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.

hi congrats on your new home ... i doubt.. hedge trimmers will solve your problem ... your inherent problem.. is that those plants are way past their prime.. you need to firmly get a grasp on the fact.. that they will need to go.. sooner or later ... there is simply no way around this ... in the mean time.. you could experiment.. and 'try' to save them ... my best suggestion.. is to get some good pruning shears .... and trace back the floppers.. into the interior.. and cut them off deep inside the plant.. insuring you ONLY cut off the flopping part .. not big gobs of greenery .... the plants will grow back and cover brown areas .. but being trees.. and talking in tree time .... it might make a year or two .... you did NOT make a rookie mistake.. you simply dont like the result .. if you were to give them a haircut.. at height.. for every one cut.. you will make 2 or 3 buds trigger into growth ... and in doing so.. over the years.. create the potential.. for more floppers ... unless you are prepared to shear them 2 or 3 times per year .... and that is either going to take up to 3 weekends per year.. or cost you a lot of money ... i dont know what your checkbook status is.. but if it were me.. having a small budget .. i might invest one weekend in digging up maybe a third of them.. and replanting some new things.. then.. perhaps in fall.. doing the next third ... and planting some more things.. and then finishing the job in the third planting season ... or in the alternative.. i would get really angry.. and just get the job done... slaughtering the whole line.. while cursing whomever planted them in the first place ... never confuse yourself into thinking the prior owner had some brilliant insight in what they created ... it would probably be closer to the mark to say they simply moved because they created a nightmare ... let me put it this way.. you bought a glorious home.. but the kitchen was tiled in fushia ... would you mess around 'living' with that problem.. or would you renovate the kitchen ... it should be no different out in the garden ... time to renovate.. IMHO ... i wish you luck ken ps: and do not think about replacing with those other plants that are so horrible in CA .... and of course.. i am blanking ... FF??

Yikes! That seems a little extreme. The fence is 5 1/2 feet tall. I had heard that you should never remove more than a third of a tree in one season? I don't mind doing the size reduction in stages over a few years.

Right^. The use of the word 'severely' by definition indicates an overzealous attack on the trees. They will-as trees always have-respond to the loss of numerous branches by attempting to right this loss. We call these suckers. Where there was one branch before, now there might be sixteen, all trying to develop around the same growing point. Much does depend on species, but when you say severely, no, that won't work for anything. Might coppicing be an option? That way, you'd end up working with all new stems, not mangled and abused things. For trees which are still basically in good shape but which have been long neglected, the "crown cleaning" that may be needed should be done taking into consideration that the tree has not had work done to it in a long time, and that therefore, they should be brought around gradually. Anything which lops off lots of foliage at any one time is likely to do more harm than good, but many tree species will resprout from the complete loss of above-ground parts.