Whether an arboretum has ten trees or thousands, many of the same management concepts hold true. Yet new collectors often progress through trial and error, as though no one else had gone through the same process. I began raising trees from seed in my garden in the late 1960s. As with many mad collectors (no matter what is being collected), I started the whole thing without much forethought—it just began one day. But I kept going and expanded the collection into neighboring woods and meadows. In 2003, I established Arboretum Wespelaar as an independent institution in a small village north of Brussels, Belgium.

My family had operated the Artois brewery for generations, so I was fortunate to have the means and the space to begin such a collection. (Artois is now part of Anheuser-Bush InBev, and we are today just long-term family shareholders.) I also had the opportunity of starting early, having good advice from my father, who loved trees, and I was curious and determined to know more. I remember my father kidding me because I did not immediately see the di_erence between a young beech (Fagus) and hornbeam (Carpinus) or, worse, a spruce (Picea) and Douglas fir (Pseudotsuga). I would not get caught again.

I returned to Belgium in 1969, after getting a graduate degree in business administration from Columbia University, with 150 seedlings in a big bag. In those days, you could carry about anything on a plane. Most of the seedlings had germinated in a wooden Borden milk box on the terrace of my apartment in New York. I had collected others during a trip to California just before my return. Fifty years later, the redwood (Sequoia sempervirens) from that trip are the tallest and girthiest trees on the estate and arboretum.

While working as a young brewery salesman in my late twenties, I visited dozens of gardens and arboreta around the world. I started buying plants at local nurseries and then European specialist nurseries. The collection spread from the garden around my house (twenty-five acres) into what was to become the arboretum (eventually fifty acres). For the first twenty-five years, I had the help of a single gardener. Now, five-full time gardeners manage the arboretum and the nearby garden at Herkenrode. Over the years, we learned by doing.

Preparing the Ground

When we began, two kinds of areas were used: meadows and woodlands. Both needed some kind of intervention. I learned this at my expense. Our first foray into the woodlands was done without concern for honey fungus (Armillaria mellea), which causes root rot. As trees were cut to open vistas, we left stumps, and the fungus soon got the best of many choice plants. We did not make this mistake when extending the collections into the old oak wood now in the arboretum: All shrubs and undergrowth were removed with a rotary cutter and uprooted. All deadwood was removed. We did not have additional honey fungus problems, but this exercise did little for the soil structure. It took years before moles arrived, finally suggesting improvements in soil structure and aeration (performed by millions of worms).

By now, the result is spectacular. You can easily see that the trees planted on mounds are at an advantage.

The old meadows required a different approach. Cattle had trampled and compacted the soils. As a result, it was necessary to plow these areas before planting. In one case, we even allowed a local farmer to grow corn for two seasons. Without soil preparation, the plants sulk, never sending roots beyond the planting hole and eventually drowning there, at least in a flat part of the world like Flanders. After plowing, we created mounds and planted the trees upon them, allowing the water to drain. Initially, it looked as if I was trying to create a minigolf course, but visitors were kind enough to say that the whole thing was not too ridiculous. By now, the result is spectacular. You can easily see that the trees planted on mounds are at an advantage, and the movement in the terrain provides some visual appeal.

Sourcing the Plants

I have long enjoyed plant propagation. Like many kids, I was fascinated with seeing seeds burst into growth. I was even scolded in school for growing wheat in the inkpot of my desk. The arboretum and the nearby gardens currently contain almost eleven thousand living accessions of woody plants. Of these, 50 percent were raised by us from seed, cuttings, and collected seedlings. Many originated from expeditions to the wild. My first trip was to Nepal in 1975, and successive annual trips (often with the International Dendrology Society) have targeted every possible temperate locale, from California to Hokkaido.

When seeds arrive throughout the autumn and winter, we place them straight into the refrigerator A numbered label is added to the individual bag and accompanies the seed through subsequent steps. The label is essential. (It is embarrassing to admit that you do not remember the origin of a beautiful plant.) The seed lots accumulate until March, when they are sown in pots. Of course, many seeds could be sown outside when they arrive (the cold, moist winter conditions are generally suitable for this), but mice will always find them and have a feast. Ungerminated seed pots should be allowed to go through another winter, because belated surprises can always be expected. Seedlings are repotted when big enough to withstand the shock (two or four true leaves above the cotyledon) but basically when we have the time. Seedlings can stay crowded in a pot for many months.

The meadows of Arboretum Wespelaar were regraded to encourage drainage and provide visual intrigue. Here, Artois Pond emerges.

As a precaution, always split a collection of rare seeds into several lots and treat each set differently. Some twenty years ago, I received a hundred seeds of a recently discovered species of magnolia (Magnolia decidua, then known as Manglietia decidua) from China. I kept fifty seeds and distributed the others in equal sets to five good propagators and magnolia enthusiasts. One morning, I had a look at my tray and realized that a fungus had killed all fifty seedlings. I was hoping that my five colleagues would have succeeded. One had died; one did not remember receiving the seeds. Of the others, Tom Hudson (of Tregrehan Garden in Cornwall) and Dick Figlar (of the Magnolia Society International) had managed to grow the seedlings and are responsible for all specimens of this species in cultivation, including the one at Arboretum Wespelaar.

