1. The soil is warm
In the autumn, the soil in your garden is still nice and warm from the summer. Plants appreciate that as they acclimatise, because it gives them the chance to form new roots. Existing roots will also stretch out happily into the warm surrounding soil, so your plant gets nicely bedded in.
There’s plenty of rain in the autumn, so you need to give little or no water to your freshly planted plants. The risk of them drying out is actually far greater in spring.
3. A headstart
Because your plants have already been bedded in for a couple of months by the time spring arrives, they're larger and will flower more beautifully when spring rolls around. If you wait until spring, they won't have half as good a chance. When the warmer weather comes, it also means that all you have to do is set out your deckchair and enjoy the sunshine — no need for hard gardnening!
ONE MORE THING
Frost-sensitive plants are the exception to the rule: they're best planted after winter. And if you want some colour on your patio to tide you over until spring, take a look at winter heather, autumn violets, Osmanthus, box, laurel, Lawson cypress, or any of these classic autumn plants.