Dr. Grow: Broadleaf burnout: Evergreens show brown, dead leaves but can survive

I have recently received reports from gardeners via telephone calls and e-mail that boxwood, nandina, and even some holly leaves are looking brown. Of course everyone wants to know what has happened and what to do about it.

Most of the broadleaf evergreens (boxwoods, nandina and holly fall into this group) that we find in our landscapes are really native to areas south of here. We have planted them here because of their beauty in both summer and winter. During recent winters the temperatures have been very mild, similar to the winters these plants are used to.

As you know this winter was much colder than normal. Most of the broadleaf evergreens planted in our landscapes are not programmed for the cold temperatures that we experienced this winter.

Broadleaf evergreens do not lose their leaves in the winter. In fact they even transpire (lose moisture through their leaves) all winter long, just like they do in the summer, although the rate of loss is much slower. During our recent mild winters, transpiration has presented no problem. Because soils were not frozen, moisture lost from leaves during transpiration was replaced by soil moisture taken up by plant roots.

This winter we had an extended period of very cold temperatures. For about a month, soils were frozen. When broadleaf evergreens transpired, the moisture lost in the leaves was not replaced by soil moisture because of the frozen ground. The net result was that leaves of broadleaf evergreens turned brown from dehydration. Their response was the same as if they had gone through an extended drought during the summer.

In some cases you will notice that leaves of broadleaf evergreens that were covered by snow did not express the symptoms of browning. They were insulated from the extreme cold temperatures, and did not transpire as much as leaves that were exposed to cold winds.

If you find that your broadleaf evergreen shrubs are showing lots of brown leaves, simply prune them off the shrub later this spring when new growth begins. If the damage is extreme, removal of dead leaves and branches may result in a shrub that is really deformed. In this case you may be better off by removing the shrub and starting all over with a new plant. I have looked at a lot of shrubs this past month. Most of the damage can be removed by shearing.

Since damaged broadleaf evergreens are stressed, you should fertilize them this spring. Use a good tree and shrub food such as 19-8-10 plus micronutrients. Good nutrition plus adequate watering will help the stressed evergreens get back on their feet.

Next year you may want to take some steps to prevent winter burn on broadleaf evergreens. In November go to your local garden center and purchase a product called Anti-Stress 2000. This product, when sprayed on broadleaf evergreens, will plug up the stomata, the pores in the leaves through which moisture is lost by transpiration. Therefore no moisture will be lost during the winter. In addition no leaf browning will develop.