Growing Hydrangeas: A Toronto Master Gardeners Guide

Hydrangeas are a group of flowering plants that are very popular among gardeners due to their large, showy clusters of flowers. The genus Hydrangea, within the botanical family Hydrangeaceae, is native to Asia and the Americas. The species grown in Toronto’s temperate climate are commonly deciduous shrubs, although vine and tree forms are also available.

Identification and Characteristics

The flower cluster form is the main feature used in the identification of hydrangeas. There are three common shapes:

• Lacecap (flattened, lacy heads with small fertile central florets and showy sterile florets around the edges)
• Mophead (globe-shaped heads with florets of similar size)
• Conical

There are six main types of hydrangeas grown in North American gardens:

• Bigleaf or French hydrangea (Hydrangea macrophylla): This popular variety has big, coarsely toothed leaves that are about 10–15 centimetres long and 8–13 centimetres wide. It is a shrub that produces large, long-lasting, lacecap or mophead flower clusters. The flower colour may be pink, blue or purple and can be altered with soil amendments. It is not as hardy as other varieties and needs winter protection in Toronto.
• Panicle or PeeGee hydrangea (Hydrangea paniculata): Available in shrub and tree forms, this variety has large, white, conical flower clusters that turn pink with age. PeeGees are the hardiest of the hydrangeas and can tolerate more sun than other types.
• Annabelle or smooth hydrangea (Hydrangea arborescens): This classic shrub is among the hardiest of the hydrangeas. In mid- to late summer, it produces large, pale green, mophead flower clusters that whiten as they mature.
• Oakleaf hydrangea (Hydrangea quercifolia): As its name implies, this hydrangea’s leaves are shaped like those of oak trees. It grows in shrub form and produces large, white, conical flower clusters that turn pink as they mature. The leaves become red in fall.
• Climbing hydrangea (Hydrangea anomala petiolaris): This is a large, slow-growing vine that needs structural support. Once established, it produces clusters of white, lacecap flowers in summer. This is a less hardy variety and may need winter protection in Toronto.
• Mountain hydrangea (Hydrangea serrata): This hardy shrub has relatively small leaves and lacecap flower clusters that range in colour from pink to blue to mauve. The flower colour can be altered with soil amendments.

Uses in the Garden

With a range of forms, habits and flower colours, hydrangeas fit numerous garden applications:

• Filling a mid-level garden bed
• Providing a focal point in a garden bed
• Planting along building foundations
• Creating borders and accents in garden beds
• Filling shady sites and other areas where grass will not thrive
• Providing a thriller plant (focal point) in a container garden
• Hedging
• Covering a fence or wall (using climbing hydrangeas)

What to Consider When Choosing Hydrangeas

Before plant shopping, determine your design requirements. What size, shape, texture and colour are you looking for in your garden? For ideas and inspiration, you may want to peruse gardening websites, books and catalogues. Also helpful is window shopping at garden centres, and touring your neighbourhood to see what grows well in your area.

In general, hydrangeas need moist, well-drained soil and protection from strong winds and the hot afternoon sun. Light requirements vary by variety, but typically, hydrangeas need at least four hours of sunlight daily, ideally in the morning. These plants don’t like competing with tree roots for moisture and nutrients, so avoid planting them directly under trees.

These questions will help you decide if a candidate hydrangea will meet your needs:

• What is the ultimate size (height and width) and shape of the plant?
• What colour and size are the flowers?
• When and for how long does it bloom? Some hydrangeas rebloom, displaying flowers all summer.
• What is the foliage colour in each season? Some hydrangeas have attractive fall and/or winter colour from the foliage or the residual flower clusters.
• How will the hydrangea relate to its surroundings through all seasons? Will the plant provide “visual balance” when positioned in your garden?
• Will the plant thrive where you intend to situate it? Check that the location meets the plant’s growing requirements, including hardiness zone, sun or shade tolerance, soil preference and moisture needs.
• What ongoing care does the plant require?
• Are disease-resistant varieties available?

Planting and Care

Planting

For the best results, plant hydrangeas in the early spring or fall. Follow the instructions on the plant tag. If there are no instructions, ask the garden centre how to plant and care for your hydrangea cultivar.

