Inner tube, or not?

Today, I dismantled my strawberry tree, as the strawberry plants in the upright 'pipe' of the tree all snuffed it. My intention now is to replant it with trailing petunias to make a feature at our front door, but I am wondering if it might be a good idea to change its construction.

Currently, it comprises of a plastic drain pipe (soil pipe size) with a smaller bore pipe in the middle; the smaller bore pipe has holes drilled into it and is then wrapped in weed suppressing membrane; the larger pipe has holes that are approximately an inch and a half in diameter, and again is wrapped in weed suppressing membrane (held in place by cable ties), and there are holes cut in the membrane for the plants to be poked through.

The original idea was to have the compost in the cavity between the two pipes, the plants would be poked through the holes in the outer pipe to be rooted in the compost and then the inner pipe was empty to be filled with water which would then leak out through the holes and water the plants in turn. The outer pipe had four 'rows' of holes cut in it to give all round planting (see my feeble attempt at a drawing below):

Now, the central pipe never did get fully 'full' to leak the water and food out to the compost, presumably because it was leaking at the bottom and into the pot and then out onto the ground; and getting the old compost out of the cavity between the two pipes was a mission and a half today (I ended up using the pressure washer and some brute force, and ended up wearing quite a lot of it!), which has in turn got me to thinking - - do I really need the inner pipe for petunias? Would I be better get shot of the inner pipe leaving much more room for compost, and then fill it with a nice mix of compost and vermiculite (50/50) and then add in some slow release fertilizer granules and maybe even a handful of chicken manure pellets to give the petunias a right roaring start?

Lastly, I have a load of purple petunias and a load of mixed petunias, and I also have the option of making a second one of these trees - - would it be best to have one tree fully purple to make a bit of a statement, and then the other with the mixed (which will be elsewhere in the garden), or would it be better to have both as a bit of a jumble up?

 

Thanks @Sheal - I think I made it worse by not adding anything to the compost at all, hence my thinking that a 50/50 mix of vermiculite might be just the job to maintain moisture and nutrient retention? Watering wouldn't really be a problem as I can either use the 'shower' setting or even the trickle setting on the hose and just water it slowly.