Hop Trellis - How Deep?

So I made a pretty pimpified (if I do say so myself, and I do say so myself) hop trellis that extends out over my 2 6' x 6' raised beds.

The design was pretty simple. All 4"x4" posts. The main support is 16' tall, with a 10' cross member at it's peak. Then it's got 2 diagonal 6' beams at 45 degree angles for added support.

My questions is how deep should I go. I put it in a hole I dug last night, but it was only about 3.5' deep. Is that enough, or should I surround the base of it with some concrete? I'll probably go the concrete route just because if this thing fell on someone, it could kill them. I figure 2 bags should be good. I'd still like to hear how deep you guys set your trellis's though. I'll get a picture of it once I run the rope.

All of the 4x4's in my setup are 3 feet deep with no concrete. My structure is all tied together so I don't have to worry about it falling, but if you fill the hole back in with dirt/clay and tamp it everything should work out fine.

Roger that. Three feet is plenty. If you want to use concrete, just pour a single bag of quick setting cement in the hole (dry). Add a small amount of water and let it set up for 24 hours or so, Cover with soil and done. If your soil is damp, don't even worry about adding water, just put the soil in the hole right on top of the dry cement. If you don't use cement, just tamp soil every few inches and you will end up with a very sturdy post.