Yes, viburnums are not caning shrubs and so the 1/3 removal method is not applicable. Dirr, in his treatise on viburnums, states: "Viburnums do not require heavy-handed, butcher-shop type pruning. Tidying and shaping with hand pruners is best. Pruning viburnums should be an exercise in retraint". Since nearly all viburnums flower on old growth, any pruning that may be required should be done immediately after flowering. But since 'Summer Snowflake' flowers pretty much non-stop through summer, that may be hard to achieve :-) You may be able to encourage denser growth by cutting back some exterior shoots to right before a node -- this typically will prompt budding of new shoots from the node. FWIW, there is a mature SS shrub at my nursery that is easily 15' tall and about 10-12' wide. I doubt it has ever seen pruning shears nor does it need to. I might look to other reasons why your shrub's growth is rangy and sparse - poor soils, too much shade, insufficient moisture?

There are at least two types of viburnums commonly known as American Cranberry Viburnum. If you have Viburnum opulus var. americanum, prune immediately after flowering (late spring / early summer). If you have Viburnum trilobum, prune in late winter or very early spring before spring growth begins. In either case pruning in "fall after the leaves fall off" goes against ALL expert advise I have ever seen on either of these plants. If you are pruning dead wood or damaged branches, you can prune at other times, but for training and regular yearly pruning, I would stick with the normal recommendations.

sure.. you can do anything you want ... but for each piece cut .. there will be.. probably.. 2 to 4 new shoots triggers.. AT TEN FEET ... and that will become a problem ... how about a pic ... google: REJUVENATION PRUNING OF SHRUBS ... and that ought to get you where you have to go ... its a 3 year process ... so if you did want to cut out one third per google .. and then top the rest ... maybe take down the second one third in fall ... and the last large part next year ... you can renovate the shrub back down to manageable size ... but that all speculation.. w/o seeing your plant... i doubt there are many Vs .. that you can kill by pruning.. that really isnt an issue ... but do understand.. that with no insult to the very mature root mass ... its going to react by growing like mad .. crazy mad ... insane mad ... and you really dont want that up at ten feet ... ken ps: have you thought about how you are going to go to town on it at 10 feet ???? this doesnt sound like it would be easy on a typical home owners 8 foot ladder ... lol.. i have had many great plans scuttled by physics.. lol ...

Since you've had your question answered, no need to repost :-) But in future, just pick ONE of the gardening forums and post there....whichever one you feel most appropriate. The multiple tagging on what used to be the GardenWeb forums - all the gardening forums - doesn't work. Except for Landscape Design.......that seems to be the only GW forum that accepts the Houzz multi-tagging. (long and involved story but GardenWeb was around long before Houzz was a glimmer in anyone's eye and the melding of the two websites since the acquisition has not gone without numerous glitches, this tagging thing still being one of the major ones. It doesn't work and it is just plain dumb anyway)