Hydrangeas of all types have always been landscape favorites. Big or small; mop head or panicled; pink, blue, white or green blooming – it seems there’s a selection for every garden and every gardener. A recent resurgence in popularity has propelled these flowering shrubs to the top of the list of must-haves. And, recent success in breeding compact cultivars provides even more opportunity for growers, landscape professionals and garden centers to offer beautiful new selections for small landscapes.
Oakleaf hydrangea (Hydrangea quercifolia Bartr.) is an ornamental shrub that is native to the southeastern U.S. Most plants grow 6 feet or taller with an equal to wider spread, but two compact selections (‘Pee Wee’ and ‘Sikes Dwarf’) are commonly available in the trade. Both reach approximately 3 to 4 feet in height and width at maturity, but they lack some of the ornamental traits found among the more attractive, standard-sized oakleaf hydrangea cultivars.
The oakleaf hydrangea breeding program at the U.S. National Arboretum’s worksite in McMinnville, Tenn., was started in 1996 for the purpose of developing attractive, compact oakleaf hydrangea cultivars suitable for use in small residential gardens. During the course of this breeding project, we have gained experience in both seed and cutting propagation of oakleaf hydrangea. This report describes the development of two new compact cultivars – ‘Ruby Slippers’ and ‘Munchkin’ – along with techniques used for propagating the species from both seeds and cuttings. Both of these cultivars were released in 2010.
The selection named ‘Ruby Slippers’ is similar in size to ‘Pee Wee’ and ‘Sikes Dwarf’, reaching 3.5 feet high and 5 feet wide after seven years of growth in McMinnville, Tenn (Zone 6b). It produces inflorescences up to 10 inches in length and 4 inches in diameter that are held above the foliage. At full flower, the exterior surface of the inflorescence is almost completely covered by large, showy sepals. Flowers initially open white but quickly begin to turn pink. Sepals eventually deepen into a bright rose color.
‘Ruby Slippers’ originated from controlled hybridizations that were made in 1998 between ‘Snow Queen’ and ‘Pee Wee’. While most of the F1 progeny had the upright, full inflorescences of ‘Snow Queen’, none had a compact growth habit. In 2001, 10 of the most attractive ‘Snow Queen’×’Pee Wee’ progeny were intercrossed, using bulked pollen from the 10 selections. A seedling from this second-generation population was selected in 2004 for further evaluation. In 2006, plants of this selection were sent for evaluation to nursery and university cooperators.
‘Munchkin’ is somewhat smaller than either ‘Pee Wee’ or ‘Sikes Dwarf’, growing to 3 feet high and 4.5 feet wide in nine years of growth in McMinnville. It produces inflorescences up to 6.7 inches in length and 4.7 inches in diameter that are held above the foliage. The exterior surface of the inflorescence is primarily covered by large, showy sepals. Flowers open white aging to medium pink.
‘Munchkin’ originated from open-pollinated seed that was collected in 1997 from H. quercifolia ‘Sikes Dwarf’. Two seedlings from this population, one with moderately compact plant habit and the other with large, upright inflorescences, were hybridized in 1999. A seedling from this second-generation population was selected in 2002 for further evaluation. In 2007, it was sent for evaluation to nursery and university cooperators.
Propagation by seed
Held proudly above the foliage, infl orescences up to 6.7 inches in length and 4.7 inches in diameter cover Hydrangea quercifolia ‘Munchkin’. The exterior surface of the infl orescence is primarily covered by large, showy sepals. Flowers open white aging to medium pink.
Seed capsules for both plants are harvested in late September to mid-October and, if necessary, placed in paper bags in the refrigerator for a couple of weeks to finish drying. A portable sieve shaker is used to extract and clean seeds. Seeds are then placed into small glassine bags and stored in a refrigerator until sown.
Although seeds can be sown immediately, we usually wait until just after the first of the year. Shallow seed trays are filled with a commercial seed germination medium, and then a very thin layer of the same medium is sifted over the top of the trays. This layer of sifted medium provides a smooth surface for seeding, but an excess of sifted material should be avoided, as it can become a hard barrier to seed germination. Seeds are sown on top of the germination medium and then placed in a greenhouse on heating mats set to 27 degrees C (80 degrees F). Seed trays are watered every two days by placing them into large, shallow trays filled with water until thoroughly moistened. If any areas of the seed trays begin to dry out between subirrigations, they are lightly moistened using a misting nozzle.
