How can ten trees be planted so that each tree is in 2 rows of four trees?

Draw a large X. Extend the feet so its base is wider than its top. Plant four trees at the ends of the X lines and one where the two lines cross. Draw two lines from the two bottom trees up to the two outer top trees and continue the lines till they meet. Plant a tree there.

Four rows of trees, with three trees in each row. QED.