PRUNING INTENSITY AND TIMING FOR Platanus, Aesculus AND Tilia MATURE TREES IN THE MARCHE REGION Minelli A.1), Pasini I.1), Polverigiani S.2), Murri G.3), Neri D.2) 1) Dipartimento Colture Arboree – Università di Bologna, Bologna – [email protected] 2) SAPROV – Università Politecnica delle Marche, Ancona – [email protected] 3) Az.Agr. “P.Rosati” - Università Politecnica delle Marche, Ancona – [email protected] Keywords: ornamental trees pruning, when to prune, callus formation, crown thinning, crown reduction Abstract Ornamental tree management should never been confused with simple maintenance, which is only part of the whole task. Correct management involves making general choices but also the definition of operative strategies and their concrete application. This research project, carried out in center of, Italy, expected to define the best time and methods for pruning ornamental trees in urban areas, to satisfy the needs of the property, using trees with the right physiological behaviour and to get the right canopy volume desired, while paying attention to the biological needs of the tree. Basic techniques were used for: the thinning of main branches, the shortening of branches with heading–back cuts leaving good lateral branches, and the removal of weak lower branches and dead branches. The research project used two different methods: “severe” pruning and “moderate” pruning, according to the size of the heading–back cuts. The pruning was performed in three different seasons: i) the end of winter (before bud-break), ii) late summer and iii) the end of autumn. A simple experimental layout was applied (season x pruning methods) repeated 3 times per treatment. The research project was carried out in two coastal resorts in the Marche region; in Senigallia (on Platanus), and in Fano (Aesculus hippocastanum and Tilia spp.). The removed branches and shoots of each plant were weighed and after 2 and 4 years from the intervention, the plants were checked for callus formation, water sprout growth (number and length) and cuts cicatrisation. Results confirmed that moderate pruning with the utilisation of heading–back cuts limited on weak branches with deviation on lateral branches, and thinning of supernumerary branches, provoked balanced vegetative growth and better cicatrisation than more severe pruning. Pruning carried out at the end of the summer proved to have a positive effect on containing growth, indispensable where the size of mature plants is not compatible with narrow planting distances that offer limited growing spaces. Introduction The architecture of a mature plant is the result of dynamic exploration of epigeous space. The plant must occupy this space by challenging other plants at first, then the plant must manage the space obtained by efficiently developing its volume there as much as possible. The canopy structure is a consequence of the branches’ way of growing that continuously evolves during its life cycle (Lauri, 1991). Natural canopy structure evolves in stages. Each stage has a different shape and organization but also a different objective (Oldeman, 1990). The first stage is that of expansion, characterized by competitive space occupation and by access to light. The second stage typically sees branch distancing through self thinning (self pruning) of the branches, the death of branch tips and increasing basiplasty (Zucconi, 2003). During this second stage, the canopy reaches its maximum size and its definitive shape and it can be considered mature. Finally plant ageing ends in the senescing stage. This stage is not reversible and drives the plant to death. Pruning plays an essential role in plant architecture evolution, allowing the moulding and directing of plants by several and various interventions. In most ornamental plants, that have grown naturally, pruning interventions should be very rare at a mature age. On the other hand, incorrect intervention, as with unnecessary intervention, is a major cause of deterioration in plants and their poor aesthetic value, especially in an urban environment. Frequent pruning is necessary in obliging plants to grow in unnatural shapes (hedges or rows) or in enclosed, restricted spaces (lines along urban roads), in order to keep volumes within the desired spatial model. The aim of plant pruning, especially for urban trees, is not just to recover damage or to assist disease prevention. Pruning can regulate mature plant development, prevents the natural self pruning, can minimize the tree’s risk of instability by cuts focused on growth direction and control, helps allow vehicle and pedestrian circulation and improve illumination, and can help avoid damage to roads and structures. These objectives are reached by reducing the cut diameter, and by respecting the branches’ natural architecture in its natural ageing evolution as much as possible (Shigo, 1989). The size of a branch section, the distance and the hierarchy of branch inclination between branches are key factors in building a harmonic skeleton. Small branch removal is generally preferable to a small number