In general, how much space should be left at the top of containers? Or is it basically irrelevant, as long as the soil is a bit below the rim?
Allow for some space for watering.
I use #1 containers and they are very tall for how wide they are. I give them a good 4 inch at the top.
It can be good to plant low if you use potting soil so roots can quikly fill the bottom where the perched water sits.
In this case I only filled half the 5 gal bucket.
The taller the soil column, the lower the % of soil that holds perched water at container capacity - so if you fill taller containers (as opposed to partially filling them), the easier they are to grow in. For example, if a soil holds 4" of perched water and the soil column is 8" deep (in a symmetrical container), 50% of the soil is saturated at container capacity; but if the soil column is 12" deep, only 33% of the soil is saturated at container capacity.
That is some very very good info!!!!!!!!!!!!!!!!!
For display purposes my soil level brings my mulch layer even with the top of the container. Is there something that can be used to fill up the bottom to change the percentage from below? That is without messing up drainage.