Earlier in the season, when leaf out starts, I would have tried pinching, adding some alfalfa meal (growth stimulating) or blood meal (high nitrogen), or composted manure or organic compost around the plant plus definitely 2-4" of organic mulch on top. But summer is now fast approaching, I am not sure where you are and if you want to add high nitrogen fertilizers just before summer. The mulch can be added and should be always maintained year around. It helps protect the roots from temperature extremes.

I see some weeds in the pictures so maybe remove these or keep them away from the hydrangea so they shallow roots of the new plant do not comingle with the weeds' roots.

It is impossible to force growth where you want it so you may just need to do some pinching for a few years as soon as you see leaf out. Maintain the soil as evenly moist as you can. Do not let it dry out, wet it, dry out, etc as this inconsistent watering will brown out some leaves from the edges inwards and make it have more parts with no leaves. I do not recommend MG unless the plant is potted because some formulations have 30% nitrogen, which is too much.

Keep an eye on the downspout to make sure that water does not accumulate where the new plant is after a thunderstorm.

Hydrangea arborescens is a very hardy variety that blooms on new growth each season. You can prune it back quite hard to the height you desire early each spring. This will eliminate the legginess you see now, encourage side shooting and increase flowering buds, making for a lower, denser, more floriferous shrub. Hydrangea arborescens also increase in size by suckering, so each year there should be more canes/stems emerging from the ground.

But don't prune now! Too late in the season and the baby plant need all those leaves now to help establish. Wait until next March or so. I'd cut that single stem back by at least half!!

Thank you both for your reply! I hand no idea to pinch leaves in spring. I do have established hydrangeas that came with the house. I trim the branches back on those every spring and they do well. Since this was just 1 branch, I didn't want to cut it back. I thought leaves would pop out all along the entire branch (like the established ones do) I will do that next year and pinch some leaves. I did add mushroom compost to the soil early spring and I think that did help to get the few leaves that I have now. I am in Chicago (zone 5)...Luis should I not use ANY fertilizer now? Just mulch and water? Thank you both so much for the suggestions!

If you received it bare-root then I would fertilize it in Spring 2020. Mushroom compost is ok. I normally add composted manure, organic compost or cottonseed meal. A chemical fertilizer rated as general purpose, slow release, with a NPK Ratio of around 10-10-10 is also ok.

If the plant did not come bare-root but the was potted and has those round fertilizer pellets on the soil, I would not fertilize it this Spring.

Feel free to give it coffee grounds now and then but stop all ferts three months before your average date of first frost (somewhere around the last week of October).

I usually try not to add any ferts when temperatures are quite warm. For example, I am now heading towards 100F so I am not fertilizing anything. I just drink the cup of java only and sprinkle the coffee grounds on the grass because I have one pooch that loves to roll around in it. Ha!. If she comes back into the house smelling like coffee or mint, I know where she has been and that is ok. If not coffee/mint smelly, she may have been causing trouble elsewhere then. Hee, hee, hee.

Thank you Luis! I'll finish the little bit of fertilizer I have left and then add the coffee ground too :) It's only in the 70, low 80s here so far. I was excited for glorious hydrangeas to surround my front porch, but guess I have to wait for next year :) Now I'll just focus on keeping them alive! Thanks so much for your help!!