You can plant it outside after 1-2 weeks after your avge last date of frost. If the plant withstands the cold in your zone, you should be ok. Unfortunately, the only way to know for sure is if the plant came with some sort of plant label. If it cannot withstand winters in your zone, you might notice a lack of blooms next year. They need afternoon shade in the South & acidic soil that is moist (not wet) and well draining. You can acidify with amendments if necessary and apply manure or cottonseed meal in June as a fertilizer (not on the first year as it probably already has fert pellets in the pot). Stop all forms of fertilizers in July and maintain the soil moist during dry winters (for a new shrub, you can reduce watering to once every two weeks during winter).

Thanks Luis. Since my plant didn't come with a label, I'll plant it in another few weeks and keep my fingers crossed!

Just be aware that many times florist hydrangeas are dwarf often the variety "Pia". When I first started gardening, I planted a bunch of these and thought I was doing something wrong because they never got past 2 feet! A florist friend clued me in! I just moved them to the front of my border.