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Several governments are proudly announcing their plans on planting thousands, even millions of trees in urban surroundings.  These initiatives arise during desperate times, when we have finally realized that there is no future for humans without trees. However, be aware that planting a tree is an unnatural activity. This is why plant material should be handled with care. Also, an strict aftercare routine should be maintained to enable tree growth. After experiencing our third consecutive dry summer, it is quite clear that we know the theory but often fail on the practice. So, let's make sure that investing in tree planting does not become a hopeless cause!

By planting trees, we take a shortcut and skip all kinds of evolutionary processes. For example, pioneer tree species such as birch, willow and poplar establish themselves in disturbed areas and prepare the soil to allow the development of “pickier” trees such as oak and beech. These natural processes take many, many years in which the soil develops into a complex system full of live and interactions with the most greatly evolved organisms. Humans? No, trees of course.

Trees as thin as five centimeters in diameter are planted at a steady, sometimes militaristic rhythm in the urban environment. In forests and shrublands, often small biennial planting material is been used. This steady militaristic rhythm, is not a problem per se. I do worry about the source of the tree and the quality of the soil though. A properly bred nursery tree and an optimal planting soil are the minimum assets for future success. Tree nurseries play an important role here. They grow and prepare trees adequately by cutting of roots to grow a compact rootsystem.  They keep the soil in optimal conditions, meeting the needs of the tree in a balanced manner. In fact, nurseries are increasingly realizing that quality planting is not a matter of forcing tree growth through nutrient over-feeding, but a matter of keeping a proper balance between soil and soil life. That’s were most benefits are obtained.

The cultivation process is targeted towards achieving a healthy growing tree with a compact rootsystem. So far, the cultivation process is going smoothly and it is in good hands. Dutch growers are known throughout Europe for their excellent open ground cultivation techniques. Actually, we should be very content with tree growers, they allow us to buy time. They grow trees of up to 50cm in diameter, and basically what you buy from them is the end product plus all the years that tree has been growing.  Indeed, nurseries may be one of the few places where time is for sale!

Looking back to the evolutionary steps, the new environment in which the tree is planted plays a decisive role in the success of the nursery tree. In this new environment, aboveground and belowground factors play their own role. I will not dig into the aboveground influences here, please read my blog about sunburn to know more about of one of these aboveground challenges. Belowground factors are an important key to success. The soil in which the tree will be placed must contain sufficient oxygen, soil life, composition and structure, nutrients and moisture-buffering capacity. We should try to adapt the choice of tree species as much as possible to the soil present on the spot. If the soil is not optimal, then we could move on to determine what is necessary to upgrade the growing place to meet the needs of the most suitable tree species. If upgrading also fails, only then should it be acceptable to exchange soil for appropriate growing mediums. Unfortunately this happens quite often in the urban environment. This occurs because the soil is often compacted, polluted, degraded and even extremely mixed with poor sandy soils. If you invest in greening the urban environment, make sure to do it well by setting up an adequate growing pit that closely resembles the forest floor layering of the chosen species.

Alright, now that the soil is ready comes the next challenge: planting the tree, and not burying it. This is the biggest mistake most people do. Even professionals tend to plant trees too deep, especially in sandy soils.  Maintaining the same depth as at the nursery is the way to go. Do not add organic material such as sod in the planting hole; place instead one or more tree stakes that still give the tree room to move. Watering right after planting might also have a positive effect on the tree establishment and sealing of the planting area. I have seen cases in which all these indications were thoroughly followed and even specified in tenders but then these contracts arrange the aftercare so badly that the tree dies after its first year due to lack of water.

Despite the fact that a compact root system is well developed in the nursery, the delivered tree will have only 20% of the initial roots. Its moisture-absorbing capacity is therefore drastically reduced and the tree must be helped with extra watering. The watering requirement should be determined in the field. Practice shows that on average a planted tree usually needs three consecutive years of water supply. Watering should not been done rhythmically or in a calendar controlled frequency, but the water requirement should be determined on the spot in the field. Given the current dry summers, there is even a great need for watering during the third growing season. For example, it is not uncommon to water 26 times over the course of three years. Normally it comes in a decreasing frequency; from roughly 12 in the first year to 8 in the second and 6 in the third year.

Creating contracts to ensure the result of a living tree is essential. These contracts should include a guarantee on regrowth and contractors should be payed based on the result: a vital tree.  In other words, after at least three to five years trees must be vital and showing an increased shoot growth after planting. Contracted arborists or landscape professionals are then challenged to make the planting a success. This way there is no perverse contract-gap incentive in the work, but a constant positive incentive to contribute to the greening of our urban environment.

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