Yellow globe willow watering problem?

While visiting in Roswell, NM, I noticed that lots of the globe willow trees are turning yellow and dying. My brother-in-law who lives there says it is a common problem for globe willows in the area. I have done a little research on the internet, and it says it could be that they are over-watered which causes insufficient oxygen to get to the roots and this causes iron chlorosis which makes the leaves turn yellow with green veins. Aside from cutting watering back to perhaps once every 15 days or so and adding some iron, do you have any other recommendations?

In soils in other parts of the country, over-watering may account for yellowing in trees by reducing the oxygen level in the soil and thereby altering the uptake of minerals, especially iron. I believe that over-watering can be the cause of problems, but unless all the homeowners and caretakers of the trees you saw were indeed over-watering, there is probably another cause. In New Mexico, the cause of globe willow yellowing is usually the fact that the globe willow can't tolerate the extremely high levels of calcium and other salts in New Mexico soils and water.

The globe willow is also subject to winter damage, especially late winter drought. The effect of late winter damage, late winter drought, and other winter injuries is that nutrients are not properly carried into the leaves, resulting in nutrient deficiency symptoms and dieback of twigs and branches. Insects also injure the globe willow. This may cause foliage problems and dieback of twigs and branches, so insects may cause the problems you observed. There are some nice looking globe willows, but they are far outnumbered by those doing poorly. The globe willow is a frustration for county agents all over the state. It can be a pretty tree but often is a problem tree.

Late winter irrigation (once a month), application of iron and acidifiers in the region of active root uptake (outside the drip line) can sometimes help a tree that is stressed. Proper watering in the growing season can also help. Irrigate once every 10 to 20 days or so (depending on soil water holding capacity) and apply the water in the zone outside the drip line during the growing season. In areas of high levels of salt (sodium and calcium salts) in the soil and water, the globe willow will not grow well and other trees should be used.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

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