Sunflowers don't like to be transplanted. That's a definite conclusion that I've reached after experimenting last year. So it's better to buy seeds of variety sunflower you want to grow, and sow directly where they are to grow.
That's one probable reason your sunflower didn't do so well. Now, looking at the photos, I'm also wondering how deep this planter is. It almost looks as though it is on the tiled base? If there is no open bottom and soil under that, this is way too shallow for even a dwarf sunflower. They grow an extensive root system. Assume needing at least as deep as 1/3 to 1/2 the height of the mature sunflower.
They are said to prefer rich soil -- like vegetable garden. But it's been my experience that they really grow well in a lawn-turned-new garden bed if you flip the sod in place, cover with newspaper or cardboard, top with a little compost and soil, mulch, then open up spaces for the seeds and plant. (If using cardboard, best to prep at least 2 weeks ahead, then cut an X in the cardboard before sowing so they can grow down below the cardboard.). Tall sunflowers will dig deep with their roots, so they are good pioneer plants to prep the bed for next year's more soil fussy plants.
To sow the seeds, I pre-germinate them. This way, you are only sowing seeds that are ready to grow.
Subject: 2014 pre-germinating/sprouting experiment Peas, Corn, Curcs
Did I mention I'm trying sunflowers and beets now? Started soaking them two nights ago and after full day of rinse and drain, the sunflower seeds have already started to germinate: I already knew it works for sunflowers after last year's experiment, but I had a couple of sunflower seed heads hanging upside down on the end of a curtain rod all this time (completely forgot about them) and I had no idea if they were still viable after being near the window and all.
Still waiting on the beets. Started nasturtiums and cleome.
Looking at the nice unbroken rows of sprouted peas and corn already growing in the garden, I'm really liking how the pre-germinated seeds hardly ever fail to grow where you sowed them.
I'm going to try growing a fence row of sunflowers (along a difference fence than the last time where they were all facing the neighbor's house when they bloomed
). I'm planting them along the fence with the neighbor who is using a lawn service. Hopefully, the sunflowers will help to provide a screen barrier against wind drift, soak up any unwanted chemicals, and also act as warning by indicating any sign of broadleaf herbicide.