No need to panic. Bobo is doing what paniculatas do and it is not dying. During the summer months, paniculatas tend to show their feet when they are heat stressed. This usually applies to foliage near the bottom/center of the shrub, close to the crown (where the stems originate from). Many stressor events can help promote this: temperatures typically much above 85°F; too much direct sun; hot drying winds; lack of water or lack of sufficient water; too much water; lack of direct sunlight; lack of organic mulch or use of rocks as mulch; new plantings (the root system is still too small); root disturbances from transplanting, gardening work or pests like voles, etc. Test the soil frequently in the summer mornings to ensure that the soil remains always damp/moist: insert a finger in several spots under the canopy or drip line to a depth of 4" (the typical depth of newly planted hydrangea roots). Water if the soil feels dry or almost dry; skip watering if the soil feels wet/soggy. Use enough water to moisten the soil at a depth of 8". There is/was no need to amend the soil to acidify it or to increase the current levels of micro-nutrients but, since the plant is close to the house, cement leeches lime and many parts of STL can have alkaline soil, it bears monitoring the soil pH once a month or less. While the roots are still growing in the potting mix, there will not be and issues. In St. Louis, there is a lot of limestone in the soil that results in an alkaline soil pH as well as a few acidic areas with sandstone. A soil pH kit can help determine the type of soil pH but if you do not have one, you can also look for colored Big Leaf Hydrangeas in the neighborhood. Also, natural stands of pines (not planted) would suggest the location has acidic soil because that is what the pine trees require for growth. Garden sulfur can be used to lower the soil pH if the plant do display symptoms of interveinal yellowing. As for fertilizing, your last application of a slow-release fertilizer should have been done at the time of your original posting; the average date of first frost for STL falls around the 2nd-3rd weeks of October so plan to fertilize for the last time around the 2nd-3rd weeks of July. But consider skipping the fertilizer too if temperatures are particularly hostile, as stressed plants should not be fertilized at that time. Note: hydrangeas are not heavy feeders of fertilizers like roses so fertilizing can be optional if the plant is well established, remains well mulched with 3-4 inches of organic mulch and the soil has no nutrient deficiencies. Enjoy Bobo. It is a nice cultivar.