Cuttings are collected between the end of May until mid-August. Every time we purchase a plant, we immediately take cuttings, given that cuttings from young plants often root more easily. For example, I took cuttings on a young Magnolia ‘Elizabeth’ three years in a row; out of five cuttings taken each time on the first, second, and fourth year, we succeeded at propagating five, two, and then none. The winter months are hard for the cuttings; even perfectly rooted cuttings will decay under the attack of fungi. Healthy white roots go brown, and the base of the unhardened cuttings does too; the cutting dries up. We have not been very good at keeping our cuttings growing, but these losses can be a relief. We still end up with too many plants: some five hundred cuttings and seedlings every year, which will have to be looked after for another three to ten years.

Small seedlings can be collected along roads and edges of woodlands. These will travel well if kept in relatively dry moss, packed in plastic bags or plastic water bottles. (Obviously, you must be respectful of rules and legal restrictions.) We also purchase plants, mainly in pots. The smaller, the better. I have had much disappointment with large plants. Small plants are, of course, cheaper and can be grown to a good size in one of our nurseries until ready for final planting in the arboretum.

A numbered label accompanies each plant from the moment it is planted at Arboretum Wespelaar. A separate inventory is used for seeds and cuttings, many of which are shared with other gardens.

The arboretum and the nearby gardens currently contain almost eleven thousand living accessions of woody plants. Of these, 50 percent were raised by us.

Numbered playing cards provide a quick visual cue when siting plants in the landscape.
Trees are a family affair. The author and his grandson participate in a conservation planting in Madagascar, managed by Madagaskari Voikaji.

Planting the Landscape

We have used three temporary nurseries around the garden and arboretum. Good woodland soil and shade from large trees provide the ideal growing conditions for our small plants and seedlings. It is ideal to observe your plants until they have suffered a bad winter. It gives you the time to decide where to plant them. They will transplant with a good lump of soil (unlike the miserable peat ball with circling roots that you find at the average garden center). We have seldom failed in transplanting a young tree or shrub raised in these woodland nurseries. On the other hand, we have lost many plants in the first few years in these sites. But better there than in the grounds after an expensive effort at planting!

We rarely place a plant directly into its final location. Most spend as long as five years in the nurseries. Few people like the idea; it seems like double work. But I consider not taking this intermediate step to be a grave mistake. Many recently acquired plants will die, and given this reality, I like them to die in the nursery. I have often thought it would have been much better to collect art of any kind and, like a dendrologist, throw two-thirds of the collection away and enjoy the remaining successes. At least, works of art generally gain value over time, whereas aging trees become an expensive problem.

When it comes time for siting the plants, we use a homegrown method involving playing cards. I do not know who came up with the idea, but we have used it for fifty years. We staple two sets of plasticized playing cards (reds and blues) onto plants in the nursery. The identity of the plant and its card is written up on a special form. A corresponding set of playing cards is placed on 104 bamboo sticks, which are reused for several years. We then take a walk through the grounds, staking locations for each of the plants. We aim to get rid of all the bamboo stakes while trying to remain intelligent and effective and still get home in time for dinner. It takes us, in general, up to five hours to place two sets of cards. Of course, we could write the plant’s name on the stick, but it is much easier to spot the cards from a distance.

We often situate the plants in taxonomic groupings. So, when we’re placing the bamboo stakes, we first attempt to place a viburnum, for instance, within the viburnum section. If there is no space left, we find room elsewhere. Obviously, you must know what condition the plant enjoys, how big it will become, and so forth. One becomes better at this with time, but the proper planting distance is always a terrible illusion. Someone once pointed out that when there was a gap between two trees and you add a young tree between them, you end up with two additional gaps. I must admit that I have found myself planting two new trees in such spaces. Discipline is essential.

Large trees should be planted at least fifty feet apart, yet we have many at half that distance. We will remove one of them in due course. Trees should not be planted near the edge of a woodland, or they will grow slanted. Likewise, groups of three—an arrangement beloved by landscape architects—should be avoided as none of the three will end up as a balanced specimen. (This is not a problem for shrubs and small trees.) These conservative approaches will make your arboretum look rather dull for many years, so you have to suffer the irony of friends and guests. Most do not understand what is going on. I like to think that I do not need to see my trees in old age; I know what they will look like. Other plant collectors are more impatient.

Cataloguing and Labeling

When beginning a catalogue for a plant collection, it is a good idea to think carefully over what software to use and then leverage its capabilities to the greatest extent. These days, you may want to consider using relational database software, but a single spreadsheet can be equally effective. Take some time to sit down and think over the structure. Some curators will suggest that at least twenty fields are necessary, but I recommend a minimum of six: accession number, name, landscape location, source, date planted, and condition. Most people will also want a field for any supplementary information. The printed catalogue at Arboretum Wespelaar presently uses nine fields, out of some twenty in our Access database.

