I have 8 outdoor trees which are going into 20" pots. I can find one half inch pine bark nuggets, but not one quarter inch, unless I buy bonsai bark.

You can use Repti Bark without screening or Agway Pine Bark mulch which requires screening to get the right size pieces.

Have you tried all stores, and garden centers? Any mulch suppliers in your area? Sometimes it's sold as Soil Conditioner. Florida should have it somewhere. Lots of pine trees there. I'm in south Georgia, and just bought a bunch in northern Florida last week. Where are you located?

I used the Agway stuff but did not screen it. Worked fine.

I am in Central PA.

I screen my Agway mulch because about 25 % is larger than 1/2 inch. I have not tried it without screening so it may be OK for me not to screen it.

I live in the Miami area, which is in Zone 10B. Zone 10 B is about the hottest, for the longest period. For that reason, I need more water retention in my potting soil. Yes, it rains a lot. However, in the case of growing in containers, the rain gets the leaves wet, but not much actually gets to the soil. Great for growing everything!

Usually if it rains for a short duration then the container may not get much water, but if it rains hard and long then the container will be saturated from the water running down the branches and leaves. Unless the canopy is very dense I do not see why the container will not get water from a heavy rain.

Although we are way up we do get periods of hot (around 100F) weather. Still I do not need to water everyday for 511 mix, especially if the container is large. Small containers will need daily watering in hot weather.

You can reduce water loss by using large container, shading the container or digging the container or mulching around it. You can also mulch the top of the container.

Experiment with increasing smaller particles in 511 mix. Add extra portion of peat, add more of the smaller sized bark, add a portion of turface or DE (napa 8822 from napa auto parts).