Container Drainage Issue?

[BayArea43]

Planted my first container and first bamboo. 5 Gallon Oldhamii. I transplanted it to a 20 Gallon Storage container for now.

See images attached- Drilled several 1/4" holes, and a 1" hole in the middle. I did not add perlite, or lava rock, or any overflow insert. I just filled it with potting soil.

But I've since noticed that the container bottom sags and all the water seems to go to the middle 1 inch hole and it slow drips like coffee all day. Not sure the other 1/4" holes are doing anything. I'm not sure if there's enough drainage, and I'd hate to have to dig it all out and redo it again.

I just got a moisture meter, it's been 24 hours since last watering, and it shows the soil as "Wet", and that's only 10" down. Bamboo is sensitive to root rot if it's too wet.

Am I overreacting?
Or should I try drilling more holes in the lower sides (not underside). Will that do much since water mostly goes to the bottom not lower-sides?

 

[Marck]

Oldham's timber Bamboo (Bambusa oldhamii) is a magnificnet speciment bamboo. I hope you are able to eventually plant in the ground so that it can reach its full grandure.

That contianer looks like it has plenty of drainage holes, but you could make them a little wider if it will ease your mind.

I am a bit concerned about your watering. If it really dripped all day with that number of drainage holes it would have to be watered to a point of super-saturation daily. Perhaps it only appeared that way because a small amount of moisture seeped out when you checked. Keep your bamboo moist but let it partly dry before performing more irrigation. It will want air to its roots as well as water. Also do fertilize regularly.

 

[Oliver Buckle]

Presumably you want to drill the sides to avoid tipping such a large container. How a bout drilling diagonally near the corners where it looks reinforced so the hole comes out underneath in the edge surrounding the bottom, which appears to be slightly lower than the bottom?