Bareroot trees and plants are sometimes not that well understood. All bareroot means is that there is no soil around the roots of a plant when it is delivered. We are all familiar with some examples of bare root plants. For example almost all flower bulbs are planted when they are dormant and have neither leaf nor root. All roadside plantings are bareroot, over 90% of all hedging is planted without soil on its roots, over half of all roses, and 90% of all fruit trees are planted barerooted.
The key word is dormant. All plants slow down in the winter as temperatures drop and light levels fall. Deciduous specimens (trees and shrubs that lose their leaves in winter) go to sleep completely and almost all cellular activity ceases. Something like suspended animation. Evergreen plants such as yew do not become completely dormant, but they can best be described as being very comatose indeed, so similar rules apply.
Because these plants are in "shut down" they have very little need for anything except sufficient dampness at the root to prevent desiccation and cell damage. This is where bareroot trees and plants come into their own. They have a number of very significant advantages over their pot grown cousins:
Without wishing to labour the point, 500 bareroot hawthorn hedging plants weigh less than 25kgs. The same plants in 2 litre pots would weigh a tonne. Literally. Or, a bare rooted 6/8 standard english oak can be easily lifted with one hand. The same tree in a suitably sized pot is a two man lift.
To paint the picture fairly, there are some (relatively minor) disadvantages as well.