Glance up, and you might notice that UT’s trees are encrusted with sinister, green-gray pom poms.
These are colonies of ball moss, or Tillandsia recurvata, which can only be described as balls of curious contradictions.
Rather than a moss, Tillandsia recurvata is a flowering, nonseasonal, inedible herb in the pineapple family. Though often mistaken for a parasite, ball moss is an epiphyte, which means it derives its nutrients from the atmosphere while its pseudo-roots merely latch onto the host tree for support. Ball moss is also commonly found on non-living structures such as fences and power lines.
Branches covered in ball moss often appear to be dying, and ball moss removal services are popular among homeowners. Some people conclude that, by anchoring itself tightly to the tree, ball moss cuts off branch circulation and increases the likelihood of bark rot.
However, Jennifer Hrobar, supervisor of Urban Forestry at UT-Austin, disagrees with this assessment.
“Ball moss prefers to grow in cool, shady areas, which happens to be the interior canopy of trees,” Hrobar said. “This is also the area where branches are overtopped by other limbs and may decline or die, and this is a natural process.”
Additionally, ball moss seeds attach best to dry, dead bark. In other words, dead branches probably precede ball moss.
Ball moss is not actively managed on campus, according to Hrobar, since it is not typically harmful.
“We may control it in some areas when populations of ball moss are heavy (to prevent branch breakage) or trees are already stressed by other factors,” Hrobar added.
This is usually done by spraying products that dry out the ball moss without damaging the trees. UT’s many squirrels, she added, often do more damage to trees with their feeding activities.
Ball moss is ubiquitous in Austin, where, according to Andrew McNeil-Marshall, arborist at the Lady Bird Johnson Wildflower Center, the weather gives it a niche.
“It is too dry here for other Tillandsia species from Texas, such as Spanish moss from East Texas, but the conditions are just right for our common ball moss,” McNeil-Marshall said.
Hrobar agreed, noting that they are part of the ecosystem of Central Texas. Ball moss improves the shade quality of a tree, providing shelter for many local creatures. Additionally, the “moss” itself is home to several insect and bird species.
“In this way, the ball moss is contributing to the general fauna of the tree,” McNeil-Marshall said.
However, when evaluating an urban tree, there is often more at stake than just health and ecological value.
“Aesthetics are obviously an important consideration at the Wildflower Center and for all tree-rich campuses,” McNeil-Marshall said. “We may also work at times to remove ball moss from high-visibility or high-profile trees simply for a ‘clean’ aesthetic.”
He hopes that when people understand that ball moss is a naturally-occurring feature of the landscape, they will come to appreciate its presence.