privet hedge

[Cheesy pete]

ok i grow my own veg have done for a while but my garden has high privet hedges ( not interested in removing them currently)

i wish to gain the meter next to the hedge to increase my yield and my garden is not massive and has many other uses currently

 

so thought of the idea of digging a trench a close to the hedge and filling with concrete as a barrier thing is i'm about a foot and half down and the soil is still very dry !

 

I know i know that's what privet does i'm just wondering what depth i need to go to stop the hedge from just growing under the concrete?

 

any idea? or more to the point how deep privet hedges roots grow

 

would prefer a permanent solution rather than a guess