There Is A Legend
At the time of Crucifixion the dogwood had been the size of the oak and other forest trees. So firm and strong was the tree that it was chosen as the timber for the cross. To be used thus for such a cruel purpose greatly distressed the tree, and Jesus nailed upon it, sensed this.

In His gentle pity for all sorrow and suffering Jesus said to the tree: "Because of your regret and pity for My suffering, never again shall the dogwood tree grow large enough to be used as a cross. Henceforth it shall be slender and bent and twisted and its blossoms shall be in the form of a cross--two long and two short petals. And in the center of the outer edge of each petal there will be nail prints, brown with rust and stained with red, and in the center of the flower will be a crown of thorns, and all who see it will remember."