When green turns to yellow: the continuing saga of "oak chlorosis"
Whoever chose the colors for traffic signals must have been a forester at heart. Green equals go, yellow sounds a caution of upcoming change, and red indicates a cessation to your system. As it is on the highway, it is in the garden and the forest. Yellow signals "caution, change en route". Yellow foliage is a sign that the chlorophyl molecule, so critical in photosynthesis, is breaking down. Yellow signals caution, change en route, because the end of photosynthesis is the end of energy "capture" by the tree and the beginning of the tree's living on stored reserves. Left uncorrected, "living on reserves" is the first thing an "unhealthy" tree does.
For that past few years in the Metropolitan Region, we have been seeing oaks turn yellow in the second half of the summer. We discussed this in the newsletter last year, but had no answers, just some "informed speculation". Well, it's back. Oak, particularly white oak, is going chlorotic all over the most of the southern and western parts of the region. If the pattern holds, we expect to see increasing numbers of other oaks exhibiting chlorosis as July moves along. This is the time of the year that oaks, having grown new leaves, shoots, and wood for the year, begin to stash starch for future use. It's really very simple, chlorotic trees don't photosynthesize. They can't. The chlorophyl molecule is "missing-in-action". These trees are in trouble. As the years go by, another "friend" of ours shows up to dot the i's in the sentence. Two-lined chestnut borer, the bane of weakened oaks, starts working over these trees, normally causing death of the branches in the very tops of trees.
We have for several years, noticed that this syndrome, nicknamed QQS for Quirky Quercus Syndrome, was largely limited to urbanized settings with 85%+ of the trees sitting within 100' of the footprint of a house, garage, driveway, or street. When pH is tested, the soil under the trees is invariably over 7.0 (ie. basic soil). The pH values under chlorotic trees is in excess of 7.5 with severely chlorotic trees having a pH of 8.0 or greater. A lot of factors go into the making of a high pH soil: leaf raking, grass catching, sod liming, watering (most cities have water supplies with a pH over 7.5, some as high as 8.5 to 9.0), road salt, and concrete foundations and driveways just to mention a few.
All is not lost in the "urban forest"; there may be some solutions and relief out there. In the long run, we should look to maintaining the natural soil acidifying factors as possible. Mulch the grass clippings and fall leaves back into the sod. Never lime sod under trees. Grow trees in mulch beds where any watering that you do filters down thought he decaying organic materials contributing the natural organic acids released by decomposition to the soil under the trees. Avoid lawn care chemicals near trees. This doesn't affect pH particularly, but does relieve "other low-level stresses" and is generally a good idea.
An innovative solution in a more active vein is being developed by Rainbow Treecare of St. Louis Park. Still in experimental testing stages, the regime is showing considerable promise at "curing", at least temporarily, the chlorosis and returning good growth to oaks. They are still working on details of timing, dosage, and general efficacy, but the technique involves the therapeutic application of several techniques including: stem injecting iron compounds, root zone soil applications of iron sulfate compounds, and the introduction of high pH tolerant mycorrhizae. All of these treatments have the effect of lowering the soil pH or of "developing" a high pH tolerant system. They look very promising in initial trials with treated pairs showing chlorosis in the control (untreated) tree and dramatic chlorosis reduction in the treated tree. The "proof of the pudding" will be seeing the results when the untreated "control" tree is treated. Does it recover? How long does the "effect" last? Stay tuned.