Merritt’s Supreme is a classic bigleaf hydrangea (Hydrangea macrophylla) prized for its enormous mophead flower clusters, which can span eight inches or more across. Like most bigleaf hydrangeas, it shifts flower color depending on soil chemistry, blooms on old wood, and thrives in partial shade with consistent moisture. Growing it well is less about intensive effort and more about getting a few key decisions right at the start, particularly where you plant it, how you manage soil pH, and when (or whether) you prune.
Choosing the Right Spot
Merritt’s Supreme performs best in USDA hardiness zones 6 through 9, though gardeners in zone 5b sometimes succeed with extra winter protection. The single most important siting decision is light exposure. Morning sun with afternoon shade is the standard recommendation for bigleaf hydrangeas, and it holds especially true for a cultivar that produces such large, dense flower heads. Full afternoon sun in warm climates scorches the blooms and wilts the broad leaves, while too much shade reduces flowering.
In cooler northern gardens, you can get away with more sun exposure, sometimes up to six hours of direct light. In the Southeast or other areas with intense summer heat, err on the side of more shade. Dappled light under a high canopy of deciduous trees works well, since it provides some winter sun when the trees are bare, helping the plant harden off before cold weather sets in.
Avoid planting directly against a south- or west-facing wall, where reflected heat intensifies sun exposure. Also avoid low-lying frost pockets. Cold air settles in depressions and can kill the flower buds that formed the previous summer, wiping out an entire season of blooms.
Soil Preparation and Planting
Bigleaf hydrangeas are not especially picky about soil type, but they strongly prefer soil that drains well while holding some moisture. Heavy clay that stays waterlogged invites root rot; sandy soil that dries out quickly leaves the plant stressed. If your native soil leans toward either extreme, amending with compost before planting is the simplest fix. Work several inches of organic matter into the planting area to improve both drainage and moisture retention simultaneously.
Dig a hole roughly twice as wide as the root ball but only as deep. Planting too deep is a common mistake that smothers the root crown and can kill the plant slowly. Set the root ball so its top sits level with or slightly above the surrounding soil line. Backfill with the amended soil, water deeply, and apply a two- to three-inch layer of mulch, keeping it a few inches away from the stems.
Spring planting, after the last frost, gives the roots a full growing season to establish before winter. Fall planting works in zones 7 and warmer, where the ground doesn’t freeze hard. In colder zones, spring is much safer.
Watering and Mulch
Hydrangeas are thirsty plants. The name itself comes from the Greek for “water vessel,” and Merritt’s Supreme, with its oversized leaves and flower heads, has a lot of surface area losing moisture to the air. During the growing season, aim for about an inch of water per week from rain and irrigation combined, and increase that during heat waves or dry spells. Deep, infrequent soaking encourages roots to grow downward; shallow daily sprinkling keeps roots near the surface where they’re more vulnerable to heat and drought.
Drip irrigation or a soaker hose at the base of the plant is better than overhead watering. Wet foliage, especially in the evening, creates conditions that favor fungal diseases. If you water with a hose, do it in the morning so the leaves dry by nightfall.
Mulch is arguably the single easiest thing you can do to keep a bigleaf hydrangea happy. A consistent two- to four-inch layer of shredded bark, pine needles, or leaf compost moderates soil temperature, conserves moisture, and suppresses weeds. Organic mulches also break down over time, slowly feeding the soil. Refresh the mulch layer once or twice a year as it decomposes.
How Flower Color Works and How to Change It
One of the most striking features of Merritt’s Supreme is its color variability. The same plant can produce blue, purple, pink, or lavender blooms depending on the chemistry of the soil it grows in. The determining factor isn’t pH directly but aluminum availability, which pH influences. In acidic soil (roughly pH 5.0 to 5.5), aluminum is soluble and the roots absorb it, producing blue flowers. In alkaline or neutral soil (pH 6.5 and above), aluminum is locked up and unavailable, and the flowers turn pink. The range in between yields various shades of purple and lavender.
At the molecular level, aluminum ions interact with pigment compounds in the flower sepals. Research on bigleaf hydrangea has shown that aluminum treatment significantly increases the concentration of specific pigment compounds in the sepals while also triggering broad changes in gene expression, including downregulation of genes involved in the pathway that produces the red and pink anthocyanin pigments.1Agronomy. Integrative Transcriptomics and Proteomics Elucidate the Regulatory Mechanism of Hydrangea macrophylla Flower-Color Changes Induced by Exogenous Aluminum The result is a shift from pink toward blue as more aluminum enters the plant.
