Hi. I have 8 tulip trees (yellow poplar) of which 5 are being attacked by European Shot Hole Borers. I have been advised that it is too late to do anything about this as far as insecticides go but to make sure the trees are well-watered and fertilized so they will be better able to fend off the attack and (maybe) survive. A couple of them are looking very stressed. I am getting conflicting information on when to fertilize them and what mixture to use. Some say to fertilize now (mid-July) with fertilizer such as 30-8-8 to pick up the tree's vitality, others say to wait until the Fall and fertilize with a mixture that is low in nitrogen so the roots can concentrate on growing for next year. In the meantime they are getting killed by the borers. Does anybody have the answer to this. Thanks in advance.
Rule of thumb in our climate is to avoid fertilizing hardy trees & shrubs after July 1. It will encourage new growth that will not harden off before fall & cause significant winter damage due to delayed dormancy. Fertilize in fall (around leaf drop) with a low N fertilizer as suggested if you must fertilize. Tulip trees in our climate are not usually stressed by nutrient deficient soil, but rather by summer droughts. Consistent watering during dry periods will do much more in increasing the tree's vigour.
