For gardeners living at upper elevations, growing the lovely hydrangea is an excellent choice. Not only does the shrub love the cool moist air in Kaloko mauka but it is also a good ornamental candidate for Waimea or Volcano Village. Hydrangeas can also thrive in a cool shady spot at lower elevations.
Though hydrangea flowers can range in color from deep pink true blue, the acid soil in our uplands usually results in attractive clusters of blue flowers. This plant, Hydrangea macrophylla, is called Popohau in Hawaiian, which literally translates to ‘snow ball’. The name accurately describes the round icy blue appearance of the inflorescence.
The Hydrangeacea family includes 17 genera and over 170 species. Most are woody plants that grow in temperate areas but macrophylla and aspera species grow well here, especially at higher elevations. Both originated in the Himalayas and southern China before spreading into worldwide cultivation.
It was the attractive long-lasting flowers of the macrophylla that made them popular for centuries in China and Japan and later in Europe and North America. The aspera variety has a long history as well, including one in Hawaiian gardens, though its flowers are slightly less showy.
Hydrangea inflorescences appear as clusters in rounded heads made up of small flowers usually smaller than an inch across. The heads can be up to a foot across creating a ‘ball’ of flowers.
Not often seen in Hawaii, hydrangea quercifolia or oakleaf hydrangea is native to the moist woods and stream banks in the Southeastern United States. Like other hydrangeas, it can grow to eight feet tall and just as wide. Both the white flowers that grow in pyramidal clusters as well as the large leaves that appear in an oakleaf shape make this an attractive member of the family.
Though hydrangea flowers can range in color from pink to purple and blue, they can also fade to lighter colors. When grown in the acid soil (pH 4.5 to 5.5) of Hawaii’s upper elevations, they will usually produce blue flowers. Color variations can be manipulated, however, by altering the soil pH during bud development. Plants grown in alkaline conditions (between pH 7 and 7.5) tend to have flowers that are pink in color. Adding lime or super phosphate to the soil can increase the rosy hue. Adding iron or other acidifying agents to pink flowering plants can change their color to purple and blue. Adding aluminum sulfate can deepen the blue color.
Hydrangea plants make excellent hedges or individual specimen plants in a shady spot in the garden either potted or in the ground. They are also very adaptable to indoor pots in moist soil and located in indirect sunlight.
New hydrangea blossoms generally appear in late spring or early summer but the flowers can last up to six months on the plant. In most locations here, hydrangeas sport flowers nearly year-round on woody branches that can reach 8 feet tall. Their large dark green leaves add contrast to the flowers at every stage.
As they age, the flowers may ‘antique’ to delicate lilac or mottled white before finally turning brown. Their longevity suits them for use in arrangements, wreaths, bridal bouquets and as dried specimens. As they dry, they are known as ‘mopheads’ and are used in many art and craft projects.
Though hydrangeas can be grown from seed, the seeds are rare. Most of the flowers are sterile and are not pollinated. This is responsible for their long life but means they don’t produce seeds. The best way to propagate hydrangea is from cuttings or by air layering.
Reproduction from cuttings is easy. Select a four-to-six-inch length from the older wood of the plant, about the diameter of a pencil. Remove most of the leaves, dip the cutting in a rooting compound and place it in a medium that is about 1/2 perlite. Keep it moist and in a cool shady spot until it produces new leaves.
Once the plant has developed strong roots and seems healthy, it is ready to plant into a cool, moist location. At lower elevations, where it might be hotter and drier, place the new plant in full or partial shade in soil that is rich in organic matter. Integrating compost into the planting mix and applying mulch in the root zone can be beneficial. Keep the site somewhat moist with a combination of mulching and frequent watering.
Hydrangeas grow well in groups, as hedging plants or as attractive single specimens and need little maintenance. The plants will do well with a small amount of a balanced fertilizer applied twice a year.
Most varieties do not require pruning except to shape or reduce size. Flowers are produced on older growth so it is best to only prune lightly in fall immediately following flowering. Some varieties do bloom on new growth, so know that hard pruning on these may reduce blooms the following year.
Though healthy hydrangeas are not likely to be attacked by diseases or pests, weak plants can develop powdery mildew fungus in warm, dry, windless locations. This can be controlled by mixing and applying a tablespoon of baking soda added to 1/2 teaspoon of liquid soap in a gallon of water. A tablespoon of light oil (neem is recommended) can be added to coat and smother the fungus. Avoid applying this in full sun. For insect attacks you can use the same mix of soap and oil, without the baking soda.
Care should be taken to grow hydrangeas where they will not be eaten by pets or children. Though some parts of the roots of hydrangeas have medicinal value, the leaves, flowers and stems are toxic and can produce nausea, sweating and vomiting if large quantities are ingested.
Add hydrangeas to you garden for long lasting ornamental beauty. Check local nurseries or garden sections for healthy specimens and plan on some lovely blossoms appearing in a few months that will last for a long time.
Gardening events
Now: Pre-order leaf resistant coffee trees from the UH CTAHR office in Kainaliu. Catimor hybrid seedlings and grafted trees are available. Conventionally grown, not organic. Trees grafted on liberica rootstock (coffee root-knot nematode-tolerant) are $15/ ungrafted seedlings $10. Available on a first come, first serve basis. Email andreak@hawaii.edu to express interest. A waiver and release form must be signed prior to purchase.
Save the dates:
Thursdays, from Jan. 9 through March: Coffee related research and management update webinars, starting at noon. Registration is required at www.hawaiicoffeeed.com/coffeewebinars or contact Matt at 808-322-0164 two days prior. Live Q&A follows each session.
Tuesday, Feb. 25: Coffee Symposium and Trade Show at the Outrigger Kona’s Kaleiopapa Conference Center with the Trade Show immediately adjacent. Sponsorships and registration available at the Kona Coffee Farmers’ Association website.
Farmer direct markets
(Check websites for the latest hours and online markets)
Wednesday and Friday: Ho’oulu Farmers Market, 9 a.m. to 2 p.m. at Sheraton Kona Resort at Keauhou Bay.
Saturday: Keauhou Farmers Market, 8 a.m. to noon at Keauhou Shopping Center; Kamuela Farmer’s Market, 7:30 a.m. to noon at Pukalani Stables; Waimea Town Market, 7:30 a.m. to noon at the Parker School in central Waimea; Waimea Homestead Farmers Market, from 7:30 a.m. to noon at the Waimea middle and elementary school playground.
Sunday: Pure Kona Green Market, 9 a.m. – 2 p.m. at Amy Greenwell Garden in Captain Cook; Hamakua Harvest, 9 a.m. to 2 p.m. at Hwy 19 and Mamane Street in Honokaa.
Plant advice lines
Anytime: konamg@hawaii.edu
Tuesdays and Thursdays: 9 a.m. to noon at UH-CES in Kainaliu: 322-4893 or walk in.
Mon., Tues. and Fri: 9 a.m. to noon at UH CES at Konohana in Hilo, 981-5199 or himga@hawaii.edu
Diana Duff is a plant adviser, educator and consultant living part time in Kailua-Kona.