I do not need to see my trees in old age; I know what they will look like. Other plant collectors are more impatient.

Accession numbers are a difficult concept for beginning dendrologists. I do not know why. An accession number is no more than a simple and unique sequential number given to each plant that comes into the collection. You can give the same number to several plants provided they are from the same source, same age, and in the same location. But otherwise, give them unique sequential numbers, or you will soon regret it. Further, there is no reason to include the date within the accession number.

Of course, everyone will want to know the age of a plant. Most curators will include the date of planting on the label, which is a good idea. But we made the mistake of including the date as part of the accession number: Our first plant bears accession number 66001, which means it was the first accession in 1966. This system was useful until the year 1999. With the millennium, we got in trouble, as the first plant of the new millennium was 00001. And it shows up first in any numbered list. We had to add two digits for the sequential listing.

When it comes to the name, it is best to refer to a single accepted list, thereby avoiding spelling errors and nomenclatural issues. The Royal Horticultural Society’s Plant Finder is probably the only document to be sufficiently comprehensive and regularly updated. Synonyms and taxonomic changes of names are clearly indicated in annual updates. It even includes cultivated varieties. Still, if you specialize in a certain taxon (like magnolias), you may want to use a recent monograph on that group.

Once this record-keeping is complete, then comes labeling—the curator’s nightmare. I have always had an average memory and have not relied on it to know anything. This is probably why I have been so determined to make sure that our plants are properly labeled. Our labels include the name and accession number and are made on a thick ribbon of white PET plastic cut to length and engraved with an automatic engraving machine (a Gravograph). Labels are inexpensive: we estimate that it cost us one euro to make a label with a reasonably long name.

Labeling problems, however, are never far away. I learned plants while visiting arboreta and botanical gardens all over the world. As I explored these collections on my own, I would go to a plant, take a picture, and then search for a name. I would be exasperated if I did not find a label and sometimes astonished at the number of wrongly labeled plants I encountered. Even so, at Arboretum Wespelaar, one of our members on a study day was surprised to find a label stating Abies rufinerve on a new maple accession. (Abies, of course, is the genus of fir trees—the tag should have read Acer rufinerve.) So problems occur even in the best houses.

Change in the Collection

Arboretum Wespelaar, like any plant collection, is in a constant state of evolution. Not only do plants grow and die, but interests and goals shift as well, changing the landscape over time. Although I fell in love with conifers initially (my first plant was a white fir, Abies concolor, accessioned in 1966), I soon switched to deciduous trees, particularly maples. Around 1969, I went to a local nursery that had a seemingly good catalogue and proudly ordered one of each maple on their list. I soon found that my collection of some twenty maples was far from what the world had to offer. Would I have given up if I had realized that there were more than 120 species, along with hundreds of hybrids and cultivars? My subsequent loves were rhododendrons, magnolias, and stewartias, as proven by the number of those plants in the collection. Today, the team at the arboretum aims to acquire all of the main species in all important genera and in particular plants from wild origin.

It is clear that gardens, if well-curated, can contribute to the maintenance of biodiversity. The Franklin tree (Franklinia alatamaha) is a memorable example: although it went extinct in the wild in the early 1800s, the species survived in Bartram’s Garden in Philadelphia. While conservation is an additional objective of Arboretum Wespelaar, our primary purpose is to ensure that people can study and learn to love plants. We have no shop, no cafeteria, and nothing for children. Dogs and joggers are not welcome. The result is that our visitors actually look at labels and take notes. I have always intended that the garden and then the arboretum should be open to the public, recognizing that I have benefited from the generosity of botanists, plant collectors, and gardeners who have opened their collections to me. In turn, it’s my pleasure to welcome others and inspire them to see and know plants.

Once a year, usually in November, we have a difficult day when we deaccession trees, removing them from the collection. This year, we will likely deaccession around fifty plants. These are painful choices but very necessary. We have planted too much with the knowledge that we would have failures and that others wouldn’t last. I am adamant that as many as possible of our trees should have lower branches on half of the crown. In due course, aesthetic considerations will always rule above other imperatives. Within a changing collection, it is always nice to have too many good things.

Philippe de Spoelberch is the past president of the International Dendrology Society, the founder of Arboretum Wespelaar, and the president of Fondation Franklinia, an organization devoted to the preservation of threatened tree species. For additional information on collections at Arboretum Wespelaar, visit the www.arboretumwespelaar.be.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

It takes resources to gather and nurture these new voices, and we depend on the support of our member-subscribers to make it possible. But membership means more: by becoming a member of the Arnold Arboretum, you help to keep our collection vibrant and our research and educational mission active. Through the pages of Arnoldia, you can take part in the life of this free-to-all landscape whether you live next door or an ocean away.

For more tree-entangled art, science, and writing, subscribe to Arnoldia by becoming a member of the Arnold Arboretum.