The ten things to know to prune plants and trees

To prune or not? This is the dilemma for those who take advantage of the holidays to tidy up balconies and gardens. The issue is divisive. And if the people of the “yes” question how and how much to intervene, those who postpone are almost always victims of feelings of guilt, fearing making a mistake. So let’s clarify the subject once and for all, to understand together if and when a pruning is needed.

1. Know the architecture of trees

Compatibly with the environmental conditions, each tree species develops its own typical architecture: a natural combination of engineering and biomechanics that guarantees plant stability and a perfect life cycle in the ecosystem it belongs to. Primary forest expert Francis Hallé established that the growth of all tall tree species can be traced back to 22 different architectural patterns of trees. If fir trees, for example, have a conical shape to let the snow flow down the branches without breaking them, plane trees spread out to ensure maximum light for all the leaves. These canopy silhouettes are related to the tree’s functions. The first question we must ask ourselves, therefore, is the following: what is the natural architecture of the plant that I have purchased? If this behavior is distorted, the tree will no longer be able to “function” well on its own and our interventions will be needed.

2. Is pruning really necessary?

Nobody performs a real pruning on spontaneous plants, yet they have been doing it for millions of years. Often it is the wind that shapes its forms, sometimes the fire can regulate the growth, in other cases the herbivores or the floods take care of pruning trees and shrubs, but regular interventions to reduce the foliage in nature are not necessary. And it’s not even true that pruning strengthens, on the contrary: “Satellite surveys attest that trees after pruning show signs of stress by lowering the rate of photosynthesis”, explained Professor Francesco Ferrini, professor of General Arboriculture and Tree Cultivation of the University of Florence. In case of hardship, moreover, the plant has the ability to “self-prun” and when there is a long lack of water or nourishment, in order not to die, it lets the branches dry which it cannot feed.

3. Why do we prune plants?

This is where agricultural tradition and gardening culture come into play. Usually we prune more for our needs than for those of the plants: to increase flowering or fruiting, to give the foliage a geometric shape, to reduce the size of the plants… Furthermore, many fruit trees and as many ornamental varieties are domestic plants created by man under certain conditions, therefore they give their best only after regular care. And again, certain trees selected for their strangeness – think of cedars and pendulous beech trees – end up growing unbalanced and need to be lightened and shored up to avoid stability problems. But it is also pruned to eliminate dead branches if there is a risk that they could fall and cause damage, or we are forced to intervene when, due to a mistake made at the start, we have placed the plants in spaces that are too narrow for their roots and foliage . So let’s find out about the size of the species before buying to guarantee it the right living space, respecting the rule of the “right plant in the right place”.

4. Forming the “skeleton”: training pruning

Let’s get into the specifics of the most adopted prunings. Formation pruning is the setting we give to the plant in the first years of life to ensure that as an adult it forms a “skeleton” of the desired shape. Nurseries perform it annually on young trees by eliminating the lateral branches along the stem up to the height where branching of the crown is desired. We do it too when we grow a tree directly from the seed, for example an oak, a medlar or an avocado, or when we want to make species that normally grow as large bushes grow in the shape of a tree: this is the case of pomegranate, strawberry tree, fig , mimosas, citrus fruits, pittosporum, clerodendron, Magnolia soulangeana and Magnolia stellata, fejoa, hibiscus, oleander. In these cases, we only let the main branch grow, regularly eliminating the lateral ones, to obtain the shape of a small tree. To form fruit trees, the top of the plant is usually cut to about one and a half meters in height to give life to lateral branches and shape a flared crown, so as to let the light enter between the fronds and easily harvest by hand .

5. Maintenance pruning: we intervene little and often

Maintenance pruning, the most common, is used to eliminate dry parts and preserve the shape and size of a bush or tree, once it has occupied all the space assigned to it; that of containment reduces the encumbrance of an overly exuberant specimen, from the bougainvillea that “rises” visibly to the hedges grown in a natural form. When we no longer have space to grow plants, therefore, we intervene once a year. Never prune drastically after several years, in order not to have to cut thick branches, doing damage to the plant. How to do? Let’s fix the architecture of the plant in mind and intervene to give it back that shape, eliminating the vigorous and out of shape shoots and pruning the interior a little (never remove more than a third of the foliage). To proceed faster, we take a photo of the specimen with the smartphone, then draw the desired shape on the image taken and follow that track. The maintenance of fruit trees involves production pruning, which consists in thinning out the foliage to aerate it and ensure that the light reaches all the branches. Each variety has specific needs, but in general the ancient fruits born before industrial agriculture can also be grown naturally.

