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This super-hardy, super easy-to-grow hydrangea goes by lots of different names: smooth hydrangea, Annabelle hydrangea, wild seven bark, hills-of-snow hydrangea, wild hydrangea, native hydrangea, and probably a few more. Botanically, it is known as Hydrangea arborescens and it is native to much of the southeastern United States, from the south of Illinois to northern Florida and from New York state to Oklahoma. If you were to see a smooth hydrangea in the wild, though, you'd barely recognize it: naturally, its flowers are small and not very showy. It is thanks to avid gardeners and talented plant breeders that we have the big, colorful flowers that now characterize this practically fool-proof hydrangea.
Hardiness/climate
Along with panicle hydrangeas, smooth hydrangeas are some of the most cold tolerant on the market, easily thriving and blooming even in chilly USDA zone 3. They are heat tolerant through about USDA zone 8, though can be grown in cool parts of zone 9, such as the Pacific Northwest.
Size
With the recent work on the Invincibelle series done by Dr. Tom Ranney and his team at NCSU, the size range of smooth hydrangeas has gone from pretty much strictly 5'+ to a huge selection in the 2-3' range, and some as small as just 1-1.5' tall and wide. With a little research, you'll find the perfect variety for any space!
Light
We recommend at least four hours of sun each day, or filtered light throughout the day. This makes a huge difference in the stem strength, as plants that are in too much shade will stretch toward the light, leading to thin, spindly growth that can't support the flower heads. For the pink varieties, sun also helps to bring out the purest, truest colors. Finally, in too much shade, you may find that mophead varieties don't fully develop and take on some characteristics of lacecap flowers. In hot climates, sun should only be in the morning, and during the hottest part of the day, they should be in shade.
Soil
Easy-going smooth hydrangeas aren't fussy about soil, but they do require it to be well-drained. They won't do well in soils that are too wet nor soils that are too dry. Soil pH is not an issue, as they can grow well in acidic to slightly alkaline conditions, and the color of the pink varieties will not change based on soil chemistry as they can for bigleaf and mountain types.
Fertilizer
Smooth hydrangeas are naturally fast-growing, and need little to no supplemental fertilizer to do their best. One application of a granular (not liquid) rose fertilizer in early spring should be sufficient in most areas.
Flower Color
For many years, white was the only option for smooth hydrangeas, but that changed dramatically in 2010 when Proven Winners introduced Invincibelle Spirit hydrangea. It was the first-ever pink flowering smooth hydrangea. Since then, new and improved color options have been hitting the market, and now pure pinks, ruby reds, mauvey purples, and even greens are available. The color on these varieties is not impacted by the soil chemistry.
Pruning
Smooth hydrangeas are one of the few types that bloom on new wood. This is one of the reasons why they are such a reliable performer even in cold climates! It's also what makes pruning them each spring the ideal way to maintain the plants, as it removes any thin buds at the tips of the stems and ensures the growth for the year comes from thicker buds that were created earlier the previous season.
Some people cut smooth hydrangeas like 'Annabelle' all the way to the ground. However, we recommend cutting the entire plant back by about one-third its total height in early spring. You can also remove any thin side branches at this time, as well as any wood that's not showing signs of life. If you prefer, you can do this in late autumn/early winter, you'll just want to wait until the plant is completely dormant before pruning. Our preference is generally to prune in spring, because this leaves the dried flower heads in place all winter, which are much nicer to look at than a bunch of cut-off branches. The plant will not be harmed with late autumn pruning if that's what you prefer.
Smooth hydrangea problems
Largely problem-free, any issues you do see on your smooth hydrangeas are most likely cultural - due to some conditions the plant is experiencing that aren't ideal for growth. That said, there are a couple of issues that may arise that you should know about.
Diseases
The leaf spots that impact other hydrangeas are usually fungal in nature. However, the most common leaf spot for smooth hydrangeas is actually bacterial. It can be avoided in the same way as fungal leaf spots: avoid overhead watering, provide good air circulation, keep the plant free of stress, and remove any affected foliage when it falls in autumn.
Insect pests
There's only one insect that you are likely to find causing damage on a smooth hydrangea, and that's the hydrangea leaf tier. That's "tie-r," as in, "one who ties," because this little caterpillar makes itself a snug little home in the leaf shoots so that it can feed safely on the developing foliage. These native insects rarely cause any issues for the plant, but once the caterpillar develops into a moth, you may see the foliage is slightly disfigured. If you see the leaves clasped tightly together at the ends of the branches, you can peel them open and smash the green larvae.
Other animals
Deer cause major issues for smooth hydrangeas: they especially love to eat the flower buds and may eat them before they are even noticeable to the human eye. They also eat the foliage, and smooth hydrangeas tend to be the type that are most severely damaged by deer. If you have deer in your yard, plants should be protected with netting or a spray, and this protection will need to be repeated - maturity does not seem to deter them.
