If you have room to allow it to grow to size - always the preferred situation - then no pruning is required nor usually needed. 'Hummingbird' is a rather compact form of clethra to begin with

Just an FYI, but this is a shrub that prefers a consistently moist and acidic soil in part shade. Is that what you are providing? It will also grow well in full sun, provided you supply adequate summer irrigation.

Thanks for the information! The plant has plenty of room so I'll let it go without pruning. Our soil here is on the acidic side and i mulch with pine straw which will help that out. The sun where it's going is full morning sun through ~1-2pm. Then is is partially shaded by a tree. I'll make sure to give this guy plenty of water during our hot summers. Thanks for your help!

Dave

If it were mine, and I decided it needed pruning, I would wait until after blooming. Am unsure how Clethra responds to such treatment, as I never prune mine. Perhaps Clethra 'Humminbird' does not sucker. Mine have plans of becoming a grove. Clethra 'Sixteen Candles' has done quite a bit of wandering. Always prefer shrubs be kept with the shape they incline to, if possible.

If you want to prune it you should do so in late winter up to mid spring as it flowers on new wood in the summer. It will probably sucker once it gets established, chop out some of the suckers with a spade if you don't want it to get too big, because it can.... It can benefit from a light application of an acidic type slow release fertilizer in the spring if you're so inclined. Keep it moist.

Just a heads up about clethra hummingbird (and others) that I didn't know. They are so late to leaf out in spring that you may mistake them for dead!