Bigleaf hydrangea (Hydrangea macrophylla) is the mophead and lacecap shrub whose flowers can turn blue or pink depending on the soil. It is also the hydrangea most likely to leaf out beautifully and then produce no flowers at all. Most Hydrangea macrophylla varieties bloom on last year’s stems, so anything that removes or freezes those stems over the winter costs you the flowers. This guide covers Hydrangea macrophylla care, why it is not blooming, when to prune, winter protection and flower color, following Clemson HGIC, NC State Extension, the Missouri Botanical Garden, Penn State Extension and UNH Extension.
Hydrangea macrophylla at a Glance
| Need | What the sources say |
|---|---|
| Light | Morning sun and afternoon shade (Clemson); dappled sun to part shade, protected from afternoon sun (NC State); full sun only in consistently moist soil (MOBOT) |
| Soil | Moist, well-drained, rich in organic matter (Clemson, MOBOT) |
| Size | 3 to 6 ft tall and wide (NC State; MOBOT for the St. Louis area) |
| Hardiness | Winter hardy to USDA Zone 6; plants in Zone 5 need a sheltered site and winter protection (MOBOT) |
| Blooms on | Last year’s stems (old wood); reblooming cultivars also flower on new growth (NC State) |
| Pruning | Right after flowering; not after August 1 (Clemson, NC State) |
| Flower color | Blue at pH 5.0 to 5.5, pink at 6.0 and up (Clemson); white cultivars don’t change (MOBOT) |
| Feeding | 10-10-10 at 2 cups per 100 sq ft in March, May and July (Clemson, South Carolina) |
| Pets | Toxic to dogs, cats and horses (ASPCA hydrangea entry; NC State) |
Sources: Clemson HGIC Hydrangea Care (updated 2025) and HGIC 1096 Pruning Hydrangeas; NC State Extension Plant Toolbox; Missouri Botanical Garden Plant Finder.
NC State Extension gives its native range as Japan, China, Korea and Southeast Asia. It is a deciduous shrub: the leaves drop in fall and the bare stems that remain carry next year’s flower buds, which is the key to almost everything below.
Why Is My Hydrangea macrophylla Not Blooming?
A bigleaf hydrangea that grows plenty of healthy leaves but no flowers almost always has one of these problems:
| Cause | What happens | Fix |
|---|---|---|
| Winter damage to buds | Clemson calls winter injury the most common reason hydrangeas fail to bloom. MOBOT and UNH say low temperatures, sudden swings, ice and late frosts kill the buds in some years | Protect stems over winter in cold areas; plant in a sheltered spot; grow a reblooming cultivar |
| Pruning at the wrong time | Cutting back between autumn and early spring removes the flowers on old-wood types (Penn State) | Prune only right after flowering, not after August 1 |
| Too much shade | Clemson lists too much shade as a bloom problem and says hydrangeas need at least 3 hours of morning sun to flower | Move it to a spot with morning sun |
| Too much nitrogen | Listed by Clemson as a cause of poor bloom; lawn fertilizer spreading into the bed is a common source | Stick to a balanced fertilizer at the label rate; keep lawn feed away |
| Too much sun or heat | Clemson’s pruning factsheet lists too much sun among causes of flower loss | Give afternoon shade |
| Florist hydrangea | Gift-shop hydrangeas are often not winter hardy, so the buds or stems freeze each year (Penn State) | Enjoy it as a seasonal plant, or replace it with a hardier cultivar |
Sources: Clemson HGIC; Missouri Botanical Garden; UNH Extension (2017); Penn State Extension (2026).
Clemson’s pruning factsheet explains why timing matters so much: the plant starts forming next year’s buds right after the summer bloom, while it is still growing. Those buds then sit at the stem tips all winter, so a cold snap or a fall “tidy up” removes them.
When to Prune Hydrangea macrophylla
The rule is the same across the sources: Clemson HGIC says the best time to remove dead canes or rejuvenate an older plant is immediately after flowering, and to avoid pruning after August 1 where possible. NC State says to do any pruning before August 1.
- After flowering: the Missouri Botanical Garden says bigleaf hydrangea generally needs little pruning; cut flowering stems back to a pair of healthy buds right after bloom.
- In spring: UNH Extension suggests waiting until mid to late spring, when you can see what survived the winter, and removing only dead canes. MOBOT also says to remove weak or winter-damaged stems in late winter or early spring.
- If you must prune in winter or spring: Clemson says to remove no more than a third of the canes, so some flowers are left for the season.
- Never in late summer or fall (UNH Extension): that removes next season’s flower buds.
