Ours grow directly behind the chicken coop ( slightly down hill ) The rain wash feeds that back area. I assume its nutrient rich soil. If you look at the furrow I dug you can see how dark brown the excavated soil is. My soil is nasty yellow clay. Chickens have been repairing it just by having the coop up hill.
We have them in the ground. I did add 2 year old farm compost + little bit of organic blood and bone meal in the furrow. Probably did not need it as my chicken coop / run is up hill of it and the runoff is probably more than enough for Sunflowers. Last year they grew great in the plain old native clay. The feeding of those fertilizers are minimal and for sunflowers is a once a year process. Not to waste the effort once the stalks are set ( 3+ foot tall ) i'll plant beans as well with them to trellis.
🌻GO BIG GROW BIG🌻At the very least you'll have plenty of seeds and a beautiful background 😍💯 , Idea just in case you haven't thought of it already , grow contest variety plus multiple others and make a sun flower garden slash perimeter or something of that nature border your Tree line anyway you got this I have 💯 faith in you as always, talk to yas later bro , be well BILLY and FAMILY , Enjoy Everything 💯 ✌️❤️ VANDAL OUT 🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻🌻