flax plant

Question

planting a flax plant in dirt with concrete underneath.

Answer 1

It should be possible to grow flax over concrete, as long as the soil is at least 6 inches deep (and 12 inches deep is better, if possible). These articles will tell you more about preferred growing conditions for flax:
https://www.gardeningknowhow.com/ornamental/flowers/flax/flax-plant-care.htm
http://www.gardening.cornell.edu/homegardening/scene12cf.html