Reblooming cultivars are more forgiving. Clemson says ‘Endless Summer’ blooms on old and new growth and can be pruned anytime without losing all its flowers, and the same goes for the lacecap ‘Twist-n-Shout’.
Hydrangea macrophylla Winter Protection
The Missouri Botanical Garden rates bigleaf hydrangea winter hardy to USDA Zone 6 and says plants in Zone 5 should be sited in sheltered locations and given extra winter protection. Two methods appear in both MOBOT and UNH Extension:
- Wrap the stems in burlap.
- Circle the plant with chicken wire and fill the cage with leaves or straw. MOBOT describes filling it 8 to 12 inches deep, and calls both methods time-consuming but an option.
UNH Extension adds that heavy snow cover also protects the buds, but it can’t be relied on, and that the protection should come off only after the danger of frost has passed in spring. A late frost after the buds swell can still undo a whole winter of protection.
If a plant keeps failing despite this, UNH suggests moving it to a more sheltered spot or trying cultivars that bloom more readily on new wood, such as Endless Summer Bloomstruck or Twist-n-Shout. Expect less than the label promises in very cold areas: UNH notes that although Endless Summer is marketed as hardy to Zone 4, harsh New Hampshire winters often kill its flower buds, and after a severe winter there may be only a handful of flowers.
Mophead vs Lacecap and Reblooming Types
Both forms are Hydrangea macrophylla and need the same care (Clemson). Mopheads carry rounded heads made almost entirely of large showy flowers; lacecaps have flat heads with small fertile flowers in the center, ringed by larger showy ones.
| Type | Examples listed by NC State |
|---|---|
| Mophead | ‘Ayesha’, ‘Blushing Bride’, ‘Glowing Embers’, ‘Goliath’, Cityline Paris (compact) |
| Lacecap | Blue Wave (‘Mariesii Perfecta’), ‘Lady in Red’, ‘Fuji Waterfall’ (double white) |
| Reblooming (old and new wood) | ‘All Summer Beauty’, Endless Summer The Original (‘Bailmer’), Endless Summer Bloomstruck, ‘Penny Mac’, ‘Nantucket Blue’, Stargazer (‘Kompeito’, lacecap) |
Source: NC State Extension Gardener Plant Toolbox, Hydrangea macrophylla.
Clemson notes that ‘Blushing Bride’ starts white and matures to pink regardless of soil chemistry, so it won’t respond to the color treatments below.
Why Hydrangeas Turn Blue or Pink
The color comes from aluminum, and soil acidity controls how much aluminum the plant can take up. Penn State Extension explains that blue flowers need acidic soil so the plant can absorb aluminum, and the Missouri Botanical Garden says flowers are blue in highly acidic soil and lilac to pink in slightly acidic to alkaline soil.
Research on the sepals themselves backs this up. A 2021 review by Yoshida, Oyama and Kondo reports that the same main pigment, delphinidin 3-glucoside, is present in both blue and red sepals. Single-cell measurements found far more aluminum per molecule of pigment in blue cells (2.4 equivalents) than in red cells (0.3). The blue color forms when the pigment, aluminum and a co-pigment (5-O-acylquinic acids) combine into one complex.
White cultivars are the exception: the Missouri Botanical Garden says soil pH does not affect white flowers.
How to Make Hydrangeas Blue
- Test the soil first. Clemson says to always test before applying sulfur or lime, and to retest every 2 to 3 years. Your county Extension office can do this.
- Lower the pH to about 5.0 to 5.5 (Clemson) or 5.2 to 5.5 (Penn State) with elemental sulfur or a soil acidifier. Clemson suggests sulfur or aluminum sulfate when new growth emerges in April; MOBOT says to begin treatments in late autumn or early spring. Follow the rate on the product or your soil test report; Clemson’s HGIC 1650 covers how to change soil pH.
- Be careful with aluminum sulfate. Penn State notes that many soils (Pennsylvania’s included) already contain plenty of aluminum, so adding aluminum sulfate is unnecessary there and may damage roots. Lowering the pH is what makes that aluminum available.
- Avoid high-phosphorus fertilizer, which Penn State says works against blue flowers.
- Be patient. Penn State says a color change may take more than a year.
How to Make Hydrangeas Pink
Raise the pH instead. Clemson gives pH 6.0 and up for pink flowers, and Penn State 6.0 to 6.2, using lime. Penn State also suggests a fertilizer high in phosphorus for pink flowers. As with blue, test the soil before liming and expect the change to take more than one season.
Watering, Mulch and Fertilizer
The name says it all: bigleaf hydrangeas have large leaves and wilt quickly when dry. Clemson HGIC advises watering deeply once a week, or more often in dry spells, and up to twice a week in summer heat on sandy soils. Drip irrigation is recommended.
