Should i use rocks in a (VERY) large container for gardening? (no drainage)

Question

Alright so, i"m starting a large project and i have two current plans, i"m not very sure which to go with.

I'm extremely new to gardening I'm in coastal north carolina, lots of rain. For my eagle scout project, I'm planning on converting a large 15x5x3 feet Brick fountain into a flower/vegetable garden in my local retirement home. There's no drainage holes except for a large concrete overflow pipe that sits about a foot off of the bottom.The bottom of the fountain is solid concrete. After a heavy rain, water seems to puddle up to about 6 inches off of the bottom of the fountain.

Plan one: Fill the first 6 inches of the fountain with Large rocks, then fill the rest with a few truckloads of compost/soil mixture. my idea is that since the water already puddles that high, it'll keep the soil from becoming oversaturated. (I'm talking big jagged 6 inch in diameter rocks) Instead of the water staying saturated in the soil, it'll fall through and make a pseudo-water-table in the bottom. However, someone on reddit commented there would be issues with the "perched water table" being too high, and the rocks not helping.

Plan two: Just fill the whole thing with the compost/soil mixture.

Any thoughts?

(edit) I'm planting things in the outer, bottom section. the middle is to be filled with smooth stones and a neat statue that's being provided by the establishment. :P

A sub irrigated planter sounds awesome! but i'm not sure how i'd go about getting a layer of air down there, i guess maybe putting a layer of mesh over the rocks could possibly work, but i'm not sure how to go about that without ripping the mesh, that's about 2 feet of dirt that'd be piled on top of it. also when it comes to the other plan of just drilling a drainage hole in the bottom, the inside of the fountain sits about 3 inches lower than the outside, so i'd have issues with puddling.

Idea 3? Fill it with dirt and place pots on top. any input is appreciated, i've never grown more than a cactus before

Answer 1

This looks an ideal setup to make a sub irrigated planter. You can look on youtube for many videos on how to construct these but you've got most of the construction already completed for you.

So, in brief, your plan looks good. But to avoid the perched water table sitting above the rocks, make sure that there are gaps to allow some of the soil/potting mix to form a continuous column to reach down to the base. That way any water from the top will go to the bottom and not perch.

With sub irrigated planters you water from the bottom so you're going to need a pipe that allows you to water the container below, and you need a way to cap it off to prevent mosquitoes breeding there. Some people have floats in the irrigation pipe so that they know how much water is in the reservoir.

Since the overflow is at one foot, and the rain never exceeds 6 inches, I'd just cover the overflow with a wire mesh and some gravel/drainage metal in case it does accidentally get flooded and will then function to drain off excess water.

Answer 2

]3No. Large rocks at the bottom, gravels, sand...none of that will help drainage in fact it will make it worse. Water has to move OUT of the system to keep pulling the water down and out or else it stays and you get anaerobic conditions. Even planting lily pads won't work with out movement of water, oxygenating the water.

I am impressed you knew about perched water tables!

You have to get a drill and make drainage holes, possibly attached to drain pipes to take the water away and down. Ideally these holes would be on the bottom of this major raised bed. Looks like drilling inch or 2 inch diameter holes at the very very bottom of the sides will have to work. Is the bottom of this planter completely level? Does it slope at all? Drill the holes on the side that is lowest at the very bottom without leaving even an inch for water to build up.

Imagine the rain you get, not talking about watering the the plants' roots right now. That rain will not evaporate. It will accumulate and make an anaerobic stinky situation. You have to some how allow any water at all to move down and out and continue down if you are using drain pipe.

Cut the top of the drain down to be the same level as the level of the bottom of that pot, raised bed. You do not want any water able to stand beneath the soil.

After you address that drainage then the only soil you should put in that container is sterilized potting soil with no waterholding gimmicks like sponges or gells and no fertilizer added! Do not add compost, sand, rocks, nothing. Fill all the way up leaving only a few inches from the rim.

I am so impressed you were even on top of perched water tables! You are the first to question that about soil physics.

Only sterilized potting soil. How deep is this 'pot'? I am sure you should be able to drill holes at the very bottom on the sides. Drainage is absolutely critical or all your work, materials, soil and plants will be wasted.

This should definitely make an incredible planter or pot. It has to be treated like a pot in every criteria. Holes made at the very bottom the lower part of the circle square sitting even with the level of the bottom. Draw a cross section and send that?

What are your plans for the 'tray' around this pot? Interesting...or is this tray the part you are wanting to plant...?

The last picture has a pond water feature but if you look at the lower left hand part of that picture are raised planters. The drainage is how I described it. The holes for drainage are extended with drain pipe, out of site and into the street drainage system, not the foundation of the home and not into that pond.