Garden Detective: Topping arborvitaes
Arborvitae used as a hedge lend themselves to top pruning: just 2 feet in early spring. Credit: iStock
You can safely remove up to 2 feet off their tops. Make the cut near a lateral branch. It's best to do this in early spring before new growth begins, so move quickly.
As a general rule, rhododendrons require at least some shade, with the larger-leaf varieties having less sun tolerance than their small-leaf counterparts. Roseum elegans is not a small-leaf variety, so more sun protection is required. Are the ailing plants more exposed?
I also wonder if you might have fertilized a nearby lawn, because nitrogen could force the plants to put energy into growth instead of flower-bud production.
They also could be suffering from chlorosis, which can occur when the shrubs, which thrive best in acidic soil, are planted in soil that has a pH above 6 to 6.5, rendering the plants unable to absorb iron. A simple pH test will help you determine whether that's the case. Plantings near concrete walls, foundations or driveways, which leach pH-raising lime into soil, can develop chlorosis, as can those in slowly draining, heavy clay soil. This results in yellow foliage, as you're reporting.
If the soil around the poorly performing plants tests alkaline, you can amend the soil with sulfur, iron sulfate or ammonium sulfate -- following package directions precisely. It's imperative you only do this if a soil test indicates it.
