March would be the ideal time to do it, but if you feel the need to do it now, do it. I would first take out interfering branches and any that are dead , damaged or deranged.That would probably be enough this year. After bloom next year you might do dome more pruning.

Viburnums are cane growers, renewing themselves from the base so attempting to arborize them may not work out that well. It depends on what the structure is of these individual specimens. The straight shoots many species produce, often in a criss-crossing pattern also limits the potential for ending up with an attractive "tree" after doing much thinning.

The former horticulturist at the local arboretum had most of the viburnum collection limbed up some years back, with terrible results. As a group these do not lend themselves to such treatment but it is possible to do it with some specimens of certain kinds, such as Burkwood viburnum that may already produce tree-like specimens on their own. Just remember that there is a tendency for new canes to come from near the base, repeated removal of these in order to maintain a tree shape is liable to result in a decline of the specimen eventually - it is being prevented from replacing old stems with new ones.

most likely .. improperly chosen stock .. planted too close to the house ....

in other words.. the cute little babes planted a decade back .... are going to eat the house... unless you are willing to beat them back with a stick once or twice a year... forever ....

its hard to say without a proper ID nor a picture... but my guess is... they should be removed and replaced... with more size appropriate stock ... planted a proper distance from the house

frankly.. you can do so much better ... in regard to the visual impact of your house ....

ken

Depending on the species and the cultivar, it sounds as though you have little to lose - if it doesn't work, you are only out a year or so, plus, you will then have some idea as to what they are, and can decide whether to do a renovation pruning to the ground or get rid of them, as Ken suggests, replacing them with more suitable cultivars at a suitable distance.

I am usually in favor of waiting for a year before doing anything major - other than getting rid of obvious weeds - in a new-to-you garden, since you don't know what the previous owner(s) had planted - sometimes there are hidden gems.

I would try limbing up - taking no more than a third of the leaf canopy if at all possible, since more might well spur a heavy growth of suckers - the root system is obviously large...

I have a V. opulus 'Sterile' that I WILL limb up this spring/winter - I have been going to get to it for the past 2 years. It seems to be growing from one main trunk, with little suckering, so seems an ideal candidate. Time will tell.

Again, viburnums are cane growers so new growth from the base is not suckers in the usual sense. Not all shrubs have the tree-like growth habit of kinds like Japanese hollies and rhododendrons, where the lower branches can be removed without it affecting the future growth of the shrub. If a viburnum moves into a phase where most of the new canes are coming from above the clear stem that is being maintained through pruning off basal or low sprouts then it probably won't matter. But if one tries to produce major new branches from the base and these keep getting snipped out for years then the shrub renewing and maintaining itself is being interfered with.