6. For those who love the geometry of hedges

The most classic maintenance pruning concerns hedges with a geometric shape (called formal) to which to restore a shape. We intervene regularly to prevent the new branches from lignifying and to prevent the internal part of the bush from emptying out. For formal hedges of Leyland cypress, pittosporum, cherry laurel, privet, thuja, viburnum, photinia, pyracantha, yew, eleagno, boxwood, berberis, we cut the new tops twice a year, in early summer and then in late autumn. The same goes for railings and walls covered with vigorous vines such as false jasmine (Rhyncospermum jasminoides), jasmine (Jasminum grandiflorum, Jasminum azoricum), bougainvillea, Hedera algeriensis, eliminating out-of-place “regrowth”.

7. The logic of when to prune: no frost or sap

With the exception of formal hedges, we prune in autumn after the leaves have fallen or in late winter, once the danger of frost has passed. In fact, the plant must not be in a moment of growth, when so much sap flows through the branches, because with the cut it would lose a lot of liquids. Furthermore, we avoid the coldest months because when the temperatures go below zero, the frozen branches can break and because the intense cold penetrates the tissues through the wounds, damaging the wood. Woe to latecomers then: we cannot prune in spring, so as not to disturb the birds’ nesting.

8. Plant by plant, when to intervene

Specifically, we need to divide the plants into groups. For the varieties that bloom in early spring on the previous year’s branches (those that are already there in winter) we intervene in the days following flowering, as soon as the petals wither and before the new shoots begin to grow: this is the case of camellias, mimosas, wisteria, forsythia, azaleas, Spiraea vanhouttei, Magnolia stellata, almond, peach and flowering apple trees. Pruning them early would mean missing out on a spectacular flowering. On the other hand, the varieties that bloom on the new branches born in spring can be pruned from late autumn until February, when temperatures are stable above zero; among them, remontant roses, classic hydrangeas, Hydrangea arborescens and H. paniculata (we will dedicate a tutorial to roses and hydrangeas), abelia, bignonia, Syriac hibiscus, buddleie, viburnum, pomegranates, lespedeza, lagerstroemia, Syriac hibiscus, pomegranates, nandina … The subtropical species grown in the South and in seaside gardens (such as citrus fruits, thevetia, Hibiscus rosa-sinensis, bauhinia, thunbergia, senna, plumbago, bougainvillea, lantana, duranta, avocado) should only be “touched up” in early spring, when it exceeds ten degrees; pruning in intense cold could even kill them. Finally, for evergreens such as conifers, the end of winter is expected only if necessary, although pines and other species resent being touched.

9. How to prune in a natural and sustainable way

In order not to stress the plants and to ward off diseases, it is necessary to act in time by intervening only on the branches less than two centimeters thick. In fact, the plant manages to “heal” small cuts by creating a callus, which it cannot do on larger wounds, which remain uncovered and become the gateway for fungi, caries and rot that make their way into the wood (damages can appear after years). Furthermore, the cut is always made one to two centimeters above the gem and not below. In summary, therefore, small branches can be cut, trunks cannot. It follows that for a correct job, shears, loppers and hedge shears are enough. If the situation has gotten out of hand and a hacksaw or a chainsaw is necessary, we risk influencing the future of the plant, so it is good to rely on an experienced gardener or a certified arborist.

10. Say no to toppings

At this point it is clear what damage the toppings cause to the plant, i.e. the drastic interventions with which we cut the heads off the trees, of which only the trunk remains standing. In addition to leaving stumps unsightly, these indiscriminate cuts made with the chainsaw open a huge gap in the plant tissues and create significant physiological imbalances. In response, fast-growing shoots are born around the topping, but the attachment of these branches to the trunk becomes a point of fragility subject to easy breakage. The pollard cuts seen in public parks are sometimes justified on the grounds of safety concerns. In fact, the living conditions for a tree squeezed between asphalt, buildings and constant injuries are not easy, but when the risks of collapse are real or the state of health of the tree is truly compromised, the intervention follows stability report signed by an expert. When you are faced with an entire avenue massacred by chainsaws, it is more likely that the operation was carried out by general maintainers and not by professionals and it is only right to be indignant because, in addition to not respecting the plants, these bad interventions are paid for with our money will create security problems within a few years.