My landscaper was grading the soil in my yard and buried the knees of my mature bald cypress tree under 6-8" inches of dirt. Will this hurt the tree? Do I need to dig the knees up and expose them to the air?
Clarification: The tree was near an embankment where the grade dropped off quite a bit. He didn't push dirt up against the trunk of the tree. The knees were about 5-6 feet away from the trunk and popping up out of the soil where the grade was lower than where the trunk enters the ground.
Practically any mature tree will die if you change the soil level around the trunk by 6".
Merely digging out the knees specific to bald cypress will not save it.
See also the diagram here for a possible way to raise the soil level drastically without guaranteed tree death (though the tree surviving it is merely improved odds, not a guarantee, either.) https://gardening.stackexchange.com/a/19928/6806
The literature (especially http://arnoldia.arboretum.harvard.edu/pdf/articles/2000-60-4-cypress-knees-an-enduring-enigma.pdf) suggests that the function of cypress knees is unknown, but does not appear to be aeration of the roots. It seems unlikely on that basis that it matters, as long as the soil level relative to the trunk hasn't changed.
I found some information regarding removal of knees, I assume it also applies to permanently covering them up
According to LSU horticulturists, you can carefully remove the knees without harming the tree: Dig a small area to expose the knee a few inches below the soil level. With a clean, sharp knife or saw, cut the knee off horizontally, 1 to 2 inches below the soil level. Refill the area.
Texas Forest Service's Mickey Merritt assesses knee removal as he does root removal. "It depends on the size of the root that will be cut, how many will be removed and the distance from the tree," he says.
Merritt has removed small knees as they developed from a 35-foot-tall cypress in his front yard. These have been about 60 feet away from the tree, well beyond the critical root zone. (The critical root zone is a circular area with a radius of 12 inches to every inch diameter of trunk, taken at 4 1/2 feet above the soil level.)
If there are a lot of knees. he suggests removing them over a couple of years and, if possible, staying out of the critical root zone. If this is not possible, it's important to make clean, smooth cuts and not chop, rip or pull the knees.
Some gardeners do give up the lawn and add other plants - or just mulch the area.
Trees for Houston's Matt Weaver does not recommend removing cypress knees. He says even though bald cypresses are tough and usually tolerate this practice, it will stress the trees like any mechanical damage.
As arborist for more than 30 years I would not recommend raising soil levels around any tree. However if the material or soil is coarse enough so as to not create anaerobic conditions and allow gaseous exchange or allow the roots to breathe and very well drained in some circumstances it is acceptable.
