Avoid pruning oaks and elms when the thermometer reads 50°F, as this is when the beetles responsible for spreading oak wilt and Dutch elm disease fungi become active. Instead, wait for a consistent period of temperatures below 40°F, ideally even colder, for a few days before, during and after trimming these trees. The City of East Lansing only prunes oaks and elms during dormancy in the winter months to avoid the impact and spread of these diseases.
More Information
More information about oak wilt and Dutch elm disease can be found at the following links: