try to 'thin' out the plant first, instead of just shearing the outside of it. What I mean, is go to the bottom of the plant or to the very end of the branches where they split, and cut off there, therfore still leaving a large number of buds left on the outside. If plant is greatly overgrown, then maybe replacement is the only way to go.
One doesn't shear Rhodies! Buds set after bloom and overwinter, so if you shear or cut them off you will lose those buds and will have to wait 1 season for the next buds. http://jollydwarf.com/beetleframe.htm this link might help!
Right on the money, but what do you do if the plants can't tell the difference between spring and winter. Im my area(San Francisco) the weather has been like spring until just wednesday.
Plants always know the realitive position of earth to sun by the rotation of the earth's axis, summer days are longer winter are longer. Hand pruning is an art and I suggest to start inside the plant and work out and also buy Felco pruners. You either have the gift to prune or you don't! Hydrangas also need west coast pruning PRIOR to our "frost"
Never use shears on Rhodys...I see shears used on a lot of shrubs they never should have been. I highly recommend that if anyone does not know how to prune properly, That they take a short coure in it or at the very least buy a book and read up on the subject. I have to agree with Powerreel...Hand pruning is an art...and a gift. I have seen to many butchered trees and shrubs out there by people thet don't know what they are doing.
I don't want to sound like I was slamming anyone, here. This is just one of those subjects that really get to me because of all the damage I see in the field. I apologize ahead of time if this offended anyone...
Hedge trimmers are for HEDGES. To many, shrub maintenance means shearing with hedge trimmers. This is from ignorance, historically. Proper ornamental maintenance is done mainly by pruning, using hand pruners, loppers, handsaws, small chainsaws, etc. Hedge trimming, or shearing, properly only belongs on a hedge, in a formal garden, or in maintaining topiary.
If the rhodedendron in question is in drastic need of pruning, it can be done any time, but fall or winter pruning will reduce the number of flowers next spring. Have to balance the need with the consequences.
Mr powerreel's suggestion that the plants can determine day length is valid, but this is not used by most plants in determining flowering. Temperature sequences are more important to the plant. I have a Mohican viburnum that has flowered three times this year, last time 10 days ago because of unusual temperature fluctuations. Also this year saw henbit (a winter annual) germinating in August and then flowering in late Sept when temp increased.
I have used http://www.rhododendron.org in the past to gleen additional information on proper planting, fertilization and pruning. The rhodo is evident in probably every customer I have. Just another source for what it's worth.