Seeds usually germinate within two weeks. At this point, a 100 ppm water-soluble fertilizer is used in place of plain tap water when subirrigating seed trays. Seeds are transplanted to 5 cm (2-inch) containers containing a commercial seedling medium when they have developed their second set of true leaves. Subirrigation with 100 ppm fertilizer every two to three days, depending on moisture level of medium, is continued until the next transplant.
When roots have filled the containers, the plants are transplanted to 13 cm (5 inch) square containers filled with a shredded pine bark medium amended with 7.6 kg/m3 (6.4 lb/yd3) 19-5-9 Osmocote Pro fertilizer (Scotts-Sierra Horticultural Products Co., Marysville, Ohio), 0.9 kg/ m3 (1.5 lb/yd3) Micromax (Scotts-Sierra Horticultural Products Co.), 0.6kg/m3 (1.0 lb/yd3) iron sulfate and 0.6kg/m3 (1.0 lb/yd3) lime. Low-volume drip stakes are used for irrigation. Plants are occasionally watered thoroughly using a hose to ensure even distribution of moisture throughout the pot.
When plants begin to crowd each other on the greenhouse bench (usually at 6 to 8 inches in height), they are pruned backed to the second or third node and large, lower leaves are removed; regrowth from axillary buds creates a full, sturdy plant. By early May, plants are large enough to be transplanted to No. 3 containers or the field.
Two problems are routinely encountered when growing oakleaf hydrangeas from seed: fungus gnats and algae. Fungus gnat populations are monitored using yellow sticky traps. Adults are controlled through spray application of appropriate insecticides throughout the greenhouse. Larval populations within the seed trays are controlled by insecticide drenches that are started at the time of seed germination. The insecticide solution is placed in the watering trays and applied in place of one of the regular watering/fertilizations.
Buildup of algae on the seed trays can become a barrier to seed germination. Even though the shade cloth is kept closed on the greenhouses during germination, it is helpful to place an additional layer of shade cloth over the seed trays prior to germination. Lower light conditions slow the development of the algae until after the seedlings have fully emerged.
Propagation by cuttings
Semi-hardwood cuttings are taken in mid-July to early August. Following a quick-dip in a 4,000 ppm IBA in 50 percent ethanol solution, cuttings are stuck into 5-inch-square pots filled with a shredded pine bark medium (same formula as used for seedlings, except controlled release fertilizer used at half-rate). Cuttings are placed in a fog-filled greenhouse. Overhead mist is used to supplement the fog, but is run less often and for shorter durations than if a fogging system had not been available. Rooting usually occurs within four to six weeks.
Hydrangea quercifolia ‘Munchkin’ is somewhat smaller than either ‘Pee Wee’ or ‘Sikes Dwarf’, growing to 3 feet high and 4.5 feet wide in nine years of growth in McMinnville, Tenn., where it was bred.
After plants have developed sufficient root systems, they are removed from fog and overhead-watered for a few days. They are then switched to low-volume drip stakes for irrigation. Plants are watered for one minute twice a day initially. The greenhouse is maintained at approximately 24 degrees (75 degress F) during the day until early December, when we gradually begin to lower greenhouse temperatures. By mid-January, only enough heat is added to keep greenhouse pipes from freezing. Watering is decreased as greenhouse temperatures drop, first to once a day and then to every two days. As plants begin to break dormancy in spring, the frequency of irrigation is gradually increased. Plants are occasionally watered thoroughly with a hose to eliminate dry spots within the pot and prevent salt build-up. Plants are ready to be transferred to No. 3 containers and placed outside after the danger of frost has passed (usually late April in McMinnville).
Availability and additional information
Like other woody ornamental plants released from the National Arboretum, ‘Ruby Slippers’ and ‘Munchkin’ are not patented, so they may be propagated and sold freely. Plants are available from wholesale, mailorder and a limited number of retail nurseries; a source list is available from the authors on request. The National Arboretum does not have plants of these cultivars available for general distribution but can supply a limited number of cuttings to nurseries wanting to propagate these plants.
As residential properties shrink and the need for compact plants grows, these beautiful and easy-to-propagate hydrangeas are sure to become a reliable part of your inventory – and a new customer favorite.
Sandra M. Reed, PhD, is research geneticist and Suzanne L. Overbey is biological science technician at the Floral and Nursery Plants Research Unit, U.S. National Arboretum, USDA-ARS, McMinnville, Tenn. They can be reached at Sandra.Reed@ars.usda.gov and Suzanne.Overbey@ars.usda.gov.
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
The author would like to point out that the propagation information provided pertains to growing the species from seed and is not meant to imply that either ‘Ruby Slippers’ or ‘Munchkin’ would breed true from seed.