Grafting and reproducing new cultivars

Responsibility in genetic replication is a necessity

In recent years, we have witnessed a large distribution of new cultivars of different tree species, some of which were discovered in Israel. Some of these cultivars are indeed “outstanding” and prove to us the importance of their reproduction. Yet others, in some cases, are merely riding the coattails of the “outstanding” cultivars’ justified fame but don’t really have much to them. Certain properties are attributed to them, though sometimes the product does not really justify their reproduction, and in other cases the features attributed to the cultivar are not really “new” but are in fact a refinement of properties that characterize the original species and are often influenced by environmental factors, such as weather and/or location and/or the tree’s age. There are cases where “new cultivars” have been put on the market without sufficient trials or knowledge. The intention, of course, is to make a tree that is included in the specifications of public projects. It is all of our responsibility to raise questions and doubts and not to be tempted by the release of each new cultivar. This phenomenon exists, and it is important to put things in a sort of order. Below are a few positive examples of grafting ornamental trees.

It is well known that columnar trees with acute branch angles are prone to breaks. With the Populus alba, for instance, breaks are only a matter of time unless the tree is carefully and properly pruned and balanced. As a result it is not considered a successful street tree, and most of its use is limited to parks, which usually have greater control and ongoing maintenance of trees, and alongside roads without pedestrian traffic. There are many columnar cultivars that, unlike P. alba, are suitable for use on urban streets. These mostly have a slow to medium growth rate, which contributes to their branches’ hardening and the formation of stronger connective tissues. Their final size potential is smaller – 8–12 meters compared to 20–25 – and they will have strong apical dominance throughout their lifespans. Some examples include the grafted columnar Quercus brutia, the columnar Terminalia ‘Shalhevet’ and the grafted columnar Pyrus calleryana ‘Aristocrat.’ Incidentally, these trees have drastically different canopies than those of their original species. The Pyrus grafted from the ‘Aristocrat’ cultivar is endowed with a host of other unique characteristics: it presents an impressive, uniform and orchestrated white-pink flowering and a late fall display that is uniform in dark fire hues. This cultivar is also free of thorns and does not bear fruit.

We also perform genetic reproduction when we want to control and schedule the flowering, especially when it comes to boulevards or groups of similar trees planted near each other. It’s important to put things in order here. The intensity of a tree’s flowering is influenced by multiple factors: its genetics, the weather and the tree’s age.

Not all trees have large and visible genetic variations that justify genetic reproduction. Good examples of this are Peltophorum, Jacaranda, Bauhinia and Delonix species. The intensity of flowering in these species depends on their reaching maturity and the microclimates of the regions where they grow. Of course, by grafting these trees, we can obtain an early flowering in young trees, but if we monitor them long-term, we see that adult trees from these genera flower at reduced intensity and uniformity in their maturity. And this, as we have said, is despite their not having been grafted; waiting for some trees to reach adulthood is all that is needed to obtain the advantages sought by grafting. However, we will perform genetic reproduction when there is a clear and visible difference in the color, size and nature of the flowering of the seed trees. A good example of this Tabebuia impetiginosa ‘Santa Barbara’: when propagated by seeds, there are variations in the hue and size of the flower clusters. Furthermore, some of the trees flower accompanied by little foliage, while others flower completely devoid of leaves. In this case, an “outstanding” cultivar, T. impetiginosa ‘Santa Barbara,’ which was brought to Israel by Sima Kagan, was chosen. This cultivar displays the maximum desired properties together: intense flowers in large clusters, unaccompanied by foliage.

Another excellent example for a non-grafted flowering tree is Bauhinia ‘Pnina.’ This Bauhinia cultivar is very reminiscent of Bauhinia variegata and purpurea, which have already been in Israel for many years. They share many similar, prominent parameters, such as the shape of the foliage, the shape of the flowering and even the tree structure. It is evident that there is great genetic closeness between the cultivars. Nevertheless, despite the great similarity between them, the ‘Pnina’ displays a few unique features that significantly differentiate it from its siblings. Bauhinia ‘Pnina’ is sterile – i.e., it does not bear fruit. This means that it never has the unflattering display of fruit that so characterizes its sibling, B. variegata. In addition, ‘Pnina’ begins flowering in November and has consistent, intense and impressive flowers in dark pink all the way until April! This is a flowering period of more than four months (the entire winter).

Fraxinus trees are also a clear example of why genetic reproduction is important. The large variations in the tree’s sex, the canopy shape, the size of the foliage, the appearance of flowers and pods and the fall colors. All these justify choosing “outstanding” cultivars. One such example is Fraxinus uhdei ‘Full Moon,’ which has a perfect, spherical canopy and large, glossy leaves. It is male and thus bears no fruit, and it is characterized by bright yellow fall colors. Another example of an outstanding cultivar in this family is Fraxinus syriaca ‘Inbali,’ which is characterized by spectacular fall colors in hues of red and has a round canopy and minimal fruit.

We would also perform genetic reproduction when we want to control the sex of the tree. The male Morus alba ‘Omry,’ which is propagated vegetatively is suitable for the urban sphere because it does not bear fruit. Thus, it does not make a mess, which accounts for its desirability. Another case is the grafting of female Ceratonia species, where we would generally prefer the appearance of the fruit over the male fruit, which is characterized by an unpleasant smell.