they are harmful to the leaf.. but not the tree ... note that 99.9999% of that leaf is fully functional ....

ignore them ... as i have forever.. lol ... i dont even know what they are.. because i have so thoroughly ignored them ... lol ..

ken

Thanks... from one "ken" to another! Do you know the kind of tree by the leaf shown in a photo? I've heard ash, hawthorn, or possibly basswood from my well-meaning neighbors!

It's a spindle gall which are common to basswood (and others) . You need to look up Tilia (basswood, lindon) to determine for yourself if that's what you have....unless you'd like to provide some better pictures.

Leaf galls very rarely cause problems for their host plants. The recommendation is to live with it.

Galls are a leaf's reaction to a bugbite, much like your skin reacts to a mosquito bite.