Answer: I am not sure why you are wanting to add aluminum sulfate to the hydrangea's soil. It is used to help maintain blue flower color, but only if the soil is lacking in aluminum or not acidic enough. In your area I would expect the soil to be acidic enough already for the hydrangea to be blue so I don't think you would need to add it. Using too much of it can create a toxicity problem, so you would not use it unless you really need to. Your local county extension should be able to help you determine if it is needed (usually by testing the soil) and if so, how much to use gradually over time to adjust the soil. It is something that is done slowly over time to avoid shocking the plant.
Planting can be done in very early spring as soon as the soil can be worked, meaning not too muddy. Container grown plants can be planted in spring, summer or fall, but bareroot plants (often used for hedges) are generally only available in late winter to early spring while they are still dormant. If you have these, plant them as soon as possible according to the directions they are shipped with.
I hope this helps.
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