Creating a billowing, cloud pruned hedge out of an existing hedge is one of the most exciting projects available to the creative pruner, as well as being relatively simple and surprisingly quick. Yew (usually Taxus baccata) is the standard for this sort of project, and occasionally box (usually Buxus sempervirens) is used on a smaller scale, but when following this method the beauty is that you get what you are given. Making the best of it is what defines and highlights the character of the hedge. Any hedge that is healthy is a potential target—even the dreaded Leyland cypress (✕Cupressocyparis leylandii), although that will keep you busy over the summer months, and of course can not be cut back too hard.
As with topiary in general, evergreens are the obvious choice, but in fact deciduous hedge species also have their place. Beech (species of Fagus) will keep its brown leaves through the winter, and even fully deciduous trees can be attractive, creating a ghost-like tension in the winter and often carrying berries or spring blossoms before coming back into leaf. Mixed hedges will also work and in some cases help in the decision making process that follows.
The big question is though, how to turn something so ordered and formal into something so spontaneous and organic? Well, the more irregularities, the better. It can help to leave the hedge unpruned for a year or two before starting, to encourage natural strengths and weaknesses to surface—the more vigorous plants within the hedge will expand, while others might fall behind and weaker plants might actually die out. Mixed hedges will naturally have different growth rates, which adds to the irregularity. A year or two of neglect will raise a few possibilities and almost suggest a form by itself.
Out of context from the rest of the garden, it would be fine to leave the design entirely up to nature, but within the garden, think about how the hedge will fit. Will it be functioning as a screen and should therefore be of a certain height, at least in places? Will it be echoing shapes beyond, perhaps drawing in contours and silhouettes of the surrounding landscape, rather like the Japanese sense of shakkei (borrowed scenery)? Will the hedge be a smooth, flowing affair or a huge, carbunculous monster? With these considerations in mind, it is a good idea to make a couple of quick sketches as a starting point, of what you have got to start with and what you intend—nothing fancy, just simple strong lines, like the hedge itself.
With a collection of tools—shears, secateurs and even mechanical hedge trimmers, set to. Rough out basic forms, following the flow of the plants. Do not be afraid to cut right back into the hedge (you ought to know by now which plants will respond well to being cut back hard and which ones die back), even removing individual plants completely if necessary, to open up the solid volume.
Think not only about height and silhouette, but also depth—will the hedge spill out onto the lawn (like the pastry on a good pie), or should it be more contained? Space permitting, encourage as much variation as possible (it can always be ironed out later if it seems too intentional). Avoid too much repetition, aiming for a natural irregularity, which is, of course, much harder than nice ordered formality. This is where your personal character comes into play—some people are inevitably more cautious, others more confident (not to be confused with recklessness, mind). Enjoy the process and take your time—come back to it over the year, digging deeper than you dared previously. The hedge will almost certainly look like a complete mess by the time you have finished with it, but that is okay, it will soon grow back and fill in any evidence of excess enthusiasm.
To come over the next few days: Making A Cloud Pruned Hedge From Scratch, Tamazukuri From Scratch & Tamazukuri Using An Established Tree Or Shrub.