Garden Detective: Topping arborvitaes

DEAR JESSICA: I have about 70 arborvitaes on the edge of my property, ranging in height from 7 to 12 feet. For the past several years I have been wrapping them with 4-inch burlap strips in the winter to protect them from splitting when it snows. In addition, the snow is knocked off with a broom. After several nasty storms over the years, the arborvitaes are looking sad, and I feel they may be top heavy. Can we trim down the height? If so, how much can be done each season and when is the best time to do this? As you can imagine, I am not up to replacing them. -- Laura Elenko, Dix Hills

DEAR LAURA: Generally speaking, it's a bad idea to top off trees. I just want to clarify that before answering, because I don't want to imply that a too-tall maple, oak or white pine, for instance, can safely be topped off to manage its height. It can't, and terminal leaders should never be cut on these trees. But arborvitaes -- and Leyland cypresses -- evergreens that are used more as hedgeline shrubbery than specimen trees, certainly can have their tops shortened.

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.

DEAR JESSICA: Can you tell us the name of these flowers? They bloom before the crocuses each year; these pictures are from March 10. We purchased them a while back and thought they were called "dwarf iris." They are about 6 inches tall. We haven't been able to locate any information on them. --Tony and Angela Pagnotta, via email

DEAR TONY AND ANGELA: Those are iris reticulata, which are commonly called dwarf iris. They can be expected to grow to 6 to 8 inches tall, and yours bloomed right on time, as they typically flower at the beginning of March. As an added bonus, they're fragrant -- but deer-resistant.

DEAR JESSICA: In 2009, I had 12 Roseum elegans rhododendrons planted in a staggered pattern along my driveway (six in front, six behind). They did fine for the first three years, but in the fall of 2012 I noticed three or four of them had few, if any, flower buds, yellowing leaves and no new leaf growth. They get ample sunlight and I watered them with soaker hoses. Should I wait to see what happens this season, or should I treat them in some way? -- Joe DeCicco, Miller Place

DEAR JOE: You don't mention whether the ailing plants are in the front row or in the rear, or how often they are watered. Rhododendrons planted in 2009 should be pretty much on autopilot by now, as these aren't high-maintenance plants. I have never watered or fertilized my rhododendron, and it's thriving beautifully.

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.