To push blooms toward blue, the traditional approach is to apply aluminum sulfate to the soil, which both lowers pH and provides aluminum directly. Studies on several bigleaf hydrangea cultivars have confirmed that higher doses of aluminum sulfate shift sepals from pink to blue or light blue, though the degree of change varies among cultivars, and some deeply pigmented pink varieties resist full conversion even at substantial doses.2Journal of People, Plants, and Environment. Control of Sepal Color and Growth of ‘Pink Ari’, ‘Morning Star’, and ‘Green Ari’ Hydrangeas by Aluminum Sulfate Treatment Merritt’s Supreme, being a standard mophead, generally responds well to aluminum sulfate treatment. Apply it in early spring before flower buds develop color, following the product label’s recommended rate. Overdoing it can burn roots.
To push blooms toward pink, raise the soil pH by adding garden lime (calcium carbonate) in fall or early spring. This ties up aluminum in the soil so the roots can’t absorb it. The change doesn’t happen overnight; it usually takes a full growing season or two before you see a complete color shift, because the plant needs to cycle through its stored aluminum. White-flowered hydrangea cultivars, by the way, do not change color regardless of pH. But Merritt’s Supreme isn’t one of those.
Pruning Without Losing Blooms
This is where more people go wrong with Merritt’s Supreme than anywhere else. Bigleaf hydrangeas bloom on old wood, meaning the flower buds for next year’s show are set on this year’s stems during late summer and fall. If you prune in late fall, winter, or early spring, you’re cutting off the buds and you’ll get few or no flowers the following season.
The safe window for pruning is immediately after flowering finishes, typically mid to late summer. At that point, you can cut back stems that have just bloomed, remove dead or weak branches, and thin crowded interior growth to improve air circulation. Don’t cut more than about a third of the plant in any one year. If the shrub has gotten leggy or overgrown, spread the renovation over two or three seasons rather than hacking it all back at once.
In late winter or early spring, it’s fine to remove obviously dead wood. Dead stems are gray, brittle, and have no green beneath the bark when you scrape them with a thumbnail. Live stems show green tissue beneath the outer bark. If you’re unsure whether a stem is alive, wait. By mid-spring, live stems will push out visible buds and you can prune the dead ones with confidence.
Some newer bigleaf hydrangea varieties, such as the Endless Summer series, bloom on both old and new wood, giving gardeners more pruning flexibility. Merritt’s Supreme is not one of these reblooming types. It sets buds on old wood only, so pruning discipline is non-negotiable if you want flowers.
Fertilizing
Merritt’s Supreme doesn’t need heavy feeding. A single application of a balanced slow-release fertilizer in spring, as new growth begins, provides enough nutrition for the season. Avoid high-nitrogen fertilizers marketed for lawns; they push lush foliage at the expense of flowers. A formulation designed for flowering shrubs, or a general-purpose product with roughly equal parts nitrogen, phosphorus, and potassium, works well.
If you’re actively managing flower color, keep in mind that some fertilizers contain ingredients that affect aluminum availability. Superphosphate, for instance, binds aluminum and can push blooms toward pink even in acidic soil. Fertilizers with ammonium sulfate can lower pH and nudge blooms toward blue. Read labels carefully if color matters to you, and be aware that your fertilizer choice and your pH-management strategy need to work together, not against each other.
Avoid fertilizing after midsummer. Late-season feeding stimulates tender new growth that won’t harden off before frost, making the plant more vulnerable to winter damage.
Common Diseases and How to Handle Them
Bigleaf hydrangeas are susceptible to a handful of fungal problems, and Merritt’s Supreme is no exception. The two most common are powdery mildew and Cercospora leaf spot.
Powdery mildew shows up as a white or grayish powdery coating on the upper surfaces of leaves, usually in late summer when days are warm and nights are cool with high humidity. It rarely kills the plant, but it weakens it over time and looks terrible. Cercospora leaf spot presents as small, circular brown or purple spots on the lower leaves, often with a lighter center, gradually spreading upward through the canopy as the season progresses.
Good cultural practices are your first line of defense against both diseases. Plant with enough spacing for air to move through the canopy, water at the base rather than overhead, and clean up fallen leaves in autumn since fungal spores overwinter in debris. If a plant has had persistent problems, thinning interior branches during the summer pruning window helps improve airflow.
When cultural controls aren’t enough, fungicide treatment can help. Research on bigleaf hydrangea has evaluated strobilurin-class fungicides for controlling both powdery mildew (caused by Erysiphe polygoni) and Cercospora leaf spot (caused by Cercospora hydrangea), finding them effective across a range of application rates.3Journal of Environmental Horticulture. Impact of Application Rate and Interval on the Control of Powdery Mildew and Cercospora Leaf Spot on Bigleaf Hydrangea with Azoxystrobin Preventive applications beginning before symptoms appear tend to be more effective than trying to knock down an established infection. Neem oil and potassium bicarbonate sprays offer organic alternatives for milder cases.
