Question
We have a cedar hedge that was probably planted when the house was built 40 years ago. It reaches 7.5 m (25 ft.) and more in height. We had it topped in 2000, but have now completely lost control of it. There’s a lot of dead wood inside, and the hedge is very thinned in places, especially in the first 3 m (10 ft.). What’s more, long branches are opening outwards. How can I prune it effectively, and with what tool?

Answer
At this stage, your hedge can no longer really be called a hedge, but rather a “windbreak” or “visual screen”. To firm it up a little, you can always trim the sides and top a little, always in the green, with the pruning tool of your choice, but you’d need scaffolding of some kind to reach the top, which requires a lot of effort. What’s more, it’s only natural that your tall “hedge” should thin out at the base: cedar (in fact, Thuja occidentalis, the real cedar is a different plant altogether) is a tree, not a shrub. It’s not its nature to renew itself at the base. In nature, it loses the lower branches when they are shaded by the upper ones, and your hedge, which is thinning at the base, is showing you its true nature.

Pruning at the Right Time
Thuja, like most conifers, doesn’t regrow from the base, or even from the old wood, if pruned too severely: so there’s nothing to be done. That’s why, personally, I advise against using conifers as hedges (with the exception of yew [Taxus], which can regenerate after severe pruning). Only deciduous shrubs, which regenerate completely when pruned in the old wood and which can even be cut back a few centimetres from the ground, if necessary, to see them grow back quickly, really make good hedges. With a conifer hedge, if you don’t prune faithfully every year and at the right time, you lose control, with the result you see.
In your case, I suggest either accepting the current shape of your now slightly open windbreak or, if you have the space, planting a row of shrubs (deciduous, of course!) in the foreground to hide its bare base.
Larry Hodgson published thousands of articles and 65 books over the course of his career, in both French and English. His son, Mathieu, has made it his mission to make his father’s writings accessible to the public. This article on bitter cucumbers was originally published in Le Soleil on October 21, 2006.
My first thought was to trim it approximately 12 inches below ground level, and start over.
These are so nice in the Pacific Northwest. I am pleased that they also perform well for us within our minor region. They are not easy to find in nurseries because they do not perform well farther inland, where most people live, and where most landscapes are. Ours are located where they do not need much maintenance, but when they eventually do, . . . someone in the future may need to replace them, rather than renovate them.