Hydrangea: how to identify it and tell the types apart

Mophead vs lacecap bigleaf hydrangea

Mophead and lacecap are two forms of the same species, not two species. NC State Extension describes bigleaf hydrangea cultivars as producing either ball-shaped clusters ("mop-heads") or flat clusters of small flowers surrounded by a ring of more prominent flowers ("lacecaps"). In a mophead nearly every floret is the large, sterile kind; in a lacecap the large ones form only the outer ring.

Whichever the form, the plant underneath is the same: a rounded, multi-stemmed shrub, generally 3 to 6 feet tall and wide, with glossy, coarsely toothed leaves 4 to 8 inches long on stout stalks. Bigleaf hydrangea blooms in late spring and summer on the previous year's stems, which is why the RHS warns that heavy pruning means lost flowers; some cultivars, sold as rebloomers, flower again on new growth later in summer.

How to tell the other hydrangea species apart

Panicle hydrangea (H. paniculata) is the big one: NC State gives 8 to 15 feet, up to 25, calls it a shrub or small tree and one of the hardiest hydrangeas in winter, and notes it is more tolerant of full sun than other species. Its heads are cones of creamy white flowers, 6 to 8 inches long, that fade toward pink and tan in fall. Its leaves are smaller than bigleaf's and, as NC State's quick ID hints note, sometimes whorled near the branch tips.

Smooth hydrangea (H. arborescens) is native to woods and streambanks of the eastern United States and is the most common native hydrangea in North Carolina. Wild plants have flat heads of tiny white fertile flowers with only a few large florets at the edge; several named cultivars have sterile flowers in ball-shaped heads. Its leaves are broad, egg-shaped to rounded and sharply toothed, 2 to 6 inches long, and the bark peels in thin layers.

Oakleaf hydrangea (H. quercifolia) is the easiest: its leaves have 3 to 7 lobes like an oak leaf, 3 to 8 inches long and felty underneath, and they turn wine, orange or mahogany in fall. Its flowers come in upright cones, 4 to 12 inches long, white fading to pink and then tan. Older stems peel to reveal cinnamon-brown bark, a winter mark. It is native to the southeastern United States.

Climbing hydrangea (H. petiolaris, which the RHS lists as Hydrangea anomala subsp. petiolaris) is a woody vine. NC State describes it clinging by aerial rootlets and reaching 60 feet on walls, arbors and other structures, with flat, fragrant heads 6 to 8 inches wide: a ring of large white sterile florets around creamy fertile ones, in early summer. Its leaves are glossy, 2½ to 5 inches long, with heart-shaped bases on long stalks, and the bark peels reddish-brown.

Why hydrangea flowers change color with soil

This is the question most people bring to a hydrangea, and the sources agree on the answer.

• The mechanism. The RHS explains that on mophead and lacecap bigleaf hydrangeas (and H. serrata and H. involucrata), soil pH controls how much aluminium is available to the plant. Flowers are blue in acid soil (high available aluminium), mauve in lightly acid to neutral soil and pink in alkaline soil. The RHS adds that blue cultivars stay blue at around pH 4.5 to 5.
• What doesn't change. The RHS notes that white, red and green-flowered cultivars stay white or green whatever the soil. NC State says smooth hydrangea's color is not affected by soil pH. Panicle and oakleaf hydrangeas are white-flowered species whose heads age to pink and tan, which is not a soil effect.
• Changing it. NC State says aluminum sulfate can be added to acidify the soil (bluer) and lime to make it more alkaline (pinker). The RHS says the "blueing compounds" sold for this contain aluminium sulphate, that you should always follow the manufacturer's instructions, and that on very alkaline or chalky soil they won't work, so growing in containers of ericaceous compost is the alternative.
• Aging is different. By late summer, the RHS notes, blue and pink heads fade to mixes of green and pink-red, and white heads often turn pink. A fading head says nothing about the soil.

So color is a clue to the soil, not to the species: two bigleaf hydrangeas of the same cultivar can flower blue and pink a few streets apart.

Hydrangeas and pets

The ASPCA Animal Poison Control Center lists hydrangea as toxic to dogs, cats and horses. The entry is filed under smooth hydrangea (Hydrangea arborescens), with hortensia among its other names; the toxic principle is a cyanogenic glycoside, and the listed signs are vomiting, depression and diarrhea, with the ASPCA noting that cyanide intoxication is rare. The RHS says all parts may cause mild stomach upset if eaten, and that contact with the foliage may aggravate skin allergies, so it recommends gloves.

If a pet has eaten part of a hydrangea, call your veterinarian or the ASPCA Animal Poison Control Center at (888) 426-4435 (a consultation fee may apply). If a person has, call Poison Control at 1-800-222-1222 (poison.org). The same ASPCA lists cover other garden shrubs in poisonous plants for dogs and plants toxic to cats.

A short care note

The RHS recommends moist, well-drained soil and light shade, with sun tolerated if the soil doesn't dry out; NC State adds protection from afternoon sun for bigleaf hydrangea. Pruning follows the flowering wood: bigleaf hydrangeas flower on last year's stems, so the RHS advises light pruning in mid-spring, cutting back to the first or second strong pair of buds below the old head, while panicle and smooth hydrangeas, which flower on new growth, can be cut back in early spring. Oakleaf hydrangea also flowers on old wood and needs little pruning. If leaves develop spots or mildew, Plant Identifier's Diagnose tool gives a first read from a photo.