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Willow Oak - Observations of a Horticulturalist

So, I was called out about a month or so ago to look at some trees that were failing to thrive. The species- Quercus phellos Hightower oak. They were planted in street-side planters. The planters were not the typical 3’ by 5’ sidewalk cutouts, but bigger. There were shrubs, ornamental grasses, etc. in the planters as well. The problem- All but 3 or 4 of the 128 trees had not grown since their installation in 2012. The foliage was yellow, smaller than normal and there was no new twig growth. Guess what? They were covered with leucanium scale. Of course, the populations were much higher on the unhealthy trees. Soil and foliar analyses were done and it looks like things are fine. It is likely that there is insufficient nitrogen in the plants. Not because it is not available in the soil, but because the root systems are not likely functioning well. It is unlikely that the soil has anything to do with why the trees are failing. The trees went in at 4 – 5” caliper. SIDE NOTE: Why do we always need to go SO big??? We are such am impatient species- we want it all and we want it now… But, that is a discussion for another day. It is likely, however, that the size has affected their growth. A plant that size takes longer to establish, making it susceptible longer to any abiotic or biotic stressors. In addition, a plant that big can be “getting busy dying” (from Shawshank Redemption- great movie! See it if you have not already!), and you will not realize really what is happening until they start going south significantly. By the way, all the trees were planted correctly. Some did not show complete root plates, but this was not limited to only the unhealthy trees. So, what the heck is going on? These trees should all be thriving. The soil was specially mixed, there is drip irrigation, and good care given the trees.

So, I put on my “Dr. Dread” hat on, and started asking around. After these discussions with knowledgeable people, I think that during planting the trees were not watered. They were watered after installation. The time they were dry may have only been a day or two, but according to one of my nursery grower colleagues, even if the root balls dry out for just one day they can die. This is one species that he indicates should be “mudded in”. He provided an example of some Hightower oak he sold to a landscaper in Virginia. The guy bought a bunch of 40 gallon, beautiful trees but did not listen to instructions from the grower, and the plants were not watered as soon as they came off the truck. The trees all died.

So, I learned two things on this adventure: 1. Do not let willow oak dry out, and Mud-in the trees until it seems you are overwatering; 2. If you spray a product to control scale, be sure to rinse the foliage off before sending it on to the diagnostic lab. Some pesticide chemistry contains high levels of manganese (potentially imidacloprid). If you do not do this the foliage will read a toxic level of Mn, but it is only on the leaf surface and does not indicate a real toxicity.

I need your help. I want to know if anyone else out there has had this particular issue with any willow oak (or other species for that matter). If so, please email me at and tell me your story. This is one way we can help each other out, and come up with some good recommendations for planting.
So, have a great summer! And keep things growing green out there!


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