114-Understanding Hydrangeas: Pruning, Blooming, Color-Forcing & More

Hydrangeas are some of my favorite ornamentals shrubs, and this week’s guest shares my love of this beautiful plant. Well actually, Lorraine Ballato has a special appreciation for all hydrangea varieties, and she has become a recognized expert on the care of this sometimes finicky species. She contributes to many respected horticultural resources, like a recent post on the National Garden Bureau website. In 2017, Lorraine wrote Success with Hydrangeas: A Gardener’s Guide, a must-have book for anyone growing this often misunderstood shrub.

Have you ever searched the internet for answers to your hydrangea questions? If so, you’re not alone. According to Google, there are more hydrangea-related queries than that of any other garden shrubs. Lorraine receives a lot of questions too, and the most frequently asked are:

• Why doesn’t my hydrangea bloom?
• How can I change the color of the blooms?
• How should I be caring for my hydrangea?
• How should I be pruning?

The answer to these and most other hydrangea-related questions depends on which type you have.

When it comes to hydrangeas, Lorraine has a commanding grasp of the information that inquiring gardeners crave. (photo: A. Ballato)

The Big, Beautiful World of Hydrangeas

There are hundreds of hydrangea varieties, but those of us living in the United States are usually growing one of five types – Big Leaf hydrangea, Oak Leaf hydrangea, Woodland hydrangea, Panicle hydrangea, and Climbing hydrangea. Mountain hydrangeas are sometimes referred to as a sixth type, but they actually fall under the Big Leaf group.

Lorraine recommends purchasing hydrangeas when they are in bloom, so visually, you know what you are buying. Hydrangea terminology is often misused and sometimes refers to more than one variety, so it’s also best to become familiar with the Latin name of each group to truly know which variety you bring home – and the care it will require during its lifecycle.

Hydrangea Macrophylla

These are the Big Leaf hydrangeas and some of the most commonly grown varieties in the U.S. They are native to southern and eastern Asia (from Japan to China, the Himalaya and Indonesia) – areas which have a typically mild maritime climate. So, it stands to reason that all Big Leaf hydrangeas – often called “mop heads” – also do best in maritime climates of North America. Gardeners in the Pacific Northwest and the Eastern Seaboard states enjoy great success with Hydrangea Macrophylla varieties.

Big Leaf hydrangeas love shade, but they do require a few hours of sun – preferably in the morning. The cooler the climate in which you live, the longer period of direct sun your Big Leaf hydrangea will be able to tolerate. That said, there are more adaptable varieties coming onto the market every year. Your best guide is to look for a good plant tag and follow its recommendation.

There are also lace cap Hydrangea Macrophyllas. Lace cap flowers feature an outer ring of petals with a center which looks like an unopened bloom. Are you looking to attract more pollinators? They love lace caps because the pollen is more accessible than in a mop head bloom.

Pink blooms, like this Hydrangea Macrophylla “Mystery” with a unique white variegation, are much more common than blue. (photo: Lorraine Ballato)

Hydrangea Serrata

These are the Mountain hydrangea varieties. Native to Japan and Korea, they are currently categorized under Hydrangea Macrophylla, but if you see Hydrangea Serrata, it’s a Mountain hydrangea.

For gardeners who live in a zone that might be too cold for other Big Leaf hydrangeas, a Mountain hydrangea might be just the ticket. They are the most cold-tolerant of the varieties which are commonly available in the U.S.

Like other Macrophylla, Mountain hydrangea blooms can be lace cap or mop head, which is why they are easily confused at the time of purchase. If you see a hydrangea marked as a “mop head” at the big box store, is it a Macrophylla or a cold-tolerant Serrata? Knowing the difference will be important for success.

Hydrangea Paniculata

These are Panicle hydrangeas. The flowers of these varieties are cone-shaped and tend to be white – sometimes turning to a pale pink. Plant breeders have developed newer Panicles which are smaller and offer deeper shades of pink.

If you place a Hydrangea Paniculata in a shady spot, you’re setting the plant up for trouble. These varieties prefer full sun, but they also require the consistently moist soil preferred by all hydrangea varieties.

Oak Leaf hydrangea (my favorite) also have panicle blooms, so many gardeners assume they are part of the Paniculata family. However, Oak Leaf varieties have a family of their own.

