Landscaper David Viljoen of Mercury Designs has this advice for planting up a container
To ensure healthy plants, the container must have drainage holes for excess water to drain away. Add a few pebbles or small stones in the bottom to prevent these holes from becoming blocked. If possible, place containers on pot supports so they are slightly off the ground.
As plants need air and water at root level, always use a growing medium that holds both, such as a good quality potting soil. Don’t use garden soil as this becomes compacted in a container.
When watering, make sure the potting soil is moist, but not constantly saturated as water displaces air and roots can easily rot in its absence.