ISSUE 7 June 25, 2009
HEAVY SEED CROP ON SIBERIAN ELM TREES
This week, I’ve received a couple of calls regarding the heavy seed crop seen on Siberian elm trees. The phenomenon is fairly widespread, as the calls came from Sargent County and Griggs County. On a trip from Bismarck to Fargo last week, I observed the same thing – Siberian elm trees that had crowns with a mixture of brown (seeds) and green (leaves). Both callers had the same question – Does this heavy seed load indicate a massive die-off of Siberian elm? After all, many trees and other plants will bear heavy seed crops right before they die. It’s as though the trees "know" they’re about to die, and so they put all of their energy into one massive, final reproductive effort before they expire.
To be honest, nobody knows for sure why the Siberian elms are acting this way this year. The idea suggested by the callers – massive reproduction effort before dying – is something I suggested last year when I saw the same phenomenon on Siberian elms in Bismarck (Crop and Pest Report, June 12, 2008). A recent phone conversation with the Bismarck city forester assured me that there was no massive die-off of Siberian elms in Bismarck last year.
Two other explanations have also been suggested. The first is that the trees are coming out of several years of drought, and with the increased soil moisture this past spring, the trees are finally able to put their energy towards reproduction instead of just survival. That explanation may hold some truth in some parts of the state, but not all locations are coming out of a drought. The second explanation is simply that conditions for pollination were perfect this year across a wide geographical area. Most elm species have a heavy seed crop every 2 to 3 years.
Only time will tell if the heavy seed crop we’re seeing is a precursor of something worse. In the mean time, a related question arises – Will the elm trees leaf out to their full potential? Many tree crowns appear thin right now, especially with the backdrop of brown seeds. I believe that, given decent growing conditions, the trees will develop fuller canopies as the summer progresses. Elms have an indeterminate growth habit. That means that some leaves are pre-formed in the bud, but others are not. After the pre-formed leaves have developed, new twig and leaf growth will continue straight through summer until the end of the growing season. So, we can expect the crowns of Siberian elms to continue to look better as the summer goes by, as long as we have decent weather.