Professor of Tree Care Vives Hogeschool Campus HIVB Roeselare is not ready for his test. His knowledge is widely spread and thanks to his clever insights we can dispel some myths.
We are very happy to add him to our Council of the Wise. Who asked again. How do you plant a tree now? Thanks in advance Wim for your tips.
Nothing is as simple as planting a tree.
Dig a hole, tree in it and then close it again. But you still see it very often that trees do not grow well. Or even die. The reasons for this can be very diverse, but the most common reason is that they are planted too deeply. There are many reasons why trees are planted too deep, but none of them are acceptable, as it is very poorly tolerated. You just have to look around in the forest, or in a park, trees fan out wide at the foot. The roots grow above the ground. Those large roots can grow a bit deeper in some species, 10 or 20 cm below the ground. But the small, fine roots that are responsible for the absorption of minerals, water and air, are very shallow. So it’s no surprise that they grow very shallow. When it rains, it is the top layer that gets wet first. Leaf or other organic matter that decomposes lies on top of the soil where it forms the litter. So that’s where the minerals can be found. And roots also need air. They also find it easiest there. But the mycorrhiza-forming fungi also grow in abundance in the litter layer. These fungi, which are indispensable for trees, need even more air than the tree roots themselves. In that top layer, just below the ground, they find everything they need there.
Did you know that a tree also has stress?
When you start planting trees, it is always a disturbance of the tree and of the soil. That tree is being cut down somewhere; roots are always lost. Especially the fine roots, the absorption roots, break down very easily. The larger roots are also lost when transplanting.
A transplant is always associated with stress for a tree. When we can plant trees in an ideal situation, where healthy soil life is active, we can limit that stress.
In practice, we rarely plant trees in optimal conditions, which only increases the stress. When the small roots are also too deep in the ground, 5 cm can be too deep or even deeper, then it becomes all the more difficult. Then the tree really has to struggle to survive.
When you dig up such a tree, you see that the small roots grow vertically upwards. The roots that are deeper are then completely dead. The tree is suffocating slowly but surely.
What is the correct depth now?
You often read that the tree should be planted at the same depth as it was at the nursery. In fact, it should be planted with the root neck level with ground level. In particular, containers grown trees can sometimes be too deep in the pot. And trees with a wire ball too often have a wire ball that is too high around the trunk. So you often have to plant them a lot higher than you would expect at first sight.
Making a wide, deep hole to improve the soil in the planting pit is not wrong in itself. It may even be necessary to break up hard soil layers. But then you should not put the tree at the bottom of the planting hole. What is also detrimental is the processing of fresh compost in the planting pit. That will continue to digest and that requires oxygen, oxygen that the roots also need. On heavier soils, that large planting pit, where the soil is loosened, is certainly not a good idea.
Sandy soils generally pose few problems. They drain quite well and allow a relatively large amount of air to pass through. Heavy loam and clay, that’s something else. If you want to plant a tree there, dig a nice pit and improve the soil in the planting pit, the tree will drown at the first rain shower. The soil improvement will ensure that the soil allows water to pass through better. When it rains, the water can easily penetrate the soil until the planting pit is saturated. Only then it can no longer leave there, because the soil around the planting pit cannot drain the water. The bathtub effect is a known tree planting problem that occurs not only on heavier soils, but also when there are hard layers in the soil. The roots will drown and eventually rot. On heavy loam and clay soils, it is better to plant the trees very shallowly and supplement the soil. Feel free to leave those trees a little too shallow. That’s not as bad as they are too deep. The only thing you have to watch out for is that the roots do not dry out.
This is a step-by-step plan of how a container grown oak was planted. The root neck was about 8 cm too deep in the pot. If you would then plant this oak with the top of the pot level with the ground, the tree is too deep.
How do you water correctly?
You often see that a drainage pipe is placed around the roots with which the tree can get water. That seems like a good idea, in reality it has a completely different advantage. If the tree is planted too deeply, such a drainage pipe can ensure that the roots still get enough air. But for watering such a drainage pipe is not suitable. It is better to make a raised edge around the tree, a ring-shaped dam that can fill you with water; the water must then penetrate the root ball.
It is sometimes said that you have to force the roots to look for the water deeper. But that will happen automatically, in the first place you have to make sure that the shallow roots do not dry out. A much layer is very suitable for this. But you shouldn’t make it too fat. the roots also need oxygen. You can go a bit further with very airy material, but five to 10 cm of wood chips is really the maximum. More is fly-tipping. The advantage of a mulch layer is that the rising groundwater condenses in that mulch layer (which is where most of the fine absorption roots are located). It also prevents the soil from becoming too hot in summer. And that in turn is very important for the soil life, which literally protects the soil against damage.
Don’t forget the tree pole !
There is one more thing that is really important about it planting and that is the tree pole. That has to be there make sure that the new roots do not tear off. If you look at how high some trees get attached, you’d swear those roots grow in the canopy. A tree pole should not be too high. A maximum of 80 cm is more than enough; the tree should be able to move freely. Attention make sure that the material does not suffocate the trunk. By the way, the tree pole can be removed after 2 or 3 years. That is quite often forgotten.
So planting trees in itself is not that difficult, but you have to know what you are doing. And be especially careful with all those weird videos that tell very complicated stories about what needs to be processed in that planting pit. In the end, only three things are really important: air, water and minerals. And those three are mainly supplemented from the surface.