I've been asked about this several times, and it's one of those jobs that is easy to demonstrate, almost impossible to describe.
However, I will try!
This technique applies to small ornamental weeping trees, usually either willow or pear - Salix caprea 'Kilmarnock', or willow-leaved pear, Pyrus salicifolia 'Pendula'. Both of these are usually grafted trees, where the ornamental weeping foliage is grafted onto the top of a short upright trunk.
This leads to two problems: shoots sprouting vertically from ground level, and overcrowded tops.
Shoots sprouting from the bottom are easily explained and easily dealt with. They are shoots from the original, non-weeping, rootstock, which are usually very vigorous, and must be removed as soon as you see them.
In a perfect world, you will spot them as soon as they are tiny green items on the lower trunk, in which case you remove them by pulling them sharply downwards, away from the trunk. Don't cut them: cutting will promote regrowth, the idea is to pull away the base of the shoot thus removing the area producing growth hormones, so it won't regrow.
(One of my Clients swears by wiping saliva on the ripped-off area, saying
that the enzymes in our saliva kill these growth hormones. I'll let you
decide whether to try it or not!)
In a less-than-perfect world, you didn't notice them until they were a foot and half long, in which case you have no option but to cut them off. Cut cleanly, with secateurs, as close to the trunk as you can, and keep an eye on the cuts as they will almost definitely re-sprout from that point. As soon as they do, rip the new growth off.
New shoots coming up from underground, near to the trunk, should also be dealt with as soon as you see them - again, don't cut them unless you really have to - try to clear away the soil until you can see the point of attachment, then rip them sharply away from the trunk or root. If all else fails you will have to cut them, but again you will then have to keep an eye on them for regrowth, and deal with the new shoots as soon as you see them.
Now we move up the trunk to the top.
The principle is to aim for a "light airy waterfall" effect. If yours is more like one of the shaggier muppets on a stick, then it is time to remove some unwanted foliage.
As with nearly all tree work, start with the three Ds: dead, diseased and damaged.
That means, go over the whole mop head, removing any branches which are obviously dead, ie with no leaves growing on them: branches which are diseased - ie fungus, pustules or other nasty things growing on them - or damaged, ie where two branches have rubbed against each other, damaging their bark. Cut out one or the other of rubbing branches - the worst damaged one, or the smaller of the two.
Usually, this will leave you with a massive pile of dead sticks on the lawn, and a significant improvement in the look of your weeping tree.
Now we can tackle the live foliage: there are three elements to this part of the work - balance, length, and thickness.
Firstly, step back and assess the overall shape of the tree. It is fairly balanced, left to right? Move round a quarter turn, and look again, to check if it is balanced front to back. This will show you where to make your first proper pruning cuts, in order to get it level.
Next, look at the distance above the ground - do any of the branches sweep the ground? If they do, they are too long and will have to be shortened. Aim to have all branches swinging clear of the ground: how far clear will depend on the size of the tree, but as a rough guide I like to have mine at least a foot clear of the ground.
Lastly, look at the top of the tree - is it still a dense mass, or can you now see through it? If it is still a dense mass, you will have to thin out the top.
Right, now we move on to the actual pruning! You can do it in any order, but I always do the thinning out of the top first, otherwise you might waste time beautifully pruning lower branches, then realise that you have just removed the whole branch. Then I do the balance pruning, to get it equal left-to-right, and equal front-to-back, then lastly I do the length pruning.
I use sharp secateurs for most of the work, and a small pruning saw for thinning out the top part, where the branches can be quite substantial.
All cuts are made to the same principle, and this is the only "technical" bit that you have to learn - again, it is much easier to show than to explain, but I'll try.
When cutting any weeping tree, always cut out the "underneath" growth, leaving a branch or twig that is "springing" up in the right direction.
Here's a photo showing a branch that has been poorly pruned: the owner just wanted to make it shorter (it's a full-size weeping pear, not a miniature one) so they cut off the branch at random, leaving an ugly stump and a down-ward growing branch.
If you haven't done this before, it can make it easier if you duck inside the canopy of the tree, then you can more clearly see which branches are springing away from you, and which part of the branch is growing straight downwards.
This is how you make every single cut, regardless of where it is on the tree: always cut just below an outward springing branch.
So, first job, thin the top: duck back inside the canopy, and work your way round the tree, taking out maybe three or four largish branches, the ones growing at the innermost part of the canopy. The idea is to keep the outer, freshest, growth, and to lose the congested old inner growth.
Once you have removed a couple of main branches, it should be a lot less congested, and you can step outside the tree to re-assess the balance: did your thinning solve those problems? If not, shorten the branches that are out of place.
Then, take a look at the length of the remaining branches, and trim any that are too long.
By always cutting the underneath growth, you will not spoil the outline of the tree: you will not remove the freshest outside growth, just the tired inner growth: and you won't end up with a terrible "pudding-basin" haircut, but will have a natural, slightly uneven lower hem.
Sometimes people leave them in the hope that as they grow heavier, they
will eventually "weep" but in my experience they rarely do, and the tend
to spoil the look of the tree for months in the meantime, so they are
best pruned off, very close indeed to the branch.
Here's an example of one (right), and the red line indicates where you should cut it.
Here is one I did earlier, as they say. This is what it looked like before I started - not too bad, as I was the one who pruned it the previous year, and you can still just about see through it, but it had grown a bit wayward so the owner called me back for an annual tidy-up.
Most of the branches were lying on the ground, so I went all the way round shortening them.
Then I went over what was left, removing all the many upward-and-outward shoots.
Here is the result, which is much neater and yet which still looks natural.
So much better than running the hedgetrimmer over it...