree well. Successive years in the nursery mean that there is constant re-potting or tilling between rows, leading to ever-greater depths of soil build-up on the stem. Furthermore, the bulk of the canopy of a tall, well-fed tree is such that a landscaper’s inclination is to bury the root-ball to increase stability.