In the previous installment of this series, I wrote about spacing plants appropriately (or not). Courtesy of my commercial neighbors in Palo Alto, here's an example with a smaller species, where tight spacing feels entirely appropriate. Betula nigra, or river birch, is one of my favorite trees — its exfoliating cinnamon bark is visually arresting, and its canopy provides delightful dappled shade. In this cluster, the trees within each trio are planted about 14 feet on center, and the trios are about 19 feet apart. This grove would be a wonderful spot for a bench, fountain, or other lingering point. The biggest downside would be the tree roots: birches are notorious for surface rooting, which can heave a flagstone patio or invade a lawn. Placing a feature inside this grove would be a short-term proposition; and planting these trees close to a hard edge could compromise the hardscaping.
This brings me to another point: when reference books talk about surface rooting — or any characteristic, for that matter — they're not just using up ink. No matter how good your intentions, you will not be the exception to the printed rule. For instance, if a tree is reported susceptible to verticillium wilt, and your soil has verticillium fungus, that's not a good tree for you. If a tree is said to grow 40 feet wide, you'll have a hard time keeping it at 20. And if a tree is said to root close to the surface, that's not a good choice next to a lawn, or next to hardscaping. Here's a Fraxinus (ash) that quite owns the lawn around it. It's a big tree, not a bad choice for screening the parking lot from the building, but definitely incompatible with turf. Can you imagine trying to mow around this without scalping the roots? If you were the maintenance crew, wouldn't you stop trying?
So, given that Fraxinus roots do this, would you use the same species as a street tree in a 4' wide planting strip? Perhaps it'll be fine, since the roots surface in an effort to find water and the sidewalk offers none. Me, I wouldn't take the chance. And given that this is in Palo Alto, whose choice of Liquidambar as street trees I curse every autumn (usually after twisting an ankle on one of the ubiquitous seedpods), I don't have high expectations.
But I'll keep watch… and learn from the big guys.