You are supposed to replace a helmet after ANY big impact. If you hit a tree with it I think that qualifies. I seem to always end up sounding like the old mom on these topics...but I usually err on the side of caution. Lots of sales going on helmets right now, I would pick up a new one. As a matter of fact, I am picking up a helmet on Monday.
More of a glancing blow at relatively fast speed. If it i hit head on the main helmet section I would definitely replace.
“I train every day of my life as they have never trained a day in theirs.” – Alexander Karelin