How can you tell if an overgrown shrub is a poor candidate for moving? A well-tended specimen with vigorous spring shoots and generous foliage will have a healthy root system. This is a good candidate for relocation, because it has stamina to withstand transplant shock when extracted from its position and energetic vigour to adapt to a new site.
Generally, woody plants more than 25 years of age shouldn't be moved. A sad shrub with many bare canes and dead twigs likely has an abbreviated root system, dead cavities in its crown and insufficient energy reserves to survive the move. Judicious pruning may improve its appearance and help keep it within bounds, and may even stimulate sucker growth that can be nurtured to take the place of the mother plant or be moved elsewhere to start a new plant.
Overgrown flowering shrubs such as lilacs, forsythia, honeysuckle, Annabelle hydrangea (Hydrangea arborescens), deutzia, dogwood and mock orange respond to hard pruning by sending up new and vigorous shoots. But most evergreen plants, such as juniper, yew, boxwood, mugo pine, false cypress and holly, won't sucker to produce additional shoots and should be shaped by less drastic pruning or replaced with a new plant of appropriate size.
Though most gardeners frequently move herbaceous perennials, many of us hesitate to relocate mature, woody shrubs that have outgrown their space. Small shrubs up to 1.2-metres-tall can be carefully dug up and moved to a new hole by one person. But larger shrubs up to 2.4-metres-tall will have a heavy root ball that requires additional hands and some advance preparation. Timing is also important. Woody plants won't respond well if moved while in leaf, so relocate them during their semi-dormant weeks in late autumn (after leaves drop) or earliest spring (before leaf buds sprout), when cool to cold air temperatures slow down plant metabolism, prevent active growth and reduce the stress of root disturbance.
If you have time to plan in advance, prune the shrub's roots. Thrust a blunt-nosed spade straight down in a circle around the shrub, about 60 centimetres out from the crown. Expect to hear or feel some snapping of roots underground. This will stimulate the growth of fine hair roots and help to form a more compact root ball that is easier to lift and less likely to break. Root-prune the shrub once at eight weeks and again at four weeks before transplanting. On moving day, dig the new hole first, estimating the size of root ball it must accommodate and providing room for soil amendments. Pre-mix the soil and amendments you will need and put them aside on a tarpaulin, covered with plastic to prevent them from drying out (or washing away in the rain). If the plant is to be moved in earliest spring before the soil is ready for digging, prepare the new hole the previous autumn, stuffing it full of straw or newspapers and covering it with plastic to keep out snow. Spread the soil excavated from the hole in the garden-fresh soil will be needed on planting day in spring.