Q: My 15-year-old mulberry tree provides wonderful summer shade, but it’s growing up into the power lines, and my neighbor says its roots are damaging his underground sprinkler system. Can I prune the roots to keep them out of his yard? Should I replace the tree? — Sandra Todd, Los Gatos, Calif.
A: Garden diplomacy between neighbors is always delicate. And in this case, responsibility for solving the problem rests with both sides. Your mulberry’s roots didn’t cause the problem — at least, not at the very start. It’s physically impossible for roots to break into pipes or to drill through them to get at water, but they will follow leaking water to its source, and then squeeze through even the tiniest existing hole or fissure. Somewhere, somehow, your neighbor’s sprinkler system must have sprung a leak, luring mulberry roots next door for a drink. Don’t bother trying to cut back the roots or digging a trench for a concrete or plastic root barrier. Wherever the water is, those roots will find it, even diving under barriers as deep as 4 feet. First, the sprinkler pipes should be fixed — they are wasting water.
The sad news for you is that the mulberry tree probably ought to go. Many people plant mulberries because they grow quickly; they can also quickly grow too tall (30 to 50 feet in only 10 to 20 years). I’d replace your mulberry with a smaller tree, which will not spread its roots as far and won’t need a periodic buzz cut from the power lineman, either. Since the canopy of your replacement tree will cover less ground, plant it a little closer to the spot where you need summertime shade. Following are some appealing alternative trees. All drop their leaves in winter, like the ulberry. They will take many years to reach a height of 25 feet and can be maintained at that height or lower with moderate pruning.
For exotic bloom you can’t beat a chitalpa, a cross between the eastern catalpa and a desert cousin, chilopsis. This hybrid tree flowers over a long period in early summer, producing abundant orchidlike blossoms. Crab apples thrive in your region, and many varieties make a wonderful small but spreading tree to sit under. Ask a local nursery for a crab apple with fruit that persists into the winter. You probably think of crape myrtle as a large shrub, but it’s easy to train into a small, multistemmed tree. The best are modern varieties developed for disease-resistant foliage: ‘Sioux,’ ‘Comanche’, and others named for Native American tribes. Flowers range from purple through a wide range of reds to white. In the winter, crape myrtles display lovely peeling bark. Japanese persimmon trees are most glorious in the fall and early winter, when the leaves turn golden and the branches bear avocado-size fruit the color of pumpkins.