In ancient times, gardeners have been long known to grow trees in pots. It is a very delicate art, as it requires intensive cultivation and maintenance of the soil to provide the appropriate degree of acidity and humidity for the proper growth of plants. In addition, extreme care must be given to water management so that there is no waterlogging or over-watering. Good drainage at the bottom is necessary because most plants cannot tolerate standing water for more than a short period of time.
The underground root system restricts a tree’s size. Trees grown in containers tend to develop much smaller root systems because they are limited by pot size and shape. This means that trees grown in containers do not have as great an ability to absorb nutrients from the soil and they cannot draw sufficient water from the soil to maintain a normal rate of growth during dry periods.
Therefore, the trees grown in containers will only develop a maximum of half or two-thirds of their expected size when growing outdoors if they are planted directly into the ground after being grown in a container. A large tree grown in pots can be reduced to a fraction of its original height by planting it out at the end of summer. If this is not done, larger trees can be transplanted from pots without any ill effects but may take years before getting established again.
In addition, container-grown plants tend to have less tolerance for windy conditions than plants that grow directly in the ground because the root system has been restricted and it is more difficult for them to maintain a stable upright position. Therefore, it is important to provide support from the wind if container-grown trees are planted in an exposed location.
The art of growing trees can be mastered through experience and knowledge on how each plant needs to be treated differently. This is because some types of trees tend to develop better than others when grown in containers. For example, plane trees tend to grow extremely well in pots while many other species suffer greatly because they need much larger pots (especially oak) or do not like root restriction (linden, etc.).
Conversely, trees that grow quickly often exhaust the soil very quickly as nutrients are consumed at high rates by large root systems; therefore, they may require repotting every two or three years. An example of such plants is sweet chestnut (Castanea sativa) and linden (Tilia spp.).
For trees that grow slowly, the soil may last longer but they require much more careful watering to prevent them from drying out or becoming waterlogged. For example, it is difficult to be sure that a slow-growing tree in a container does not dry out between waterings so frequent checking of the soil moisture level is required even when there has been no rain.
A tree grown in containers require high levels of nutrients. When choosing container supplies for each type of plant, people should bear in mind that many species differ markedly in their nutrient requirements. Some plants have very specific needs and will require specific nutrients while others can cope with a wide range of soil conditions.