marina phillips wrote:Snow drift (tumultuous circular wind patterns in general) will be worst at the downwind distance of at two to three times the height of the windbreak.
The suggested within-row spacing of trees and shrubs is 4 to 6 feet; 10 to 16 feet for evergreen trees, and 12 to 20 feet for deciduous trees. For between row spacing, distances of 10 to 12 feet between shrubs, 15 to 20 feet between shrubs and trees, and 15 to 20 feet between trees are recommended. Windbreaks should be at least 100 feet from the nearest building
Is this windbreak for protecting the house from cold north winds? Are you also trying to create warmer micro-climates for spring and fall? ... Still might have room for a row of evergreens in that 21'.