We like to say the best time of year to plant any type of plant is within the dormant season. Here in the UK, the dormant season runs from November-March.
Though you can plant potted plants at any time of the year, the plants will require much more care and attention regarding watering, feeding, and weeding etc., during the hotter months.
Autumn is one of the best times to plant larger hedges/trees/shrubs, as it gives the plants the best chance to establish their root systems and soak in nutrients/water over winter. Though many believe spring is the very best time to plant, we still like to recommend autumn as the weather is cooler and wetter, making the maintenance side of things much easier for yourself! As well as this, you even get the chance to help pollinators who are struggling to find food at that time...
A good strategy for planting in autumn is by picking out your plants at the same time as your bulbs. This way you'll get them to your front door at the same time and are able to do your gardening all at once (or over a few days!). As well as this, you won't have the same struggle with weeds in autumn, as you would in spring- you can quite often clear the site a week or so before you plant! The weeds are dormant, and therefore won't take over the site before you've even got to the planting stage.
We'd just like to note that most plants will go dormant during autumn and winter, except evergreens that never go fully dormant. This will mean that the root systems on your plant will not be growing much new roots, exactly, but will instead build up lots of energy to help them flourish during the warmer months!
With that all being said, spring is still a good time to plant, and so is summer (for container grown plants). If you're not ready to plant until spring/summer, it's not a problem- just ensure you give your plants plenty of attention. Water the plants well and fertilise/feed them regularly.
Of course, you can plant during winter, too! All root types can be planted during winter, you just have to be aware of frost and ice etc. that could delay your planting scheme...and avoid very wet, waterlogged areas!