Only urgent in the sense that I've just bought a new cordless hedge cutter and must must must make use of this glorious weather. 
We have, what I call, "red sticks" growing in the front garden. Google tells me they are some form of dogwood. But not a tree, they seem to have an enormous root system and pop up independently, not from a trunk or any main branches iyswim.
They are taking over and I need to know how to contain them if you please? RHS website states to cut back to knee height which is fine . But what about the ones that are spreading and growing up through surrounding trees and hedges? Not sure if I can dig them out easily as their are so many other shrubs.
Sorry it's so long, tl:dr version: how do I stop dogwood spreading all through the garden?