Chlorotic oaks: an August update
We have been talking about a problem of chlorotic oaks, particularly white oaks, for over a year now. It's not the "Iron Chlorosis" that is so well known in pin oak (Quercus palustris), a non- native oak sold in Minnesota nurseries. This chlorosis appears mostly in urban areas, within 100' of the house, street, or driveway, mainly in areas where the parent soil material is strongly
calcareous (high pH, ie. basic), and where local water systems (used for irrigation) commonly have a pH in excess of 8.0. Since our last newsletter, two things have happened.
First, it has started to rain, heavily. In fact, over much of the problem area, July rains were three to four times normal. July rain in the Twin Cities was almost THIRTEEN INCHES !!! This has clearly helped the trees by providing adequate available moisture during the critical photosynthesis month of July. Moreover, this moisture is "pH balanced" (sounds like a shampoo commercial, eh?) for the job at hand. No more high pH "irrigation" water. Far from returning trees to normal, but it has clearly helped.
Second, in the last newsletter we indicated that a commercial arborist (Rainbow Treecare, St. Louis Park, MN) was experimenting with an injection system using iron compounds. Trees injected last fall had shown clear improvement this year, so mid-summer (early July) injection was tried with mixed results. First, the material seems to be very metabolically active. It was quickly translocated within the tree and seemed to concentrate in the "most active, greenest leaves of the tree, which were summarily shed by the tree over a ten to fourteen day period. Ooops! Second, the most chlorotic of leaves and trees quickly began to "green up" and two to three weeks after injections were even beginning a new flush of growth with "green" leaves. As of mid-August, summer injected trees were looking decidedly "better" in that they were much greener than before ( and presumably undertaking photosynthesis).
The final regime isn't set yet, far from it, but it looks as if mid-summer injections may be too metabolically "hot" for the oaks to tolerate with any grace (the local teenager would say "its just way too cool for the room"). Late summer and fall injections would appear to be more appropriate, but I can't help but speculate if a mid-summer injection of a dilute solution might be useful in attempting to save an individual tree that has serious crown dieback and residual foliage that is more-or-less, totally yellow.