In general, dig a hole wider than and as deep as the root ball, and position the plant in the hole so the top of the root ball is level with the soil surface. Spread out the roots and fill the hole with the soil you removed.

Water thoroughly after planting. Spread organic mulch (such as shredded leaves, bark chips or compost) to a height of 5–8 centimetres around the plant to enhance moisture retention and reduce weed growth.

Watering

The genus name Hydrangea comes from Greek word hydor, meaning “water,” a clue that these plants need plenty of water to thrive. Do not let the soil around hydrangeas dry out. They have shallow roots and tend to dry out faster than other flowering shrubs, so monitor these plants regularly, especially in hot, dry weather, and water deeply when needed.

Fertilizing

Apply compost or a general-purpose organic fertilizer in the spring.

Flower Colour

The flower colour of some bigleaf and mountain hydrangeas can be controlled by applying a special-purpose fertilizer for acid‐loving plants, such as aluminum sulfate. Follow the package directions carefully to avoid plant injury.

More acidic soil has increased aluminum solubility, which impacts flower colour. Flowers will be pink in soil with pH greater than 6.5, mauve in soil with pH 5.5 to 6.5 or blue in soil with pH less than 5.5.

Pruning

Some hydrangea cultivars flower on new wood, meaning the buds form on branches produced in the current growing year. Pruning of these cultivars should be done in late winter before new growth begins. In this group are panicle hydrangeas (H. paniculata) and smooth hydrangeas (H. arborescens). For the latter, periodically cut back some older branches to the ground and shape the remaining ones.

Other cultivars flower on old wood, with the flower buds forming on branches grown the previous year. Prune, if necessary, just after the flowers start to fade in late summer, before next year’s buds start to form. This type includes bigleaf hydrangeas (H. macrophylla), oakleaf hydrangeas (H. quercifolia), climbing hydrangeas (H. anomala var. petiolaris) and mountain hydrangeas (H. serrata).

Some bigleaf and mountain hydrangeas are reblooming, producing flowers on both old and new wood. Prune these cultivars minimally (only to remove dead branches or shape the plant) in the spring after growth has started.

Why Hydrangeas Fail to Bloom

If your hydrangea does not flower, consider the following common explanations:

• Late summer pruning has removed the flower buds of a hydrangea that blooms on old wood.
• Winter injury has occurred to the flower buds of a hydrangea that blooms on old wood.
• Hungry deer have eaten the branches bearing flower buds.
• The hydrangea is not receiving enough sun. (Hydrangeas typically need at least four hours of sun a day, and some varieties need even more.)
• The hydrangea is receiving too much sun. (Yellowing or browning of leaf edges and tips is another sign of this problem.)
• Excessive application of nitrogen fertilizer has encouraged leaf growth over flower production.
• The hydrangea is not receiving sufficient water.

Winter Protection

Hydrangeas that flower on old wood need protection during winter, particularly the bigleaf hydrangeas. Cover the plants with 15 centimetres of mulch (e.g., leaves or straw) in late fall. Remove the winter mulch only after the chance of a late spring frost has passed.

Recommended Hydrangea Cultivars

There are hundreds of hydrangea cultivars. Some hydrangeas produce sterile flowers (e.g., mophead bigleaf hydrangeas) and do not support pollinators. If you choose one of these varieties, also include plants that support pollinators in your garden. Look for cold-hardy hydrangea varieties, and consider a reblooming cultivar.

Bigleaf Hydrangeas

Endless Summer Hydrangea (Hydrangea macrophylla ‘Endless Summer’)

• Multi-stemmed, reblooming shrub
• Glossy, oval, dark green leaves
• Large, deep pink, mophead flowers (sterile) in mid-summer to fall, turning blue in acidic soil
• Height: 1.2 m; spread: 1.2 m
• Hardiness zone 4a
• Full sun to partial shade; prefers morning sun and afternoon shade
• Prefers evenly moist, well-drained soil
• Requires winter protection

Panicle Hydrangeas

Bobo Hydrangea (Hydrangea paniculata ‘ILVOBO’)

