Can I move my rhododendrons now, in late winter to early spring?
According to A Plantsman’s Guide to Rhododendrons by Kenneth Cox (Ward Lock Ltd., 1989), “rhododendrons are generally quite easily moved, most even in full flower and at considerable age. […] Size is really no problem, provided you have the means to do the digging and the moving. Obviously, the more rootball you can take with the plant the better, but usually you can reduce it considerably without too much harm being done. If you end up with a disproportionately small rootball, you can reduce the size of the top somewhat to compensate. The roots of a rhododendron generally extend to about 50% of the plant’s foliage diameter […] it can be far more or much less. The roots are usually less than 18 inches deep, even on a very large plant. To move a large plant, start digging […] quite far out from the stem, and continue towards it until you meet roots. Then dig all round underneath the rootball […] gently rocking the plant to ease the rootball from the soil. Watch out when lifting a plant by its main stem; it may not be strong enough to carry the weight of the rootball. The root can best be reduced by prising soil from it with a fork. […] A rhododendron can remain out of the ground for considerable periods if you keep frost and sun from the roots, and ensure that it receives regular watering. Heeling it into the ground, or covering the roots […] usually gives adequate protection. Although rhododendrons can be moved during the growing season, they will require extra watering after transplanting.”
In addition, you may find the Royal Horticultural Society’s directions on moving a mature tree or shrub helpful.