Hydrangea Offshoot - What do I do Next?

I found this growing under the parent tree so decided to take advantage and pot it up. I dug down about 30cm before I gave up and severed the attaching root. Since then I've kept it close to where it was growing and as it's been in it's pot for about 3 weeks now I'm confident that it seems to have survived. It's not a particularly straight growing main stem but I'm not complaining as I have a free plant ....
What do I do with it next please? Just leave it?

 

I've done the same and just left it to its own devices and it still seems quite happy

 

I had one of these taken from a smallish plant last year. Both shrubs are now 2' x 2' (!) and covered in flowers so it should catch up quickly. I left it in the pot until late winter then planted it out in well prepared soil.

 

That's sounding hopeful if it grows fairly quickly. I have a friend that has admired the parent plant ever since we've lived here (it's in the background of my original photograph) so I want to give it as a surprise (it gets hidden from view whenever they're here at the moment) to plant in their garden when it's an established plant.

 

When I visited a neighbour I commented on his wonderful hydrangeas, he immediately pulled up a shoot and told me to just stick it in the ground and water it in well and to keep it watered which I have..... this was probably only a couple of months ago.... mine now is actually in flower.... just goes to show how quickly they adjust and come on, I can't wait to see it next year

 

That sounds so easy just to put it straight in the ground, I've always taken about a six inch cutting and treated like a baby in the greenhouse, I'll try your method next time. It's so helpful what we find on GC.