Hydrangea Quercifolia

These are the Oakleaf hydrangea varieties, and they are native to the United States, which is one of the many things I love about them. I also appreciate that they offer four seasons of interest. Not only do their gorgeous blooms continue through spring and summer, but the foliage turns beautiful shades of red and orange in fall. Once the foliage drops, I get to enjoy their exfoliating bark all through the winter.

Oakleaf hydrangeas are also one of the more cold-resistant and dependable classifications of hydrangea. If you live in a cooler zone and struggle with Big Leaf hydrangeas, give an Oak Leaf a try. They’re available in a variety of sizes, and I’m betting you’ll fall in love with them too.

Oakleaf hydrangeas are my favorite landscape plants – for their beautiful blooms and four-season interest. I can’t add enough to my GardenFarm landscape.

Hydrangea Arborescens

These are the Woodland varieties, also known as the Smooth hydrangea. The most popular of this classification is Annabelle. Hydrangea Arborescens are also native to the U.S., and if you need a shrub for a shady spot, Woodland hydrangeas are a good choice. They are very tolerant of shade and, surprisingly, drought.

Some Hydrangea Arborescens varieties have lace cap blooms, which can be mistaken for lace cap Hydrangea Macrophylla.

Hydrangea Petiolaris

These are the Climbing hydrangeas, which are quickly climbing my favorites list. Hydrangea Petiolaris varieties are under appreciated for all that they offer. For one thing, they love the shade. In fact, place a Climbing hydrangea in sun, and it will turn to find the shade. Plus unlike many climbing shrubs, Hydrangea Petiolaris won’t harm brick or mortar, but they sure will transform a wall into a showstopper. The fragrant blooms of these species are lace cap in form.

Don’t have a shady wall or arbor for a Climbing hydrangea? Not to worry, they are also gorgeous as ground cover or to grow over – and camouflage – a tree stump or other eyesore in your landscape. Speaking of trees, these varieties will climb tree trunks too. With so much versatility, you can’t help but begin to visualize ideas for adding Hydrangea Petiolaris to your landscape.

Climbing hydrangea will require a little patience on your part. They need to be well-established before they bloom. Actually, it can sometimes take years, but when the flowers begin, they are profuse and well worth the wait.

The delicate blooms of Climbing hydrangeas are incredibly fragrant once they finally make their appearance.

Hydrangea Care

Gardeners love to fertilize. The fact is both watering and fertilizing are two steps many gardeners are quick to take even when neither one is necessary and could even be detrimental. You’ll be most successful – no matter what you’re growing – when you are more thoughtful about how and when you water or fertilize.

Instead, start by amending with compost. Compost can do so much for your soil health that you may never need to fertilize at all. The nutrients in compost are absorbed slowly, as the plant requires them. There are microbes and other beneficial aspects in compost which will do much more to promote a lush plant than any fertilizer can offer.

Next and before you ever add a fertilizer product – get a soil test. A soil test will tell you which nutrients – if any – your soil is deficient in for the plants you are trying to grow.

For example, some soil is naturally high in potassium. It would be important to know that before you add a fertilizer containing potassium which would create an excess. Another example is overdoing the nitrogen. Too much of that key nutrient will promote foliage growth instead of the gorgeous blooms which are usually our goal when growing hydrangeas.

If you do need to apply fertilizer, pay attention to the nutrient ratio on the packaging. (You can learn more about fertilizer types and ratios in an earlier podcast.) Lorraine suggests “rose fertilizer” as a good choice for hydrangeas – depending on the results of your soil test, of course. Rose fertilizers are highest in phosphorus, and they also tend to be absorbed more slowly than other options. You can overdo it with this important but less-sustainable nutrient too, so get that soil test first.

As another word of caution, always check the ingredients of fertilizer products and avoid anything containing imidacloprid, which is lethal to beneficial insects. Some rose and flower bloom fertilizers, for example, are known to include this systemic pesticide.

When it comes to watering your hydrangeas, keeping the soil consistently and evenly moist will promote the healthiest shrubs. Amending with compost will help with that too, because it improves the ability of the soil to retain moisture properly. Also, top the surface around your hydrangea with a good layer of mulch. A 2-3” layer of natural mulch will also go a long way toward maintaining a proper soil moisture level.

