How to know if you can revive thinning, off-color plants

I noticed that three of my boxwood shrubs are looking thinner than normal with browning foliage. Some of my yews have a yellow cast to the leaves. This seems to have happened over the last three weeks and some shrubs have brown areas just on the outer leaves while others are brown all over and throughout the plant. Is there anything I should do now?

— Lydia Russell, Morton Grove

I have been observing off-color yews and boxwoods in home gardens like what you are describing and think there are different factors in play for different gardens. Evergreens are usually among the earlier plants to show visible signs of winter injury in the spring. Signs of winter injury include golden or brown needles as well as foliage that has developed an overall yellowish or off-green color. Leaves may appear bleached. The damage commonly occurs on the south or southwest side of the plant or on the windward side of a plant that is exposed. I have also recently seen winter burn on a boxwood hedge with an east exposure. During the cold winter months, evergreens continue to lose water vapor through their leaves (or needles, which are modified leaves). The leaves must replace the water by pulling it up from the roots. But when the ground is frozen, the plant’s roots cannot absorb water to supply it to the leaves. If the weather turns warm and sunny while the ground still is frozen, evaporation from the leaves increases and the water cannot be replaced. Discolored or “burned” foliage will start to appear on the plants in spring as the weather begins warming up. The symptoms tend to show up quickly in spring when days are sunny and warm.

De-icing salts can also damage plants with the degree of damage varying from year to year depending on the frequency and duration of snowstorms that affect the amount of salt used. The salt affects plants through the soil as well as through contact with the foliage from spray kicked up by passing cars or from salt-laden snow shoveled onto plants. The faster the speed limit on the road, the further spray can carry. Evergreens damaged by salt spray develop brown or scorched foliage that is usually worse on the side facing the road. Evergreens that are at the edge of or out of their hardiness zone can also be damaged or killed after a difficult winter.

Simply trim off any brown tips of evergreens that are showing minimal damage. If you are unsure whether the plant is dead, give it more time to see what happens. Scratch a few branches to see if there is any green. If the entire plant is brown and you do not see any life in the lower stems, it is likely your shrub is dead. Some evergreens may be green below the snow line with the foliage above completely killed. If your evergreen has an off-green or yellowish color, it is best to give it more time with an extended period of warm weather. It may green up. Look for new buds swelling along the stems — if you see them, it is a good sign that your plant will recover with time.

Yews and arborvitae that have an overall yellow cast with thinning foliage may be struggling because of wet soil conditions that damage the root system, providing an opportunity for a fungal disease called phytophthora root rot to establish. The wet springs over the past years have contributed to the development of this disease. Work with an arborist to determine if there is a program that can be implemented to save these plants. Action steps might include air spading, drainage installation and fungicide treatments to the root system.

I have also seen boxwoods that seem to be in a state of decline with thin foliage along with dead and yellowing foliage. Generally, these plants have been thin and not growing well in previous years. Boxwoods that have been in decline over the years are not likely to rebound, so it is best to plan to replace them.

The final decision as to whether you need to replace any of your plants will depend on what kind of plant it is, the extent of the damage and how long you are willing to wait for it to recover. For example, a winter-burned yew is more likely to recover than a more difficult-to-grow broad-leaved rhododendron. A boxwood with a yellowish cast to its foliage may be OK once the weather consistently stays warm, while a yew with yellow foliage due to wet soil will likely continue to decline unless measures are taken to improve the growing conditions.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.