The Washington Park Arboretum rang in the new year with a series of windstorms that broke limbs, downed trees and dulled chain saws. What the storms didn’t do, however, was cause extensive damage to collections, structures, or visitors. “Lucky” might be your first thought, but luck had little to do with it. Proper tree care and a knowledgeable and observant tree care crew allow us to consider our recent tree ‘failures’ successful.

Our biggest break in January was one of the Hemlocks that line Arboretum Drive. Years ago it developed a ‘double leader’, or ‘codominant stem’ (2 or more main stems with similar diameter that emerge from the same location on the main trunk). Codominant stems can be challenging as the tree grows because the stems push against each other as they grow together, causing deformity that often results in compressed wood and ‘included bark’. These factors can often lead to a weak spot in the tree that may be susceptible to failure.

UWBG Arborist Chris Watson had been monitoring this Hemlock for years and decided to place a cable in the tree a few years ago to prevent any serious breakouts. His decision and placement were both great moves, as this tree did succumb to the wind, but the broken leader remained cabled to the stronger leader and no damage occurred.

Anybody who has ever taken a class in the Arboretum with Dr. Bob Edmonds has likely heard him discuss the fungal pathogen Armillaria mellea, commonly called Armillaria root rot, shoestring root rot, or honey mushroom. Unfortunately we have this pathogen in our soils and occasionally when a tree we suspect has the disease falls, we get a chance to investigate. We (and Dr. Edmonds) suspected this tree had Armillaria.

Tell-tale signs of Armillaria mellea include: White, fan-shaped mycelium growing on the inside of the bark and over the sapwood, soft, spongy and stringy wood that has a lighter yellow coloring, and finally and often most noticeable, black shoestring-like ‘rhizomorphs’ in the dead and dying wood at the base of the tree. Upon investigation of this tree, we did find multiple signs of Armillaria including rhizomorphs, white fans of mycelium, soft spongy yellowed wood, and a column of rot in the center of the trunk that the tree had compartmentalized pretty well over the years.

UWBG’s horticulture staff’s diligent monitoring and tree care regimes turned this failure into a great research and teaching opportunity in our living classroom called the Washington Park Arboretum. Come discover and learn with us.