I normally recommend to my clients that we plant trees, shrubs and perennials in a hierarchy of sizes. That means with slower growing items such as trees and shrubs, its best to plant them at least a certain size so we can enjoy the garden the way it is meant to be as soon as it is finished. If the plant needs to be pruned specifically or a particular shape developed, this is another time it’s a good idea to plant a mature specimen.
I don’t believe in planting large trees for no reason but when the size of the job is large enough then it seems a real benefit to plant large specimens straight away. Thankfully there are numerous large plant and tree nurseries in the uk and the continent.
Here you can see the process of large tree planting, The main issue is the size of the root balls to sustain such a large plant, this makes moving the plants difficult requiring low loaders or cranes, and a particular issue is the delicacy of the root ball vs the weight of the plant/tree, this means the planting holes must be dug very accurately in order for the plant to as much as possible go into the hole the first time.
Normally I like to plant my perennials in very small P9 pots, these seem to root out very quickly in the first year, then plant smaller shrubs which I hope to become 1-1.5m high usually as 50-60 cm in height when first planted.
Trees I want to be 2-5m high I plant as mature specimens usually 2-3 m in height already. However, as I have said, when a site is very big there is a benefit to planting large topiary and mature trees to really create impact on a scale that reflects the garden, as the job will probably have a high cost to create the higher proportional costs of planting larger plants is balanced to the job costs and thus makes sense.
Heavy equipment is often used to achieve the planting and these larger specimens go in before a lot of the ground work is done requiring accurate drawings for positioning before the garden is built to get them in place. Drainage and irrigation must be considered for these larger root balls. The drainage was checked and improved during planting, for this box ball garden we see above and below.