Ask Mr. Smarty Plants

The first thing we do when we get a question about a particular plant is to determine its nativity. At the Lady Bird Johnson Wildflower Center, we promote the care and propagation of plants native not only to North America but to the area in which they are being grown. We learned that the genus name for mock orange is Philadelphus, a member of the Hydrangeaceae (hydrangea) family. There is no member of this genus native to West Virginia, according to our Native Plant Database, but two, Philadelphus inodorus (scentless mock orange) and Philadelphus pubescens (hoary mock orange) are native to Virginia. Close enough, as far as we are concerned. If you purchased this plant commercially, however, it's more likely that it is Philadelphus x virginalis, the "x" meaning it is a hybrid. We ordinarily don't consider hybrids true natives, because of the difficulty of determining their parentage and knowing how the hybridizing affected the qualities of the plant. In this case, we don't think that matters so much, as you are basically asking how to prune a particular woody plant. We found this website from Ed Hume Seeds  Mock Orange, from which we quote a paragraph on Pruning the Mock Orange.

"This is probably the most important step in the care of any mock orange variety. On an established plant one should prune out about one-third of the old growth. On newly planted shrubs, wait until after the second year to do any major pruning. Then at that point it is a good practice to remove old, flowered out wood each year after the plant has finished flowering. Newly developed shoots should be thinned, encouraging those that add best form to the structure of the bush. This pruning will help keep the plant healthy and vigorous and at the same time will confine its height."

Your question concerned pruning to encourage bloom. Of course, pruning the old wood away and clipping off smaller shoots will encourage more bloom on new wood the next year. One thing we might mention about encouraging bloom on a woody plant is that you should be careful to use a fertilizer that is formulated for roses, and avoid high nitrogen lawn-type fertilizers. Too much nitrogen will cause an abundance of leaves and divert energy from blooming. 

Philadelphus inodorus

Philadelphus pubescens