Get a 20” piece of black plastic flexible perforated drain hose. 4” diameter.
Pick a low spot outside your garden area that is 6” below the center of the garden. Take a narrow tile spade and dig a trench from that point to a low spot in the center of the garden where you can dig a small sump hole. Place the drain hose in the french drain trench and then fill in with small clean gravel to have a french drain to take water out of your plot.
Conversely, you could dig a number of deep small diameter holes down into and through that 18” deep clay and fill with clean gravel to allow your soil to drain. I had a spot I wanted to plant a tree that was always wet and took a post hold digger and dug two 30” holes a couple of feet either side of my tree hole and the tree is now 30” tall.
Your tree is 30 inches tall?