Would it be possible to grow in the ground? Loosen and amend the soil. It sounds like you grow organically which is tough for a guerrilla . I use slow release ferts. To each his own though.
Im from the midwest the lowlands are always watershed areas for me. The farmers dont plant here because of flooding in spring. These areas have enough ground moisture all year to produce giant trees without ever once watering them. I grow right in the soil after loosing it up 12-16 inches down and 16" diameter hole. I use 20-20-20slow release ferts. The 4-6 months stuff. I put a little over 1/2 cup per hole. My soil is black gold but I have grown in sandy areas that get the same moisture content with out problems. I always try an amend the sandy holes with soil from the area. What ever I find near by.
Since you have a mix of clay and sand it should hold water well. You just need ferts or organic matter , depending on your methodology, to amend the holes.
I only use containers in swamp grows because I have very little concern of them being found. Of course its also impossible to plant straight in the ground. I actually have had more success with 5 gal buckets with many 1/2" holes drilled in the bottom. I fill the buckets with promix .
Because of the height of the 5 gal pail the water doesnt wick completely to the top of the container giving the roots a place to hang out while the ground water level is high (1/2-2"). In the heat of the summer the area drys out the roots find their way out through the holes and anchor the 5 gal pail to the ground. Its pretty cool how hard they hold on. Never had one blow over once they are anchored.
This same technique can be used with 5 gal smart pots but their reduced height and ability to wick water can lead to root rot in a wet year. The roots would also grow right through the bottom of the pot. You can imagine a 5' plant with a 2" diameter stem has a pretty good size tap root( all plants grown from seeds).
Core gardening? I looked it up and maybe I was mistaken on what moisture levels you have. Does your lowlands area contain water?