Welcome to NGA, Charlotte123 . Planting any type of hydrangea, including the sun tolerant panicle hydrangeas like Bobo, in the middle of the summer, can result in this problem as growers coddle them in greenhouses with plenty of shade and humidity. Then they get planted in the outside world with summer temperatures and a still small root system. They sometimes also get inconsistent or insufficient soil moisture.
Start by providing 7-10 cm/3-4 inches of organic mulch past its estimated width at maturity of 3 feet. If you use rock mulch around Bobo, consider removing the mulch until the shrub is well established in 2-3 years.
Consistent watering: in the summer, aim to keep the soil always moist, never dry nor soggy/wet. Test the soil often at a depth of 10 cm/4 inches so you can immediately water the soil -never water the leaves- as soon as the soil feels almost dry or dry. But never water if the soil is already soggy prior to starting to water.
Sufficient watering: hydrangeas need copious amounts of water but not so much that you accidentally trigger root rot. After watering and allowing the water to percolate, periodically ensure the soil at a depth of 20 cm/8 inches feels moist; if soggy/wet, use less water the next time; if dry, use more water.
Direct sun: while hydrangea paniculatas are very sun tolerant, they are still hydrangeas. Environmental conditions can promote premature bloom browning: lack of diffident moisture; inconsistent moisture; a small root system that is not used to temperatures at or above 100F, etc. Temporarily consider providing supplemental late afternoon shade or even all afternoon shade until temperatures recede and typically stay at or below 29C You can use large umbrellas, some outside chairs, sheets, 35% shade cloth, etc.
Blooms: blooms that have turned brown are spent and will not undergo the proper set of color changes that Bono's bloom undergo so you can deadhead them now if you wish or you can deadhead them at any time of the year that you prefer: prune above the first of leaves or, if the stem is already dormant/leafless, cut the string that attaches each bloom to the stem.
Emergency: in a wilting/sun emergency, carefully extract the plant and dunk the plant in a pail full of water in full but bright shade. Let it stay there rehydrating the hydrophobic root ball until you see no more air bubbles, about 2-4 hours. Or wait until night time and then carefully replant it.
Store returns: it depends on just how much injury Bobo has sustained. It is common for hydrangeas to survive; they may be leafless for a while too (until spring 2024 and then either leaf out or sprout new replacement stems) and that is critical as the leaves provide food for the roots during winter. Also important is the state of the rootball but that cannot be inspected unless one directly and carefully views the roots to see if they remain healthy.
