Selecting a tree planter
Usually any planter larger than around 80 x 80 x 80cm would be deemed a ‘tree planter’, but obviously the right size depends on the type and maturity of the tree that is in need of a home. It is generally acknowledged that trees are happier when planted in the ground, but provided they get enough TLC they can survive well in a planter. You would look to choose a container which is 2 or 3 times the size of the pot it comes in from the garden centre or nursery, which will give it plenty of growth room and allow it to thrive for many years. It is worth remembering that a square planter of say 100 x 100 x 100cm contains a larger volume of soil (1000 litres) than a round planter dia.100cm H100cm (785 litres).
Another reason to choose a larger rather than smaller tree container is temperature control. Trees in pots are more susceptible to extremes of heat and cold, and the smaller the container the more likely it is to overheat or freeze the roots. Larger pots will minimize the stress to the tree caused by fluctuating temperatures.
For stability, look for planters that are wide and low rather than tall and slim. Trees have a lot of wind resistance when in leaf, and you really don’t want to worry about the planter being blown over. In terms of height, a proportion of 1/3 planter to 2/3 tree works well aesthetically and practically.
We cannot claim to offer any expert advice on the type of trees that are suitable for planting in a pot, but there's a lot of information widely available online. However for residential gardens (as opposed to commercial schemes) we do know that our customers are most often looking at planting a Japanese acer - one of the many smaller growing species - in a pot. Another popular choice for pots is an olive tree, or perhaps a palm. Smaller fruit trees such as lemon or fig are also good in containers as they can be moved inside or to shelter during the winter.
Planting the tree
What you will need:
Peat free general purpose compost (or preferably a specialist tree and shrub mix). Compost is measured in litres, as is the volume of planters, so you will need the litre volume of your planter (less the space taken by drainage material) minus the litre volume of the pot the tree is currently in. Get more than you think you need, as you can always find use for spare but it is intensely frustrating to be short.
Slow release fertiliser, usually in granules
Drainage material – the purpose of this is to create some air gaps at the base of the planter to allow water to percolate out through the drain hole/s. You can use a few carefully placed irregular stones/crocks just around the hole/s, taking care not to block them, or a layer of fine gravel or covering the base of the planter to about 5cm deep.
Optional: If your tree planter is to be sitting on pale coloured paving and you are concerned about keeping the paving as clean as possible you could add a permeable membrane layer which will cut any soil run-off. Look for ‘permeable geotextile membrane’, usually sold as weed suppressant membrane. It can be placed in the base of the planter to cover the drain holes before the drainage material goes in.
Optional: mulch to top-dress, such as bark chip or pebbles. Mulch will help to retain moisture, suppress weeds and can add a rather smart or finished look.
First, water your tree really, really thoroughly while in its plastic pot. This is particularly important if you are planting in the summer months. If you can, stand the pot in water for an hour or so to make sure the soil is completely saturated.
After putting in your drainage layer (and membrane if using), fill the planter with your compost mix up to about the depth of the pot the tree is in, so that the base of the trunk will be 10cm or so below the top of the planter. At this stage, add your slow-release fertiliser, following the packet instructions. Then carefully remove the tree from its plastic pot, which is likely to be a two person job. If the tree has been in the pot for a while It may be necessary to cut (or saw) it off. You’ll see a tight network of roots, which retain the shape of the pot. To help the tree establish, try to tease them out a little at the sides and the base so they will more easily spread into the new soil (we call it ‘scriffling the roots’, not a technical term!) You don’t need to be too cautious here, and if the roots are very fibrous and tightly wound you may need to use a knife/secateurs to cut in to them in order to open them out a little.
When the tree is in position in the planter, fill around the rootball with compost, tamping it down lightly but firmly as you go to avoid any air gaps. Then water in well, and if the compost level shrinks back when wet, top it back up. The root ‘flare’, where the roots first emerge from the trunk should be just below the surface of the soil. Now add the mulch if using, and stand back and admire!!
Tree planter care
Trees planted in pots must be maintained and cared for, unlike trees planted in the ground which after the first year or so will generally thrive without further input. Watering is critical, and it may be necessary to water daily and thoroughly in a warm summer, particularly if the planter is in a sunny and/or exposed spot. Container-grown trees should be given slow-release fertiliser granules annually (usually in the spring), and this can be combined with refreshing the compost – scrape off as much compost as possible from the surface and replace with fresh, mixing the fertiliser in. Eventually, the roots may completely fill the planter, in which case fertiliser granules will just sit on the surface and be of less use, so a more regular liquid feed can be given instead.
See our Tree Planters category for containers of a suitable size for tree planting.