Issue: June 14, 2003

Watering tree through the bark?

I had a conversation with a nurseryman in Santa Fe yesterday. He promotes providing supplemental water to stressed trees by simply watering the exterior of the tree, particularly the bark. He says water can penetrate through the bark into the cambium, where the water will be distributed throughout the tree, providing additional internal water over and above that taken up by the roots. Could you please comment on this interesting concept? George Santa Fe

Although it may be possible to get water into a tree in this manner, I think it would be very inefficient. There are a number of barriers to water uptake by plants in this manner. The bark will absorb water, and only after the bark has moistened through to the phloem layer underneath will water be available to living cells for uptake. Most of the water absorbed by the bark will be returned to the air by evaporation and not available to the plant. Some plants, such as pinon, contain resins in the bark which will prevent water ever reaching the living cells below the bark, so almost all of that water will be lost to evaporation. Other trees with thin bark have waxy coatings on the bark. These coatings will cause the bark to shed rather than absorb water. In either case this seems like a very inefficient way to water trees. Another factor to consider is that the layer of cells just inside the bark is the phloem layer which carries dissolved sugars and other material from the leaves to the roots. Thus, even if water is absorbed, it must go first to the roots. The roots are designed to absorb water, so it makes sense that the best way to get water to the plant is through the roots. Water from the roots must go through a special "active uptake" process which limits what minerals are absorbed with the water. This is for the good of the plant and also suggests that water uptake is best done by the roots that are designed for this. I also checked with New Mexico State Forestry Division Urban Forester, George Duda. His comments included: "If that were true, various pathogens could enter the tree with water (rain), and perhaps we would not have trees at all. The purpose of bark is to keep things in and keep other things out of the tree (quoting Dr. Alex Shigo). That includes water. Watering with domestic water (ground water), directly on a plant, can introduce the various minerals in the water to the surface of the plant. It is not uncommon to see trees with the whitish cast on the bark, indicating mineral deposits. With our typically low humidity, there isn't much time for water to hang around long enough to do anything but evaporate." In summary, I think it is most efficient to use the plant part designed for water absorption to provide water to the plant. This means that we must irrigate the trees properly (at and outside the dripline of the tree, not inside the dripline). In times of water shortage, there are many people looking for more efficient ways to provide water to our plants. This is good, but we must be sure that what we are doing is right based on the anatomy and physiology of plants.

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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