As soon as these buds swell and just begin to open the roots should be pruned and the plant repotted. In general, maples are among the earliest to be repotted and oaks among the latest. This usually works out to somewhere between the first week of March and the first week of April. Deciduous spring flowering plants should usually be repotted immediately after flowering. Hardy conifers have a wide range of appropriate pruning times and may be repotted up until June.
Conifers are subject to root rot if their roots are pruned while still dormant.
Pines can be root pruned any time after the candles begin to suddenly elongate. Junipers will tolerate late root pruning even into mid June. Tropical trees should be root pruned only well after they are actively growing and the root temperature is above 60 degrees. I have successfully repotted a large ficus in the middle of July. Protect newly repotted trees from the wind and direct sun for at least three weeks.