My Japanese birch is suffering die-off in some of the smaller branches and twigs. This is concentrated in the lower and interior portion of the tree. There's a sparse area where dead material has been pruned away.

The tree is planted in the lawn area next to the driveway. It has southern exposure, receives moderate watering with the lawn and is fertilized intermittently with tree fertilizer.

It should be noted that a similar Japanese birch is planted 15 feet away and is doing very well. What is the problem and can my tree be saved?

– Marvin Shaver, Sacramento

According to UC Master Gardener Bill Pierce, you should inspect the tree for signs of insects or disease. Look for symptoms such as wilting or sawdustlike deposits from borer activity.

Perhaps it needs more water than it gets from the sprinklers. The concrete area next to the tree is limiting its root area. With a limited root area, the tree's roots cannot take in the large quantities of water that birches need.

Dig down and see how deep the soil moisture is. The tree may need slow, deep waterings.

Did you use a weed-and-feed type of fertilizer? Birch roots are close to the surface and they will take in the herbicides in this product. That can cause dead branches or even kill the tree.

Thank you for your column in The Bee about iris (July 2). It answered some questions that have bothered me. However, one major question was not answered. When should iris be dug up and replanted? Is it based on the calendar (certain time of year) or condition of plants (they don't seem to die back enough for that) or what criterion should I use?

They haven't been dug up for some years, so they probably should be attended to now.

– Melvin Shore, Sacramento

Traditionally, bearded iris are divided and replanted in August in Sacramento. If the tops die back, they also can be dug up in July. Or you can wait until September.

According to the Sacramento Iris Society, you should divide them every three to four years to get the most bloom. When they get too crowded, they stop blooming or produce few flowers.

Irises bloom off the new plump rhizomes; a node that bloomed last year won't bloom again. So cut off and discard the shriveled-up sections of the rhizomes and replant the healthy, newer roots.

When you replant, dig a trench about 8 inches deep and dig compost or aged steer manure into the bottom of this trough. Put some soil over that bottom layer and spread the rhizomes across the top, so they'll be barely covered with topsoil (less than an inch).

Then, water thoroughly. That will get them off to a good start. In the spring, give them a little sprinkle of bone meal and they'll be happy.

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