of big cuts, because it enables rapid injury recovery, limits wood alteration risks, preserves a more natural shape and induces a smaller number of water sprouts. To respect apical control (Wilson, 2000) the apex of the branch must be preserved as much as it is possible, or it can be substituted by a lateral lower one able to maintain the natural hierarchy in the branch even after the head back. Urban tree pruning starts with the elimination of branches that could be dangerous for traffic circulation. To maintain and/or to improve the plant skeleton along with plant ageing, pruning should prevent the weakening and the natural failure of the supernumerary branches in each phase. The pruning of the rest of the canopy is done working from the top to the bottom. Pruning intervention can also be helpful in canopy recovery for previously topped trees. Intervention is repeated over several years in order to restore a shape as natural as possible and this consists of the thinning of the limbs. Remaining branches are trimmed by heading back the limbs, removing any parts protruding from the top thus creating a lower terminal. The same process is applied to lateral ramification. Over the following year the thinning is repeated until the desired density has been reached, by taking care to eliminate at any time, up to one third of the branches in order to avoid water sprout growth. The experiment was aimed at defining the best techniques and periods to prune rows of trees in urban areas, in order to balance urban spatial needs and the physiologic characteristics of the tree. The period of pruning along the year can be crucial to get positive results because it not only deals with plant pathogens interaction, but it also strongly interferes with the natural annual carbohydrate allocation and partitioning, modifying root to shoot interaction. Materials and Methods The research project was carried out in two coastal resorts in the Marche, on Platanus s.pp. in Senigallia, and Aesculus hippocastanum and Tilia spp. in Fano. The research project used two different methods: “severe” pruning and “moderate” pruning, in two different years. There were three seasons of pruning: the end of winter (before bud-break), late summer and the end of autumn. Choosing the right season depends on the aim of the pruning. A simple factorial experiment layout was applied (period x intensity) with three replications at least. Basic techniques were used for: the thinning of skeleton branches, the shortening of branches with heading–back cuts and the removal of weak lower branches and dead branches. The difference between the two methods lies in the intensity of the intervention, therefore on the amount of branches removed and, above all, in the different pruning methods used. In particular, the trees that underwent “severe” pruning, had their large branches and shoots pruned (diameter > 7 cm) and were heavily thinned and shortened. Whereas the trees that underwent “moderate” pruning, had shoots and small branches removed (diameter < 7 cm), using heading-back cuts to preserve the tree architecture. However, in both of the methods all the branches which were competitive, in a bad position or in too large a quantity (because of earlier incorrect pruning) were removed. The light penetration inside the canopy, was encouraged through thinning so that the branches were covered with leaves. In addition to this the removed branches and shoots of each plant were weighed. Two to four years after the intervention, a survey was carried out to check the results, taking into consideration the shoots growth (number and length) and cicatrisation. Results and Discussion This experiment shows that plants react differently, but in a predictable way according to the intervention. Considering intervention intensity, it is evident that severe pruning causes a larger number, and length of shoots (figures 1, 2) and a higher average annual shoot growth than moderate pruning independently from the species. Whereas light pruning is characterized by smaller cuts and promotes better callus formation reducing flaws such as caries or missing callus formation. Severe pruning is faster and costs less, but cicatrisation of cuts required longer increasing the risk of pathogens. In detail the lower percentage of callus formation defects corresponds to diameter cuts smaller than 20 cm (figure 5). Severe pruning caused a higher number of shoots to grow, and more frequently these shoots were weakly inserted and promote assurgent and co- dominant bifurcations. In terms of the intervention period, summer pruning reduced the strength of the reaction and shows the lowest shoot number and total length (figures 3, 4). All the results were set against the TSA (Trunk Section Area measured at 1.5 m) in order to neutralize variability of vigour among the subjects considered. Considering this correction factor, the intervention carried out in the end of autumn was placed in an intermediary position and, in particular, had a number of shoot that was very close to the values determined by spring pruning.