When and how does one prune these? I have two at the entrance to my home. They are the type with a long trunk and branches just near the top. They become top heavy in rain and snow and have to be supported at this time. They have been in place for 2 years. Soil is clay and they are in sun.
The hydrangea you have, on a long trunk, is a Hydrangea paniculata which is the only species that can be pruned into a standard (think “standing”) or tree form. The flowers can become heavy with rain or snow, causing them to droop. Some people like the look of the dried flower heads on the plant over winter. If this doesn’t suit you, though, you can simply cut them off – if you simply want to tidy up the plant, cut below the base of the flower.
This type of hydrangea produces flowers on branches that are produced the same spring (known as new wood). That means you can cut the branches back in fall, winter or in early spring. I find it’s easiest to do in spring, when you can see the new buds developing. Cut each branch back to just above a set of buds, without leaving a long stump. If you want the crown of the tree to stay compact, then cut branches back closer to the main trunk (but not all the way). If you want the crown to be larger, cut off less of each branch. Except for removing clearly dead stems or crossing branches that might rub other branches or crowd the crown, you don’t really need to prune H. paniculata at all, and it will still flower.
If you want to retain the tree form, however, do not cut off all the top branches, and avoid damage to the trunk. Occasionally, you’ll get a sprout from the base or side of a tree-pruned hydrangea, too, and these should be removed.
Here’s a link to a very good website, all about hydrangeas, including pruning tips and photos. It’s called, quite fittingly, Hydrangeas hydrangeas.
Please let us know how you do with your hydrangeas, and if you have any further questions. Happy gardening!