Q: On my deck I have two birch trees in big planters, which have done well for many years. Now I’m beginning to worry about them. I prune them every year, but is it time to repot them or prune their roots? — Jay Cohen, Brooklyn, N.Y.
A: Call off the surgery! Root pruning is a nearly impossible job and more likely to hurt your back than help your trees. The root balls are extremely heavy and hard to handle. The mass of soil and roots is hard to get out of the pot, and it’s even worse to get back in. The roots will be heavily entwined on the surface of the root ball, making it difficult to tell which ones to cut and which to leave. Realistically, large trees like birches don’t attain a great age in patio containers. White birches, which tend to be attacked by borers and leaf miners, won’t last as long as Japanese maples or crab apples (although any tree that’s pot-grown will eventually need to be replaced). If you get 10 years out of your birches, you’re doing well.
Birches prefer it cool and moist, so you’ll want to keep them in partial shade and water them well. Because the roots are concentrated right next to the sides of the container, with little extra soil to insulate or retain moisture, hot, dry summers can leave birches stressed and vulnerable to pests. Where winters are cold (20º F or lower), these uninsulated roots will also suffer — much more so than the trunks, branches, and buds. The best way to protect the roots from the cold is to build a larger wooden box around each container, big enough to accommodate 1 or 2 inches of foam insulation on all sides. If your tree shows signs of decline (limbs dying back or many leaves turning yellow prematurely) you can try to rejuvenate it next spring by cutting off the whole thing close to ground level before it leafs out. Birches grow back fast, sending out two or three new brown shoots from the base. Leave these shoots in a clump or prune out all but the strongest one. The “new” tree should reach 3 to 4 feet by the end of the first summer after the radical pruning; by the second year, it will grow to 6 feet. If you think your birches’ time is up, I recommend replacing them with ‘Heritage’ river birches. This species is highly resistant to the common birch pests and more heat-tolerant than other white-barked birches.