This Hydrangea Paniculata “Tardiva” has been pruned into a tree form and is quite a focal point in the landscape. The layer of mulch under the canopy helps to keep soil moist. (photo: Lorraine Ballato)

Big Leaf hydrangeas are notorious for drooping in direct sun, and plenty of internet resources advise that the best corrective measure for that is to water. Well, those internet resources are wrong. Drooping in direct sun is a natural reaction which will correct itself after a few hours of shade time.

Hydrangeas are designed to recover by pulling moisture from surrounding soil which is then pushed through the stems to return to an upright form. Too much water – just like too much nitrogen – will promote foliage growth rather than flowers. Just another reason it’s always better to err on underwatering versus overwatering.

So, have a little patience, and wait a few hours – or even overnight – before you decide that your hydrangea needs water. Oh – and when it comes to internet resources, there is a lot of misinformation out there, so stick with university websites (.edu) and trusted experts in the world of horticulture.

Common Hydrangea Problems

The first step in addressing hydrangea pest or disease issues is to focus on growing a healthy plant. Understand which type of hydrangea you have and the conditions it prefers – and, then, provide those conditions along with healthy soil. A healthy plant is genetically programmed to be less vulnerable to pest and disease attack, and it’s better able to recover should an attack come.

The stronger your plants, the less intervention they will require from you to address an issue.

The voluminous blooms of Hydrangea Arborescens “Annabelle” start off lime green and brighten to white as they reach full size.

So, which diseases affect hydrangeas most? Leaf spot disease, Chlorosis (generally caused by an imbalance in soil chemistry), Cercospora Leaf Spot and Powdery Mildew (both generally caused by lack of air circulation).

If you’re struggling with a fungal disease like Cercospora or Powdery Mildew, avoid wetting the foliage. Moisture on foliage from rain or overhead watering – especially late in the day – is exactly what fungal spores need to form. You can’t stop the rain, but you should always water near the base of your hydrangea.

If you opt to remove any foliage and stems which become infected by disease, don’t compost them – get them off of your property. Many diseases can overwinter or survive the composting process and, when conditions are just right the following year, will infect your plant again. However, disease damage to hydrangea is typically aesthetic. So although removal is the right approach on your edible plants, it will negatively impact the main value of a hydrangea – its beauty. For that reason, it’s usually better practice to treat hydrangea fungal disease.

If you do treat, be sure to follow the instructions carefully and know that these treatments aren’t cures – they’re just treatments. They can prevent spread but will not cure disease. That’s why it’s best to try to stay ahead of the game with a healthy plant and proper watering.

Lorraine has been experimenting with Bacillus amyloliquefaciens as a treatment for fungal disease and seeing encouraging results. This product is a synthesized version of a naturally-occurring enzyme which can kill soil-borne fungal disease. It’s a promising newer option. As with all treatments, it’s important to follow instructions carefully and be targeted in your application.

Hydrangea Serrata “Blue Billow” isn’t just beautiful. It’s also the most shade tolerant variety in the hydrangea world. (photo: Lorraine Ballato)

Of all the diseases, Powdery Mildew is the most treatable. There are several good and effective organic treatment options on the market. Lorraine prefers bicarbonate-based products, and like me, she recommends Green Cure, in particular.

As for pests, Lorraine says that webworms are becoming a bigger problem than in years past. Fortunately, webworms are fairly easy to eradicate. Open the web up to expose the worms to birds and other natural predators. It won’t take long for the worms to disappear. Their webs are easy to breach, so even if they are high up, it won’t take much contact from an extension pole or other long object to get the job done.

Prefer not to wait? You can also cut out the area containing the web and get it off of your property. Yes, even on a hydrangea. The cut area will be minimal, and so, will cause little (if any) impact to next season’s blooms.

Japanese beetles can be a problem on sun-loving hydrangeas because the beetles love the sun too. Fortunately, Japanese beetles are least active and, so, most vulnerable in the morning. During those early hours, it’s easy to spend a few minutes knocking them off the plant into a cup of soapy water. Stick with it, and you can keep damage from these pests to a manageable level.

When it comes to pest treatments, it’s important to understand the unintended consequences which come even from organic products. Lorraine has been having a lot of success using Beauveria bassiana (B.b) which infects and kills some species of insect pests.

