Root Barriers - set above or bellow order? I just purchased a feature 18" depth root barrier for a Chinese...

I just purchased a feature 18" depth root barrier for a Chinese elm, burch, jacaranda, and Hass Avocado. The garden supply place that I bought the barrier from said I should install them bellow the grade so I can achieve the barrier as low surrounded by the ground as possible. This video that I found online (link attached) claims that barriers call for to be installed above grade for aggressive root systems. Do the trees that I own fall into this category? What would you recommend?


http://www.centuryrootbarrier.com/video/...
What munificent of Bug is this!?!?
Answers:    It is usually obedient to leave the top snake above ground level. Generally in the order of 4" to keep mulch and such within place.

I'm not thinking that it would matter much for the plants you mention. They are not plants that put roots above ground, lke next to bamboom and such.

Happy ditching.