Garden beds on old drain field

The best place to put my garden beds appears to be on an old drain field with lots of drainage rock, so drainage is clearly high. Actually, most the surrounding area of my house is an old drainage bed because the original owner put septic tanks on every side of the house.

There is lawn growing on top of the drain rocks, but the drain rocks go down fairly deep. As you can see, the main concern is the drainage. I do not want have to depend on irrigation for my home garden bed.

I have access to palms, trees, and branches to use for a hugel bed. I do not have access to topsoil, unless I wanted to buy it which is not an option for me. I am creating 2 meters of compost right now (hot compost with lots of turning so I can get material fast.)

My original thought was sheet mulch the area with pockets of compost to plant in. I do not believe that idea will allow me to plant in it right away if I plan to use hugel beds. I imagine it will be too much nitrogen tied up in the carbon, probably resulting in some seriously slow growing and possibly diseased plants.

I would love to hear your opinions. Feel free to ask questions for me to be more specific

totally not in your climate, and don't have the rocks, but I have gardened in the perimeter of my drainfield extensively. My drainfield is a raised drainfield about 60' long and wide and is raised 4' above the existing grade with a gradual slope from the drainfield to the grade. I have a small lawn circle on top of the drainfield  and it is surrounded by a "toilet seat shape" garden , and north of that another small lawn and more garden...at the hinge part of the toilet seat shape.

south of the two curved areas are paths going down the drainfield and one off the extra lawn.

I have perennials, shrubs and trees planted on all the sides..they grow quickly as they are watered by the moisture coming off the drainfield, however, the lawn on top is tricky as it is so dry on top..so you gotta put  in a lawn that requires very little water..right over the tank I have a lot of sedums as the soil is only a couple inches thick thre.

you can see my drainfield gardens on the blog ..link below..but I have mostly ornamentals and wildlife plants on mine, but you couild grow about anything..I have gobs of daylillies, lupines, gaillardias, iris, and on and on with lower perennials, and then I have roses, lilacs, barberries, spireas, honeysuckles, cotinus, and other shrubs and a few vines over some arbors, have grapes, clematis, cl roses, etc..there are cherry trees away from the base of the drainfield nearer the house, dwarfs, and pear trees and peaches around the sides and front of the house..

IMO you safest best in enclosed raised beds.  You can use pure aged horse manure as bedding material.  Perhaps a kind person will deliver you a load if you shovel it for them.  I have heard of people building beds with bales of hay instead of lumbar.  If you try to sink beds straight into the drainage you are going to have a heck of a time retaining carbon and water.  Good luck!

Brenda, my whole yard is a similar level, aka flat, including the drain fields. Is your drain field still being used? Thanks for the link!

Hanley, my intentions were to create a sheet mulch garden beds with Paul's favorite cardboard to retain moisture. I plan to use palm fronds, dry hay from my yard, straw, manure, food scraps, and maybe some tree mulch as it is so easy to come by. My thoughts were that the above mixture should retain enough water before it reaches the drain bed.

I guess this will be an experiment, and hopefully  positive one