This might not be the right place to ask this if not can someone point me to the right forum...

I have a couple of patio trees Syringa meyeri 'Palibin'. Does anyone know what they are grafted onto? We lost one but a shoot came up from the rootstock, when it was big enough we grafted a piece of Palibin back onto it, now it's back in business. More suckers keep coming from the rootball, I would like to know what this rootstock is and how to propagate it so I could make another patio tree.

A......