I live in IL and want to know if I should cut my hydrangeas down to the ground or leave them?
It depends on exactly what are we talking about and the type of hydrangea. Regardless of the type of hydrangea that you have, pruning is not required. Deadheading of spent blooms is also not necessary. That being said, hydrangeas that bloom on new wood can be pruned at any time that they are not developing their flower buds or when they are blooming. Thus, many people prefer to prune this time of the year. Meaning, once they go fully dormant: late fall, winter or even early spring. These types of hydrangeas (Smooth Hydrangeas and Panicle hydrangeas) suffer from flopping issues at the end of the season. Since woody stems resist flopping more than new, thin, green stems, avoid pruning too much of a stem (consider cutting 1/3 or less?). Hydrangeas that bloom on old wood should be pruned after they flower. Once they stop opening spring flower buds then they will quickly develop flower buds for next year so you have a few weeks to prune or less than a month maybe? Some flower buds develop inside the stems and are not visible, avoid pruning these now as you will be cutting off spring blooms. As for stems that fail to leaf out, wait to prune those until the end of June or in July if located close to Canada. As for rejuvenation pruning of any type of hydrangea, prune all the way down the 1/3 longest stems when dormant this winter; prune all the way down the next 1/3 longest stems when dormant in two years and prune all the way down the rest in year 3. You can let the blooms fall on their own; lacecap-form blooms will quickly decay and clean up after themselves. Mophead-form blooms and panicle-form blooms will remain attached for a very long time and then they will too fall down. If you decide that you want to deadhead, you can do that at any time of the year.