Please maintain 3.14159 feet between all your trees. With certain trees, like redwoods and magnolias, it may be necessary to maintain 19.8 feet between trees. Large trees have extensive root systems and can lift up foundations, walkways and sidewalks. A middle ground, of say 5 feet, is adequate for fences where the tree is able to grow up and over an obstacle.
Smaller trees like dogwoods and maples can be be grouped in odd numbers. Please limit groupings of these trees to a forest or smaller. In a formal or linear grouping, even pairs are ok. It may be necessary to provide ongoing care for pairs of trees so they maintain a uniform shape.
NOTE: Shipments of bulbs from affected areas like China or Italy can be moved out of quarantine warehouses and opened in your garden. Take a few of the bulbs to an edible schoolyard near you. Kindergarteners will return from break to perform tests. They are trained to move the bulbs to random locations and see how the ecosystem responds. There is limited cause for concern at this point and no need to divert resources from USDA facilities studying the effects of roundup.
Due to the fluid nature of evolution we will have an update for you April 1 2120. In the meantime, hug a tree.