We have 2 different types of ash trees planted near each other. Is it OK to...

Asked April 27, 2015, 4:43 PM EDT

We have 2 different types of ash trees planted near each other. Is it OK to trim low branches off in early May? Due to the shallow root system, grass has died out around the trees. I read we shouldn't add dirt to get grass to grow due to the shallow roots. Would it be OK to put mulch around tree base and if so, what type is best?

Stearns County Minnesota

1 Response

Thank you for the question. It is ok to prune dead branches or those that pose a danger to people or structures at any time, but according to the University of Minnesota Extension publication "Pruning Trees and Shrubs" the late dormant season is best for most pruning. Pruning in late winter, just before spring growth starts, leaves fresh wounds exposed for only a short length of time before new growth begins the wound sealing process. Another advantage of dormant pruning is that it's easier to make pruning decisions without leaves obscuring plant branch structure. Pruning at the proper time can avoid certain disease and physiological problems.

Here is the link to the entire publication: http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/pruning-trees-shrubs/ For safety reasons, a large tree may be better pruned in the hands of a certified arborist.
You are correct in your assumption that adding topsoil under trees can be harmful to the tree. 50-75% of feeder roots are in the top 1 foot of soil to have best access to oxygen so add mulch no greater than 4 inches thick. The type of mulch is really personal preference and using a vegetative mulch is preferred over something like rock or gravel. Mulches like shredded leaves, shredded bark or wood chips, compost, cocoa hulls, or pine needles are all good and have their pros and cons but they all eventually break down, enriching the soil and protecting it. Whatever you choose, imagine a doughnut when you are applying it and keep it 6 inches away from the tree trunk. This publication from Colorado State http://www.ext.colostate.edu/mg/gardennotes/245.html focuses on how to correctly plant beneath trees but a bit lower down there is a very nice explanation of the different types of mulch and how to apply it.

Thank you for contacting Extension.