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?

Are you willing to sacrifice some flowers to prune them now? If not, then wait and prune them after they bloom in the spring. Another choice is to lightly shape them now, not removing all that you want, but enough to get by until after bloom time. Depending on what kind you have (and I know you're going to try and figure that out), viburnums generally flower on the branch tips, so be aware of what you're cutting off. Are they too close to the house and brushing up against it?

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 ...

bazinga.. agree .. look at what grew.. since last years trim ... those stragglers up high.. look like they grew 2 foot this year ... grab one.. and track back to see where it was pruned before ... with that annual growth rate ... you are going to be hard pressed to keep it tamed ... you can try ... you wont kill it.. just go to town on it.. and with no insult to the root mass.. dont be surprised if in 2 years.. its the same size ... probably another example of a homeowner planting a plant.. in a space.. for the size it was when bought .. with no real concept of its vast potential ... get rid of it.. unless you just want to learn how to prune ... ken