It is only relatively recently that the emphasis on gardening has been to plant in spring. Before the current rash of gardening gurus all trying to find an angle, any gardener worth his/her soil knew that if you planted in the autumn, the following summer would see earlier and stronger flowers on plants that established better and more reliably. There are some obvious (and some less obvious) reasons for this.
Advantages of planting bareroot trees and hedging early include:
The autumn sees lower temperatures which means there is less loss of water from foliage and so less call from water from the roots. Falling temperatures mean that the enzymes that drive biochemical reactions like photosynthesis slow right down. The combination of this with shorter days mean that trees refocus their energy into their roots rather than their leaves. Since you want your trees to establish from their roots up, all this plays into your hands and makes autumn a great time to move or transplant.
Second, there is still some warmth in the soil. Tree roots continue to grow a little during dormancy and especially so when the soil retains a little warmth, as in autumn. The consequent root growth means that in spring a tree’s progress is more advanced because it is better placed to take up water and nutrients.
Thirdly, autumn is often wet, and is certainly dewy and so there is less need for watering once you have planted.
All of the above especially applies to pot grown, rootballed and evergreen trees which should ideally be planted in October/November. It makes less of a difference to deciduous trees but they do also benefit from early planting although for them that means November as a general rule. It is important to wait until the tree has drawn down all of the nutrients it can from its leaves. It is the transfer of nutrient to the root that is important here, not the shedding of leaves. Some trees, such as beech for example, hold their leaves for months after they have dried and become (nutritionally) useless. So for most deciduous trees the first opportunity to plant is normally in November, when the soil is still surprisingly warm. However some species, notably Alder, Oak and Pear become dormant later and should not be planted before winter has arrived.
Disadvantages of Planting Early:
Anyone living in a cold area or who is subject to a freezing winter (will we ever see one again in the South-West?) risks subjecting their newly planted trees to several months of bitter weather. Well planted, this should not damage roots, but frost lifts the soil and means they will need checking and refirming when the temperature is above freezing. That can hinder tree establishment. So you will need to check newly planted trees regularly through a hard winter and again in spring once it has thawed. Not necessarily a disadvantage as it gets you out of the house and a hard winter can also be stunning beautiful.
Don’t plant early on clay unless there is drainage. If winter is not cold it will be wet and planting holes on clay become water-logged. When that happens the roots are deprived of oxygen and can rot and die. Yew, which is normally bombproof is especially subject to this in the first year after planting.
Exposed and windy areas also are prone to become windier in late autumn and winter and so if you do not plant your trees well they can suffer from windrock. Evergreens in particular need protection from the drying and deleterious effects of a cold, north wind before they have had a chance to settle in.
For the reasons above we would, therefore, recommend planting small conifers such as seedling Christmas trees late in the season as spring approached; they are too small to survive being soaked and blown about all winter.