I love big mophead hydrangeas, they do really well in shade or part sun, and get big bright flowers on them. They don’t do well north of where I am, but I’m pretty much at the northern border of where you’ll reliably get blooms every year, so I can grow them.
I always want more, and the easy answer is to propagate them. There are two main ways, layering and cutting. I’ll explain layering briefly but this post is mostly going to be about cutting.
Laying involves picking a flexible cane on the side of a shrub, carefully stripping the bark in about a one inch segment as if you were stripping the vinyl from an electric wire, bending the cane down against the ground, and layering something heavy like a brick on top to hold it in contact with the soil. It will root and thus form a new plant directly adjacent to the old one. This is a good method to create a hedge.
To propagate with cuttings you simply cut off a section at the terminal growth of a cane and pot it up.
Okay, it is simple, but let me give a little more detail.
You’ll want to snip off a section of stem about six inches long with one set of opposing leaves attached. Make the cut on a diagonal to maximize surface area. Dip the cutting in rooting hormone (available at any well stocked garden supply store, Lowes, Home Depot, etc). You’ll want a small pot or other container with premoistened potting mix. You can buy a special rooting mix, vermiculite, or something like that too, but plain potting mix should work. Using a pencil make a small hole, and then put your cutting into the hole, and gently firm the soil.
You now have a cutting in a pot, but your work is not done. Without any roots the cutting will not be able to suck up water, and so it’ll dehydrate. Since plants respirate and lose water through leaf surfaces you need to minimize leaf surfaces. So, there should be two leaves attached to your cutting, cut each leaf in half. It seems harsh, but it is necessary.
The final step is to do something to hold moisture around the plant. You need containment, but also volume, anything that isn’t rigid could hold moisture up against leaf surfaces and promote infection. You can put wooden sticks in the pot to prop up and open a plastic bag, or what I prefer is a clear two liter pop bottle, cut in half. The world’s cheapest greenhouse, and it works. Don’t worry about sealing the bottom, you’re not trying to make it airtight or anything.
Give your cuttings a few weeks to form good roots before planting them up or removing the cone. Then you can share them with friends or use them to expand your own garden.