[scuvnut]

This particular unit handled the rocks without any problem. Roots are a mere afterthough. You can see that I went by a large ceder and it chowed up the roots like nothing. I dug 2 feet deep, but these units generally can go up to 4 feet deep (below the frost line). If you look at the second picture, you'll see the trench is not perfectly straight (and I did use a rope and a can of spray paint to give me a straight line). When you hit a really big rock, it shifts a little, but its pretty inconsequential.

I have rented a trencher three times in the last four years. The first time, I used a walk behind unit and trenched 200 feet. Again, it took down roots and handled rocks quite well, but bounced around alot.

The second time, I rented a ditch witch unit the was a bit bigger - it had tracks instead of wheels and was significantly heavier than the walk behind with wheels (another brand with this setup is a Toro Dingo). This was a great unit, barely bounced around and encountered lots of roots/rocks.

However, I was able to rent the driveable Vermeer unit this time for the same price as the one with tracks. The only differential is that the drive on unit may not work in close quarters.

Hope this info is of some value to you.