- Mulch: 2 to 3 inches (Clemson); MOBOT suggests 3 inches of shredded bark, peat or compost year-round.
- Fertilizer: Clemson suggests a balanced 10-10-10 at 2 cups per 100 square feet in March, May and July in South Carolina, watered in right away. Use your own soil test and local timing where you live, and avoid excess nitrogen, which Clemson lists as a cause of poor flowering.
For acid-loving neighbors, see our rhododendron care guide.
Are Hydrangeas Toxic to Pets?
Yes. The ASPCA lists hydrangea (its entry names Hydrangea arborescens, with hortensia as another common name) as toxic to dogs, cats and horses because of a cyanogenic glycoside. Signs include vomiting, depression and diarrhea; the ASPCA says cyanide intoxication is rare and the usual result is stomach upset. NC State lists H. macrophylla specifically as a low-severity poison for people, cats, dogs and horses, with hydracyanosides in the bark, flowers and leaves. If you think a pet has eaten some, call your veterinarian or the ASPCA Animal Poison Control Center at (888) 426-4435. For safer choices, see our cat-safe plants list.
Common Problems
| Symptom | Likely cause | What to do |
|---|---|---|
| Healthy leaves, no flowers | Winter bud damage, late pruning, too much shade or nitrogen | See the causes table above |
| Spots on leaves | Cercospora leaf spot or anthracnose (Clemson) | Rake up fallen leaves; keep foliage dry; water early if using sprinklers |
| White powder on leaves | Powdery mildew (Clemson, MOBOT) | Keep foliage dry and improve air flow |
| Curled, sticky new growth | Aphids (Clemson, MOBOT) | Hose them off; see our aphid guide |
| Leaves tied or rolled together | Hydrangea leaftiers (Clemson) | Prune off the tied leaves |
| Wilting | Dry soil; large leaves lose water fast in heat | Water deeply, more often in summer heat (Clemson); mulch |
| Flowers changed color | Soil pH drift, lime from nearby concrete (Penn State) | Test the soil and adjust |
Sources: Clemson HGIC; Missouri Botanical Garden; Penn State Extension.
For other garden shrubs and trees, see our guides to magnolia trees and bonsai.
Frequently Asked Questions
Why is my Hydrangea macrophylla not blooming?
Usually winter damage to the flower buds, which Clemson calls the most common reason, or pruning between autumn and spring, which removes the buds on old-wood types. Too much shade and too much nitrogen are the other common causes (Clemson).
When should I prune Hydrangea macrophylla?
Right after it flowers, and not after August 1 (Clemson, NC State). In spring, remove only stems that died over the winter.
Can I prune hydrangeas in March?
Only to remove dead or winter-damaged stems (MOBOT). Cutting back live stems in March removes the flower buds on old-wood types; if you must, Clemson says take no more than a third of the canes.
How do you change hydrangea flower color from pink to blue?
Lower the soil pH to about 5.0 to 5.5 with sulfur or a soil acidifier after a soil test, and avoid high-phosphorus fertilizer (Clemson, Penn State). It may take more than a year. White cultivars won’t change.
How do I protect hydrangeas in winter?
In Zone 5 and colder sites, wrap the stems in burlap or circle the plant with chicken wire filled with leaves or straw, 8 to 12 inches deep (MOBOT), and remove it only after the danger of frost has passed (UNH Extension).
Are Endless Summer hydrangeas better than regular hydrangeas?
They are more forgiving, because they bloom on both old and new growth (Clemson, NC State). In very cold areas they still lose flower buds in harsh winters, and UNH Extension says to expect only a few flowers after a severe one.
Are hydrangeas poisonous to dogs and cats?
Yes. The ASPCA lists hydrangea as toxic to dogs, cats and horses; the usual signs are vomiting, depression and diarrhea.
Sources
- Clemson HGIC: Hydrangea Care in South Carolina (updated 2025)
- Dabbs, A. Clemson HGIC 1096: Pruning Hydrangeas (reviewed 2021)
- NC State Extension Gardener Plant Toolbox: Hydrangea macrophylla
- Missouri Botanical Garden Plant Finder: Hydrangea macrophylla
- Gibson, M.J.R. (2026). Why Doesn’t My Hydrangea Bloom? Penn State Extension
- UNH Extension (2017): How do I protect my Endless Summer hydrangea over the winter?
- Yoshida, K., Oyama, K. and Kondo, T. (2021). Insight into chemical mechanisms of sepal color development and variation in hydrangea. Proc. Jpn. Acad. Ser. B 97(2): 51-68
- ASPCA: Hydrangea