Oakleaf hydrangea foliage is showstopping in the fall. (photo: Lorraine Ballato)

Hydrangea Pruning

This is the most misunderstood topic of all aspects of hydrangea care. Did you know some varieties don’t need to be pruned? It’s true. Big Leaf hydrangeas are an example. As long as you provide the right growing conditions, they never need pruning to perform. That said, cutting out dead, unproductive stems at ground level after three or four years can promote a more productive and healthier plant.

When it comes to pruning, you need to know which variety you’re dealing with and whether it is a “new wood bloomer”, “old wood bloomer” or a “rebloomer”.

New Wood Bloomers

Any variety which blooms on new wood – meaning the flowers develop on stems which have grown during the current season – will perform best with yearly pruning and deadheading. You can shape them and cut them back once they go dormant without the risk of losing blooms. The ideal time is during the months of January, February and March. If you live in a cooler zone, April is another good option.

It’s best to avoid pruning in the fall before the shrub goes dormant.

Why? With all plants, pruning stimulates new growth, and new growth which forms in fall is much more susceptible to damage from cold winter temperatures. So if you prefer to prune in fall, wait until leaves drop to make your cuts.

If you do prune during the first 3-4 months of the year, that’s also the best time to fertilize.

New wood blooming hydrangeas are also good choices for colder zones, because flowers will form even if stems are killed back during frigid temperatures. Panicle hydrangeas and many Woodland hydrangeas bloom on new wood.

Make deadhead cuts just underneath the spent bloom and above the nearest set of leaves. (photo: Lorraine Ballato)

Old Wood Bloomers

When a variety is an old wood bloomer, that means the flowers will bloom on stems which grew during the previous year (and sometimes, which grew a year or two earlier).

Never prune these varieties after early to mid-August (depending on your zone) or before buds appear in a new season. Lorraine calls this period the Hydrangea Danger Zone™. Wait to prune until after new buds (called “broccoli”) have formed in early spring to avoid pruning off sleeping flower buds.

Oak Leaf, Mountain, Climbing, and many Big Leaf hydrangeas all bloom on old wood, so leave these varieties be during the Hydrangea Danger Zone™ period.

If severely cold temperatures kill back growth on one of these old wood blooming varieties, you might get few or no blooms that year, because the plant will need to develop new stems which will bloom the following year.

You may be wondering whether or not to deadhead an old wood bloomer. Personally, I like the look of spent flowers on my hydrangea shrubs. I feel they add to winter interest, and birds use Woodland hydrangeas as a food source.

If you prefer to deadhead spent blooms, Lorraine says you can cut directly below a spent flower head without impacting next year’s blooms. Look for the spot where there are two leaves or leaf buds coming off the stem. There, you should see new growth. You should cut above that point, because it’s the new growth bud which could be a flower next season.

Sometimes, your old wood hydrangea may have so much new growth that it obscures the flowers. Lorraine suggests “pinch pruning” – but timing is everything. In July, cut back selective stems (just enough to allow the flowers to shine through) by about a third or 4-5”.

This unopened bud is no longer viable due to winter’s freezing temperatures, but it could be “pinch pruned” to stimulate dormant flower buds further down the stem. (photo: Lorraine Ballato)

Rebloomers

In 2004, reblooming hydrangeas were introduced. These varieties produce flowers on both new wood from the current season and old wood from previous years. Rebloomers, like old wood bloomers, shouldn’t be pruned until you see the new buds in spring.

Have you heard of “apical dominance”? That term refers to the ability of a plant to control the flowers at the tip of the plant’s stems (called the terminal flower bud), and it’s a key factor in the development of reblooming hydrangeas. When the terminal flower is pruned off, hormones in the plant are pushed to the rest of the stem where sleeping buds are waiting to be triggered to develop.

So, you’ll be encouraging multiple blooms on a stem by pruning off the terminal flower or a dead tip once you determine that the bud was frozen off or somehow damaged while waiting in the wings.

Here is a good visual of the bloom structure of a reblooming hydrangea stem. Notice how the blooms extended from the side of the stem. Once the flowers have faded or the plant goes dormant, these stems can be safely pruned out. (photo: Lorraine Ballato)

There are three exceptions to pruning restrictions for any type of hydrangea, and those are commonly referred to as the three D’s – dead, diseased or damaged growth. Regardless of variety, that growth should be removed immediately, all through the year.

