cursed is the ground because of you;
in pain you shall eat of it all the days of your life;
thorns and thistles it shall bring forth for you;
and you shall eat the plants of the field.
Before the Fall, no thorns existed on earth. After the Fall, because of sin, thorns and thistles grew.
and twisting together a crown of thorns, they put it on his head and put a reed in his right hand. And kneeling before him, they mocked him, saying, “Hail, King of the Jews!” (Matthew 27:29)
The only reason thorns grew is because of sin. The man sinned and as a result, the ground was cursed, and thorns came. The thorns on Jesus' head are a visual reminder of our sin and the curse of it He came to release us from.
He is a good, good God.