Welcome to NGA. In your zone 4a, I would have planted the hydrangeas in the ground earlier this year or at least protected them from frost by bringing them into a protected location. Make a note in your wall/electronic calendar for next year that the average date of first frost for Limestone falls around the 2nd-3rd weeks of September.
Since your Big Leaf Hydrangeas are now blooming, I assume that these are rebloomers. These rebloomer varieties produce two flushes of blooms throughout the year.
The early spring flush occurs when this year's stems leaf out and the flower buds that are now inside the ends of the stems open. In zone 4a, I would expect that those stems do not survive unless one winter protects. Thus the only new growth originates from the base of the pants in the crown.
The second flush occurs in late summer/fall when this year's (2022) new growth gets tall enough to produce flower buds and immediately open them. From this year's example, it appears that they are not getting tall enough, quickly enough to bloom early enough so early first frosts get to the blooms. I accelerate growth a bit, you should immediately try fertilizing once with a slow-release fertilizer such as Espoma Holly-tone, NPK 4-3-4, as soon all danger of frost has passed. For Limestone, the average date of last frost is around the 2nd-3rd weeks of June. Afterwards but before your first frost date, switch to a quick release, low nitrogen (the lower, the better), high phosphorus fertilizer like -for example- Scotts Super Bloom Water Soluble Plant Food, NPK 12-55-6. Make 1 or 2 additional applications but well before your average date of first frost in mid September. You can also sprinkle some coffee grounds in between the last frost and the first frost dates. Additional applications of Holly-tone are not recommended as each application lasts about three months and the last one should be applied three months before your first frost date or around Limestone's last frost date; that is why I recommend switching to a quick release fertilizer. By using a high phosphorus content with Scott's product, you may get the plants to do their blooming slightly earlier than they did this year. So maybe apply it in July and in August.
It is up to the plant to decide what to do now with the injured flower buds. They sometimes open partially. Other times they turn all brown. Just wait and see. But if you leave them outside, the upcoming frosts may get to them so that leaves playing a game of musical chairs: bring the pots indie to protect the flower buds and then quickly outside as soon as temperatures climb. Ugh. Of course, you can deadhead the blooms and be done with that: cut off the peduncle, the string that attaches each flower bud/bloom to the stem.
The material of which the pots are made will not protect the hydrangeas/hostas from zone 4a cold temperatures. You can move the pots into a protected zrea that will be kept warmer though, like a heated garage, a heated greenhouse or in a pinch, a basement or inside the house (away from cold/hot air vents). You can also try digging holes in the ground for each pot so the heat from the soil will help them and then cover the stems with a pile of mulch. The ends of the stems is where early spring flower buds develop so this area needs to be protected with the mulch. You can use chicken wire to help keep some shape to the mulch as cold, desiccating winds may blow the mulch away and dry the stems otherwise.
Note: if you plant the hydrangeas deep in a container, make sure the container has plenty of water drainage holes to drain the water as humidity levels will be high inside the container and this may promote the germination of fungal spores and lead to powdery mildew or leaf spots in hydrangeas leaves.
Use slug bait for the hostas.
Luis
