By planting this autumn you'll ensure that the tree puts down some good roots through the winter and gives you some great rapid growth next year.
The type of tree you plant will depend on the size of your garden. But there is a tree for every garden whatever the size. Visiting Cannizaro Park this summer to see Hayseed Dixie (you should have been there - what a night!) I was given the honour of being able to shelter from the rain under one of their champion Sassfras trees. This is a prince among trees and very rare. I'd compare it to the tulip tree Liriodendron tulipifera, the sweetgum Liquidamber styraciflua and the Persian ironwood Parrottia persica for autumn colour and pure glamour. But these are all pretty big trees if you're going to let them grow to maturity.
For small gardens we have some great choices of trees that are readily available from nurseries. Everyone loves cherry blossom and varieties such as Prunus Kanzan and Prunus Shirotae are well-loved and planted across the UK. But consider a native cherry like bird cherry Prunus padus and wild cherry Prunus avium for their wildlife value or the winter flowering cherry Prunus x subhirtella Autumnalis for a really long period of flower just when you need cheering up. There's another family member, the common almond Prunus dulcis which does well but is a little larger.
Two other species that are always worth a look are Sorbus and Malus. Generally there are a lot of trees within these groups that are great for small gardens. Sorbus commixta and Sorbus cashmiriana are usually planted for their autumn colour and winter berries which the birds always seem to leave alone until they've stripped other plants. There's also Sorbus aria Lutescens a variety of Whitebeam that gives the best downy silver leaves you can expect.
The Malus species is equally prolific for garden trees from the crab apples such as Malus John Downie and Malus x zumi Golden Hornet to fruiting apples. At this time of year you can visit RHS Wisley Garden and Brogdale in Kent to sample the apples before you buy the tree and you'll get advice on which trees to buy to ensure you get the best fruit.
All these trees are great but what if you fancy something more exotic? Well have a look at the ornamental French pear Pyrus calleryana Chanticleer or the snowy mespilus Amelanchier lamarckii with rich coppery red autumn tones and fragrant white flowers in spring. These trees earn their space in the garden all year round. And a final word for tree enthusiasts like me - get a seed of Paulownia imperialis the Imperial Princess tree but be prepared to prune it every year to get the largest tree leaves you've ever seen!