• Multi-stemmed, dwarf, long-blooming shrub
• Conical, white, upright flower clusters, turning pink in early fall
• Height: 0.9 m; spread: 1.2 m
• Hardiness zone 3b
• Full sun to full shade
• Prefers average to moist soil
• Prune regularly for best performance

Limelight Hydrangea (Hydrangea paniculata ‘Limelight’)

• Large, multi-stemmed shrub
• Oval, green leaves; conical, lime green, upright flower heads in mid-summer, fading to pink, red and burgundy in fall
• Height: 2.4 m; spread: 2.4 m
• Hardiness zone 3b
• Full sun to partial shade; prefers morning sun and afternoon shade
• Prefers average to moist soil

Smooth Hydrangeas

Annabelle Hydrangea (Hydrangea arborescens ‘Annabelle’)

• Fast-growing, multi-stemmed, rounded shrub
• Forest green, heart-shaped leaves
• Enormous, ball-shaped, white flower heads in mid-summer
• Height: 1.2 m; spread: 1.5 m
• Hardiness zone 3a
• Full sun to full shade
• Prefers average to moist soil
• Prune back to 15 cm in spring

Incrediball Hydrangea (Hydrangea arborescens ‘Abetwo’)

• Multi-stemmed, rounded shrub
• Dark green, heart-shaped leaves
• Huge, white flower heads in mid- to late summer, aging to jade green; the flowers are more upright than those of the Annabelle cultivar
• Height: 1.5 m; spread: 1.5 m
• Hardiness zone 3a
• Full sun to partial shade
• Prefers average to moist soil
• Prune back to 15 cm in spring

Climbing Hydrangeas

Climbing Hydrangea (Hydrangea anomala var. petiolaris)

• Self-clinging, multi-stemmed, slow-growing, long-lived, woody vine
• Glossy, heart-shaped, green leaves
• Fragrant, white, lacecap flower heads in mid-summer
• Height: 12.2 m; spread: 0.6 m
• Hardiness zone 6a
• Full sun to full shade
• Prefers average to moist soil
• Requires a supporting structure

Oakleaf Hydrangeas

Snowflake Hydrangea (Hydrangea quercifolia ‘Snowflake’)

• Multi-stemmed, deciduous shrub
• Unique double florets (sterile) in large, white, conical flower clusters throughout summer, gradually changing to pink and then red in fall
• Large, fuzzy, lobed leaves turn red in fall; peeling, brick-red bark
• Height: 1.8 m; spread: 1.8 m
• Hardiness zone 5a
• Full sun to full shade
• Prefers average to moist soil

Pee Wee Hydrangea (Hydrangea quercifolia ‘Pee Wee’)

• Compact, multi-stemmed shrub for smaller gardens
• Fuzzy, green, lobed leaves turn brick red in the fall
• Fragrant, conical, white flower clusters in early to mid-summer, fading to pink
• Height: 0.9 m; spread: 0.9 m
• Hardiness zone 6a
• Full sun to full shade
• Prefers average to moist soil

For More Information

General Guides

• Ohio State University Extension. Ohioline. Selecting hydrangeas for the home landscape. https://ohioline.osu.edu/factsheet/hyg-1263
• Proven Winners. 6 types of hydrangeas: demystified. https://www.provenwinners.com/Hydrangeas-Demystified
• Willowbrook nurseries. Care for hydrangea – planting and maintenance. https://willowbrooknurseries.com/how-to/hydrangea-care-guide/

Flower Colour and Fertilizer

• University of Massachusetts Extension. Growing tips 5 – Hydrangeas: Color and fertilizing https://ag.umass.edu/sites/ag.umass.edu/files/fact-sheets/pdf/FS5_HydrangeaColorFert14_0.pdf

Pruning

• National Garden Bureau. When to prune hydrangeas. https://ngb.org/2020/03/05/hydrangea-pruning/
• University of Massachusetts Extension. Growing tips 4 – Hydrangeas: pruning for blooms. https://ag.umass.edu/sites/ag.umass.edu/files/fact-sheets/pdf/FS4_HydrangeaPruneBloom14.pdf

Winter Protection

• Savvy Gardening. How to protect your hydrangea for the winter. https://savvygardening.com/how-to-protect-your-hydrangea-for-the-winter/

 

Date Prepared: February 2022