Answer: It is true that healthier plants will bloom better. Since it is in a container, your hydrangea will need extra care to stay healthy.
This hydrangea is considered winter hardy into zone 6 so you are at the cold end of its range. When you grow plants in a container, above ground, there is additional winter cold exposure because the roots are not insulated as they would be in the ground. Any steps you can take to minimize winter cold will be helpful. This is a big container but moving it into a sheltered place with protection from winter wind would help. Or, you could consider wrapping the plants for winter.
These hydrangeas bloom on old wood that grew the year before, so winter damage to the stems will reduce flowering. Spring frosts can also damage the flower buds. And, avoid pruning in fall, winter or spring as this will remove flowering wood. If you need to prune, trim back individual overly long stems on a selective basis. Do this in summer right after it blooms.
During the growing season, the soil must be kept evenly moist yet be well drained. This means always damp like a wrung out sponge, not sopping wet and not dried out.
You also need to fertilize regularly using either a slow release granular or a water soluble fertilizer such as 10-10-10 per the label directions. Top dressing with compost and using a mulch would also be helpful.
These should do well with four hours of morning sun, or bright dappled light all day. Four hours of early afternoon sun would be too intense and hot leading to heat stress.
Finally, if the container is on a paved surface reflecting summer heat upward, you should raise it up a few inches to allow air to circulate under it during the summer and help keep the roots cool. In winter, set it back on the ground and if possible, heap mulch or other insulating material around it or move it into an unheated garage or shed. Good luck with your hydrangeas!
I hope this gives you a few tips.
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