When choosing the shrub / tree consider what you want from it!
For example, has it to flower, be evergreen, bear fruit or even all three.
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Once this is established, pot size will have to be considered.
In many cases, growing a shrub / tree in a container will restrict the plants ultimate size.
Another factor is stability!
Lightweight plastic pots allow plants to be moved around with relative ease, whereas a heavier terracotta pot would be more difficult to move but would have better stability.
If the latter is selected, ensure it is frost proof.
Another consideration is, will the plant require indoor winter protection?
If so then you should strive to have a pot no larger than 500mm (18”) dia. /square.
Where shrubs / trees remain in the one spot all year round then larger pots could be considered.
When filling the pot with compost firstly place 50mm (2”) of crocks, gravel, or broken up polystyrene over drainage holes.
Keep the base of the pot off the ground by standing it on purpose made feet or bricks.
This allows surplus water to drain off and prevents the pot sitting in pools of water during the winter months.
One should consider filling the pot with loam/soil based compost rather than soil-less and this will give more stability.
This may not always be practical if lime-free compost is required, in this case one would use ericaceous compost.
Mix in a controlled slow-release fertiliser to sustain the plant during the growing season.
Leave a gap of 25-50mm (2”-3”) between the top of the compost and the rim of the container to allow for watering.
Container-grown shrubs / trees are more prone to drying out and need regular and thorough watering to wet the compost to the full depth.
Feed and water potted shrubs even in damp weather because their canopy of foliage acts like an umbrella, and may stop water from reaching the roots.
Watering may be required twice a day in summer.
Use rainwater for ericaceous plants.
Apply a balanced liquid feed regularly.
Pot on, or re-pot (small) trees in late winter to early spring.
Alternatively, renew the top 50mm (2”) of compost including mixing in a controlled slow-release fertiliser to sustain the plant during the growing season.
Trees may require removal from their pots every three to five years to replenish the compost.
Tease out the roots, trim the thicker roots, re-pot with fresh compost and controlled slow-release fertilizer into the same size pot.