Issue 4, May 27, 2014

When It Comes to Yew, You Had Better Know This!

Yews (Taxus sp.) are a common woody ornamental staple in the yards of homeowners across America. Yews are used as hedges, windbreaks, topiaries, barriers, and more as their moderate growth rate and easy maintenance make for a relatively maintenance free implement in landscaping designs. This very prominent figure in homes across America has a finicky nature, though.

Yews must be planted in well-drained soils, as they do not tolerate overwhelming amounts of water present in poorly-drained or over-saturated soils. Yews planted in areas of water-logged soils are often said to have "wet feet." One of the main symptoms of a yew with wet feet is edema. Edema comes in the form of bumps or blisters on the epidermis of the undersides of needles that turn tan and corky. These bumps are the result of ruptured cells. 

Edema on yew, exemplified by the dark spots on the undersides of the needles. Symptoms of "drowning" demonstrated by necrotic needles.

In poorly-drained soils, yews can also be victims of "drowning." Without enough air, the roots begin to die off and rot, which in turn affects the above ground growth. Symptoms of drowning include needle discoloration and death. When these symptoms are observed with edema, it is understandable to suspect a pathogen-caused disease. However, these symptoms are all due to environmental problems.

Remedying over-saturated soils may be as simple as reducing watering or diverting a rain spout run-off, but fixing poorly-drained soils is not as easy. If you are planning to place yews in an area that accumulates water, improving drainage before planting is critical. Other suggestions include planting the bottom of the root ball no deeper than 8 to 10 inches below the soil surface.

All this being said, yew care-takers should be aware that in extended periods of dry weather, yews will need additional water. Clearly, it is easier to maintain yews in soils that are too dry than too wet.

Along with edema and needle dieback, yews that are stressed due to water conditions are also more prone to infection by pathogens. There are a number of weak fungal pathogens that can affect yew, including Pestalotiopsis blight caused by the fungal pathogen Pestalotiopsis sp. This disease is favored by unfavorable growing conditions and stressed plants.  Compound the cool, wet weather from last week on top of the hard winter we had and Mother Nature has brewed up a great recipe for Pestalotiopsis blight. Symptoms of this disease are similar to those of drowned yews: needles become discolored, changing from green to yellowish, to brown, normally starting at the base of the plant where the foliage is most dense. The discoloration will begin at the needle tip and progress towards the base of the needle. 

Though this disease can kill small twigs, it is usually considered a relatively minor disease because infected twigs can be pruned and discarded to prevent further disease progression. Pestalotiopsis blight can also be avoided by keeping yews as stress-free as possible, including minimizing winter injury caused by dehydration, removing accumulated snow near the base of the plant, and allowing individual yews plenty of sunlight and air flow by not overcrowding them.

A popular choice for use in the landscape of homeowners across America, yews are appealing to the eye. Understanding their finicky nature is essential to keeping them healthy and beautiful. (Chelsea Harbach & Diane Plewa)

Chelsea Harbach
Diane Plewa

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