The soil is warm - roots love warm soil.
The soil is moist - normally we have had or are expecting to have a good amount of rain, which means once planted and watered in you can just leave your plants to take care of themselves.
The plants have time to get their roots down prior to the onset of winter and have a head start on spring planted perennials.
You can see the gaps in your border and maybe more importantly you can still remember how the border looked at the height of summer and throughout the rest of the season ....
It's the ideal time to move and split existing perennials so if you wish to change the look of an entire border you can.
You can also plant spring flowering bulbs amongst your perennials to help extend your season of colour.

But avoid planting slightly tender perennials such as Penstemon, some Salvia, Gaura, ... these are far better planted in the spring once the last of the frosts has passed.
Don't plant when the soil is saturated - you will damage the soil structure by walking on it when it is very wet. If your soil is continually wet and you need to get on it work from a builders plank or board to spread your weight across the soil surface.

To help you get started Rosy has come up with a couple of small planting plans for a 3m x 1.5m border one in a shady situation and with a moderately good, free-draining soil and the other for an open sunny site with a moderately good, free draining soil.

Firstly ensure that the plant you are about to put into the ground is well watered.
Position your plants within the border; we give a guideline height and spread on the label. The height is obviously important as you want to be able to see your newly acquired plant above the plants in front of it. The eventual spread is very important when setting out your plants - if we say your plants have a spread of 30cm (1ft approx.) you will need to plant them at 25cm-30cm apart to allow them to eventually meet but not choke one another.
Prepare a planting hole ideally a little larger than the pot. Pot grown plants should in general be planted to the same level in the soil as they are in the pot; however it is important not to cover the rhizomes of bearded Iris which sit just out of the ground so they may be baked in the sun; likewise Nerines will not thank you if you bury their bulbs below the soil. If the ground is very dry, a watering can full of water poured into the planting hole and allowed to soak away before planting will help your new plant get a good start. Remove the pot, tease out the roots and place your plant into the planting hole, back fill with the soil and firm around the plant either with the heel of your hand or gently with your boot. Then water the plant in and go and make yourself a cup of tea.