Cutting back before moving will have the advantage of reducing the number of leaves which will help the plant adjust to the move by decreasing natural water loss.

Many plants benefit from root pruning or require it before attempting a move. I've never found it to be necessary for rhododendrons at all. Wait for a period of damp, overcast weather. Cut around the roots a good distance from the trunk with a sharp square nosed spade. Gradually undercut the roots by pushing the spade toward the trunk, gently prying as you proceed around in a circle. Once you have loosened the roots, slide the plant onto a tarp and drag it to its new location. Keep it well watered for the entire summer after the move.

Rhododendrons have dense fibrous root systems close to the surface. There is no need to root prune as you would with many other genuses. In fact, digging the "trench" as you would in conventional root pruning would expose the rhodendron roots to air drying which will rapidly kill them.

Much thanks, Mainegrower!

For clarification, are you saying that a hard prune and a relocation/transplant can be done almost simultaneously?

I was aware that rhodos have shallow fibrous roots. In general, would you say 6, 9, 12 inches deep?

There are two schools of thought on hard pruning and moving woody shrubs including rhododendrons...one as described above, and a second that suggests its no longer thought to be a good plan to do both same time, or even same growing season. You can prune the shrub back a little if necessary to make it more manageable but it is no longer widely recommended that woody plants be pruned severely when they are transplanted.

Horticulturists used to suggest that pruning the topgrowth back hard at transplanting helped balance the severely reduced root system. More current research shows that transplants may need as many leaves as possible to photosynthesize and produce the carbohydrates necessary to regrow a complete root system.

I've seen it done both ways and lean towards moving them intact for faster recovery. But you have to understand too that my climate is so hospitable to rhododendrons, its hard to lose one under any circumstance.

Whatever you decide, prepare your new planting site first, and don't plant any deeper than was originally growing...this may mean allowing for settling of the soil. You may move successfully any time the ground is not frozen and the rhododendron is not in active growth (the period immediately following bloom). If its a large plant, you can take a smaller rootball (roots will extend at least as far as the drip line)...try for a diameter rootball 2/3's the plants height if possible, even if that means asking for help in moving it.

Interesting post from morz8. A good compromise might be to trim off enough to make the plant manageable, leaving the rest. In any case, I'd avoid cutting back into old wood. When you do this it can take a long time to regrow and I would not do it before moving or in the first year or two after moving. Trimming back long awkward shoots, but not all the way to old wood is fine.

It can vary, but I'd say root depth would be closer to 9" than 12" on average. Rhododendron roots need oxygen, so they never go deep into more compacted soil. A 40 year old plant will, however, have a large mass of roots and it will be heavy.

Hmmm...now I'm stumped. Even if I prune this baby back by 1/3 all around, it's still a pretty large plant. I was hopeful that I could knock it back even further. A big rhodo in the next town over (also probably came from the local woods as was the practice with the old timers here) was chopped down to the ground and the entire thing came back beautifully in a year or two with a far better shape.

So either this moving thing is going to be a two year (or more) process for me (prune hard, wait a season, prune aggressively again, and then move next season) or I'm going to have to enlist professional help, which I hoped to avoid, as I'm kind of a DIY gal.

Thanks so much to both of you for your advice.

I'm not sure you'd find it helpful to hard prune, wait, prune aggressively then move.

It's true that a healthy, well established rhododendron will usually respond successfully to a hard renovative pruning and come back after being cut quite low - although that practice is not without some risk. But pruning a second time to move? You're sending plant energy in two directions again...establishing roots into the soil, and sprouting top growth. Most of the weight is going to be in the root ball whether you move before or after pruning, and moving it won't be a job one woman or man could do with most mature shrubs. An 8' rhododendron cut back to 2' is going to have a diameter root ball that coincides with its original height :)

You would need a few strong friends, a tarp or large sheet of cardboard, and drag to the new location as mainegrower suggested. Easiest on backs, and on the root ball which could pull loose and be damaged by its own weight. After loosening the root ball from the ground, you would rock/tip the plant to one side and slide the tarp under, you (and your friends) might find that easier with better 'handholds' left by not pruning.