Beyond fungal issues, watch for aphids and spider mites, especially in hot, dry weather. A strong spray of water from the hose dislodges aphids, and insecticidal soap handles heavier infestations. Deer are also fond of hydrangea foliage in many regions, and no amount of spray repellent is completely reliable in areas with heavy deer pressure. Fencing or netting during the growing season may be necessary.
Winter Protection in Cold Climates
In zones 7 and warmer, Merritt’s Supreme generally overwinters without special protection. In zone 6, and especially in the colder parts of it, protecting the flower buds through winter becomes important. Remember, the buds for next year’s flowers are already on the stems. A hard freeze or desiccating winter wind can kill them even when the plant itself survives.
The most effective low-tech method is to build a cage of chicken wire or hardware cloth around the plant after the first hard frost and fill it loosely with dry leaves, straw, or pine needles. This insulates the stems and buds. Remove the cage gradually in spring once temperatures are consistently above freezing, but don’t rush it. Late frosts in April or May can catch unwary gardeners after they’ve already unwrapped their hydrangeas.
Avoid wrapping the plant tightly in burlap or plastic. Tight wrapping traps moisture against the stems and can cause more damage than the cold itself, encouraging mold and rot during winter thaws. The goal is loose insulation that lets some air circulate while buffering temperature extremes.
An additional layer of mulch, four to six inches deep, protects the root zone through winter. Pull it back to the normal two- to three-inch depth in spring so the crown isn’t smothered as growth resumes.
Growing Merritt’s Supreme in Containers
Merritt’s Supreme can be grown in a large container, which gives gardeners in marginal climates a useful advantage: you can move the pot to a sheltered spot for winter. Choose a container at least 18 to 24 inches in diameter with drainage holes. Use a high-quality potting mix rather than garden soil, which compacts in containers and drains poorly.
Container-grown hydrangeas dry out much faster than those in the ground, so you’ll need to water more frequently, sometimes daily during hot weather. A self-watering planter or a drip system on a timer can save you from having to babysit the plant through summer. Feed container plants with a liquid fertilizer every two to four weeks during the growing season, since nutrients leach out of pots with each watering.
Managing flower color is actually easier in a container because you control the entire soil volume. You can mix aluminum sulfate directly into the potting mix for blue flowers, or use a lime-amended mix for pink, without worrying about your native garden soil buffering the change. The tradeoff is that containers offer less root insulation in winter. In zones colder than 7, either move the pot into an unheated garage or bury it in the ground up to its rim and mulch over it until spring.
Propagation From Cuttings
If you want more plants without buying them, Merritt’s Supreme propagates readily from softwood cuttings taken in early to midsummer. Select a healthy, non-flowering stem and cut a section about four to six inches long, just below a leaf node. Strip the leaves from the lower half, dip the cut end in rooting hormone, and insert it into a pot of moist perlite or a perlite-peat mix. Cover the pot with a clear plastic bag to maintain humidity, keep it in bright indirect light, and mist the cutting if the medium starts to dry out.
Roots typically develop in three to six weeks. You can check by gently tugging on the cutting; if it resists, roots have formed. Transplant rooted cuttings into individual pots and grow them on for a full season before planting out in the garden. Young plants are more cold-sensitive than established ones, so give them their first winter in a protected spot or bring container-grown starts indoors.
Layering is another low-effort option. Bend a low-growing branch to the ground, wound the underside slightly with a knife, pin it down with a landscape staple, and cover the wounded section with soil. Leave the tip exposed. After a season, the buried section will have rooted, and you can sever it from the parent plant and transplant it. Layering is slower but has a high success rate because the stem stays connected to the parent’s water and nutrient supply while it forms roots.
Why Blooms Sometimes Fail
Even experienced gardeners occasionally end up with a lush, healthy Merritt’s Supreme that simply refuses to flower. The most common causes, roughly in order of frequency, are:
- Bad pruning timing: Cutting stems in fall, winter, or early spring removes the buds that were set the previous summer.
- Winter bud kill: A cold snap or late frost destroys the flower buds while the plant’s roots and stems survive fine. The plant leafs out normally but produces no blooms.
- Too much shade: Heavy shade reduces the energy available for bud formation. The plant grows leaves but doesn’t set flowers.
- Excess nitrogen: Over-fertilizing with nitrogen-heavy products pushes vegetative growth at the expense of reproductive growth.
- Deer browsing: Deer can nip off stem tips where the flower buds sit, especially in late fall or early spring when other browse is scarce.
If your plant hasn’t bloomed in two or more years, work through that list systematically. The pruning and winter-damage explanations account for the vast majority of cases. For gardeners in zone 6 or colder who find that winter bud kill is a recurring problem despite protection efforts, switching to a reblooming variety that flowers on new wood may be a more practical long-term solution than fighting the climate every year.