The question comes to our Extension office on a regular basis, with increased intensity this time of year: “When can I prune my Hydrangea” (or in some cases “Hygeranium,” one of our Southern terms for the shrub)? The usual response is another question: “What kind of hydrangea do you want to prune?” “Oh, I don’t know…the pink kind,” or even, “I’m not sure, but I think it’s got white flowers.”
While many experts have slightly different advice, I’m going to take the bull by the horns, or more accurately, the hydrangeas by the pruners, and make some suggestions that may help guide the answer.
When hydrangeas bloom impacts when they can be pruned; once we identify what we have, pruning becomes (relatively) easy.
Most of the hydrangeas we grow in Central Alabama fall in one of four groups: (1) Mopheads and Lacecaps,  (2) Oakleaf, (3) Paniculata (PG) or (4) Snowball (i.e.’Annabelle’). I won’t go into particular varieties or cultivars; there are far too many to address in this column, but if you recognize the bloom and/or leaf shape of your shrub, that will get you started.
First of all, pruning is not required to keep hydrangeas growing and blooming.  If gardeners weren’t around, hydrangeas would tick along just fine. It is our desire to improve on nature that causes us to go wild with hand pruners and loppers. We want plants to conform to our vision. Since plants can’t “talk back,” their recourse if unhappy is to sulk, avoid blooming, or in extreme cases die. However, there are certain instances when pruning is advised. Removing dead stems or branches should be done every year at any time. Branches that cross and/or rub against each other should be removed at any time. And for hydrangeas that bloom on old wood (“old wood” are stems that were formed the summer before the current season, or last year’s growth), pruning can be done in the summer. However, the hydrangea will quickly grow to its former size, the No. 1 reason it is best to plant a hydrangea where it won’t have to be pruned because “it got too big.”
OK, if you have a mophead or lacecap hydrangea (ones that are usually blue or pink but sometimes white): prune these in the summer before August. Some experts argue it’s safe to prune into August, though the risk is these hydrangeas may have set their bloom buds for next year. That means if we don’t get to prune when it was “safe” and decide to prune in late fall, winter, or early spring, more than likely there go blooms for the coming spring/summer season.
Note: generally speaking, mophead and lacecap leaves are relatively thick and crisp, often heart-shaped. Leaf edges are coarsely toothed, leaf surface somewhat shiny.
Oakleaf Hydrangea, which is Alabama’s state wildflower, is one of the hardiest, most versatile of the lot. Growing in sun or shade, the Oakleaf, named for leaves that resemble an oak tree, exhibits four seasons of interest. In addition, they remain attractive without any pruning other than removing dead or damaged limbs. If the plant was stuck in a too-small space and pruning is needed to keep it in bounds, wait until after it blooms in summer before you prune.  Refer to mophead/lacecap suggestion to prune before August in previous paragraph.
If your hydrangeas are paniculatas, i.e. PeeGee, “Limelight” or arborescens, i.e. “Annabelle,” which usually bloom white and bloom on new wood or stems, the good news is they can be pruned anytime except (you knew it was coming) in spring (for ‘Annabelle’) or summer (PG) as they’re about to bloom. That leaves most of the year safe for pruning, but keep in mind that pruning on live wood or stems initiates growth, which can be zapped if that growth occurs close to freezing weather. In fact, when these plants fail to flower it is usually due to a late frost killing new growth.
Note: leaves are somewhat heart shaped but are thinner and floppier than those of mopheads and lacecaps. “Annabelle” leaves have a matte (not shiny) surface and are courser textured. Annabelle’s have a unique trait among hydrangeas: their bloom opens green, turns white for 2-3 weeks then gradually green again.
If you don’t have a clue as to what kind of hydrangea you have, call your county Extension office for more tips on how to identify it. Identification might entail taking notes on your hydrangea(s) for a season: when it blooms, leaf shape and texture, or taking samples of a branch to an Extension office.  Nurseries selling hydrangeas often can help with identification; however, the best tactic if purchasing a plant is to make sure yours has a tag in the container or attached to the shrub that provides this information.
Learn more about growing and maintaining hydrangeas by downloading or picking up a copy of ANR-1276 “Hydrangeas” from your Extension publications site or office.