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I have this long, very tall leylandii hedge, I guess the taller ones are approaching 40 feet high and one for thick trunks.
Especially the older ones are quite sparse... Fine as trees but this is my privacy barrier from a main road both for noise, and people seeing through.
The other day I saw another leylandii hedge that was very neat and very dense - it looked totally solid, almost fake!
What can I do, if anything, to get a denser foliage?
It seems your trees (yes, Leyland cypresses are trees) were never properly trimmed, more likely planted once and then left alone.
Which means they behaved like all trees planted closely together: Gained height quickly, losing density. And all conifers tend to become bare on the bottom and inner branches.
Now, the "almost fake" looking hedge you are talking about is the result of the exact opposite gardening approach: It requires constant trimming of the outer branches (a bit like just nipping off the tips) to encourage more and more sideward growth which will result in that super dense green wall.
Typically the first trimming should happen in spring, just before or right when the trees start to grow. A second trimming is optional, if you choose to do so, late August or early September would be a good time to neaten up your hedge. Always choose a frost-free period and, especially for the second trimming, ideally an overcast day to minimize stress, both for the tree and the gardener.
Unfortunately, your options are limited: Unlike most deciduous trees, the cypresses won't regrow if you cut back to the bare branches. If you want to keep your trees, you can start to trim the sides, but as I said, do not cut into the bare wood. The hedge will become a bit denser, but especially the older trees will never give you full coverage again.
If privacy is really an issue, you might consider biting the bullet and replace the old overgrown tree with new ones. Leylandii grows so fast, even with regular trimming you should have the desired green wall in just a few years. Especially if you buy larger (=pricier, unfortunately) new plants.
And if you are looking into replanting, you could also consider other hedge plants - just an encouragement to take a unsatisfactory situation and turn it into an opportunity.
Everything that Stephie said is fine. However, the trimming would most likely work for the right side of the hedge (right side of the last photo), but for the left hand side, it would be a difficult uphill battle.
Simply, there is not enough branches with green leaves in that area that would be able to produce desired new growth, no matter how often you trimmed them.
My advice is to consider an alternative design that would produce the desired screen effect with much less pain and much more probability.
Such as this:
(smaller blue circles are young piceas, larger represent how big they will grow)
Picea pungens would provide incredibly good windbreak and optical insulation. Leylandiis would, on the other hand, serve as an excellent background for piceas light bluish color. You would get more interesting area in your garden.
But this time, it would be desirable to trim piceas from time to time, to get denser foliage (so that the same mistake is not repeated).
Additional comment: Judging by the pictures, if the height of leylandiis is around 40ft, the distance between centers for them is 3-4 ft. this is fine for the low hedge, but if the trees are left to grow without intervention, as it was more or less in your case, that distance is way too small. The leylandiis suffered! This is one of reasons your hedge doesn't look so good. Note that I placed piceas at much larger distances between them.