Does your hydrangea flop under the weight of large blooms? It turns out, that’s often a result of incorrect pruning. You may have seen suggestions to cut a Woodland hydrangea shrub back all the way to the ground, but that’s not a good idea. Lorraine recommends against pruning stems lower than a height of 2-3’ to create a framework to support blooms. It also encourages the stems to become sturdier over time, which will be better able to weather punishing winds or heavy snowfall.

What if you need to rejuvenate a plant which hasn’t had proper care? Old wood blooming varieties only produce on stems between one and (usually) three years old. Any stems older than three years can and should be cut out to make room for new growth, but never remove more than one-third of the plant in a season.

Maybe you’ve bought property with hydrangeas in the landscape, and you don’t know if they are new wood bloomers, old wood bloomers or rebloomers? Your best approach is to study the shrub for a season before you make any pruning decisions. Lorraine’s trick is to mark each stem with colored twist ties. For example, she’ll tie a red one around growth from the current season and a blue one around growth from a previous season.

Old wood stems will stand out from among the new growth due to peeling, stippling and/or bumps. (photo: Lorraine Ballato)

Old wood stems should be easy to differentiate from new growth. The bark on old wood may be peeling, have stippling or have bumps. New growth will be smooth.

As the season progresses, watch which stems produce a flower. If an old wood stem doesn’t develop a bloom, it never will and can be earmarked for removal when you’re ready to prune.

Changing Hydrangea Bloom Color

Odds are good that if your Big Leaf hydrangea is pink, you would like to change it to blue – and vice versa. We tend to want what we don’t have, right? Well, not all hydrangea colors can be changed.

If your Big Leaf hydrangea is white, it will always be white. Some of the pink and blue blooming varieties are pH-dependent or pH reactive. Here again, it all comes down to knowing your variety. In clay soil (like my home state of Georgia) or naturally acidic soil, pH-dependent varieties will be blue. In alkaline soil (here’s looking at you, Midwest gardeners), those same varieties will bloom in pink.

You can add sulfur or aluminum or another amendment designed to alter soil pH, and that will have an impact on bloom color. However, the color shift will take a season or two, and you will need to continue amending. Soil will always revert to its natural pH level, so if you are sold on a color not supported by your native pH, you will need to remain diligent.

Rather than fighting soil pH, try looking for a variety which is not pH reactive. There are several beautiful pink blooms in the Panicle family. Mathilda Gutges and Rhythmic Blue – two Hydrangea Macrophylla varieties – offer blue blooms which will remain true to color, although they may take on a purple hue in highly alkaline soil.

The elusive blue bloom is a challenge for gardeners who live with naturally alkaline soil, but consistent amendments can have an impact. (photo: Lorraine Ballato)

I hope this episode has provided some clarity for you when it comes to growing and caring for hydrangeas. I also hope you make time to listen to my conversation with Lorraine by clicking the Play icon in the green bar under the title at the top of the page. Lorraine shares more specific variety recommendations, and her confidence in the world of the hydrangea is contagious. Which variety of hydrangea are you hoping to add to your landscape soon?

You can continue to learn more through Lorraine’s book and blog. She emails a new update every week or two to share what’s going on in the hydrangea garden as well as new developments that will be good to know in seasons to come.

Links & Resources

Episode 008: Organic Pest Control with Jeff Gillman

Episode 016: Composting Guide A to Z: The Quick and Dirty on Everything Compost

Episode 063: Garden Fertilizer Basics: What to Know Before You Grow

Episode 110: Why Mulch Matters in Every Garden: What You Need to Know

Episode 108: Easy Ways to Make More Houseplants, Vegetables and Flowers

joegardenerTV YouTube: How to Take a Soil Test

joegardener Online Academy: Master Pests, Diseases and Weeds – my newest online course! Just $47 for lifetime access.

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Sign up for Lorraine’s hydrangea newsletter on LorraineBallato.com (Look for the mailing list subscription option at the bottom of the page)

Success with Hydrangeas: A Gardener’s Guide by Lorraine Ballato

National Garden Bureau: Getting the Blooming Hydrangeas of Your Dreams!

Bacillus amyloliquefaciens

Beauveria bassiana (B.b)

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