Is it possible to plant a tree such as a European Hornbeam in a very large ornamental pot that does not have a bottom so the roots can eventually grow into the ground. The purpose is to increase the off the ground height of the tree.
Thank you for contacting the Toronto Master Gardeners regarding container grown trees.
My initial response would be that a tree always does better when planted in the ground and if you want to increase the height off of the ground you can certainly always limb up the tree. This way you get the added benefit of underplanting around the tree with various perennials. If planned properly, this will give you season long colour.
However, if you are set in planting a tree in a container there are numerous consideratons to keep in mind. First, trees that are planted with their roots above ground feel the cold more than trees which are planted with their roots in the soil. My recommendation would be to pick a tree that is cold hardy to two zones below your regular hardiness zone. For example, if you live in Toronto then your hardiness zone is 6. As a result you should choose a tree that is hardy to zone 4.
European Hornbeam is hardy to a USDA zone 4 which is equivalent to a Canadian hardiness of zone 5. This tree is best grown in medium moist, well-drained soils in full sun to part shade. It is a is a medium-sized, deciduous tree that can reach 40-60’ tall with a pyramidal to oval-rounded crown. The growth rate for a hornbeam is moderate/slow so you could expect about 1 foot of new growth per year
Second, your container choice also has a big impact on how much cold the tree roots will experience. As a result you should not plant in plastic or metal containers. Not only are these containers quite thin but the plastic will crack overtime. Ceramic or clay pots should also not be utilized as the freeze thaw cycle that happens over the winter will also cause these to crack. The container should also be large enough to accommodate both the growing tree and its roots and wide enough to provide the best possible insulation for the roots. The bigger the container, the more soil around the roots the better the insulation.
The soil for your container grown tree is also important. It should provide enough aeration and drainage while maintaining suitable amounts of moisture. Caring for your tree in the container is also different from when it is planted in the ground. Container grown trees tend to dry out quicker and as a result need regular and constant watering until the roots become established in the soil. Make sure to not plant the tree too deep. The tree must be placed in the soil so that the soil grade is level with the bottom of the root collar. When planted at the proper level you will see the gentle flare of the trunk rise just above the soil line.
Trees planted in pots run the risk of girdling roots. If a tree has been in a container too long, the roots may hit the pot edge and begin to grow in a circle, becoming matted and tangled. Circling roots will become girdling roots and the plant will die from lack of air, water and nutrients. Hornbeams have taproots rather than a lot of roots close to the surface. This tendency for their root systems to grown down rather out may help with circling roots.
If you decide to plant your tree directly in the ground our Garden Guide on Planting a Tree for Life gives detailed information on the proper planting technique to ensure its survival.
The following links contain more information on growing trees in a container.
Overwintering Unplanted Trees, Shrubs and Perennials
Good Luck 23.09.23