| Oakleaf hydrangea | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Asterids |
| Order: | Cornales |
| Family: | Hydrangeaceae |
| Genus: | Hydrangea |
| Species: | H. quercifolia |
| Binomial name | |
| Hydrangea quercifolia | |
Hydrangea quercifolia, commonly known asoakleaf hydrangea oroak-leaved hydrangea, is aspecies offlowering plant in the familyHydrangeaceae.[2] It isnative to the southeastern United States, in woodland habitats fromNorth Carolina west toTennessee, and south toFlorida andLouisiana.[3] Adeciduous shrub with white showy flower heads, it is grown as a garden plant, with numerouscultivars available commercially.
Itsspecific epithet combines theLatin wordsquercus ("oak") andfolium ("leaf"). However, it is not closely related to oak species (Quercus).
Hydrangea quercifolia is a coarse-textureddeciduousshrub growing to 3–12 feet (0.91–3.66 m)[3] tall with an open crown. The plant sprouts shoots from undergroundstolons and often grows in colonies. Young stems are covered in a felt-like light brown bark, and the larger stems have attractive cinnamon-tan-orangebark that shreds and peels in thin flakes.
Theleaves are yellowish green to dark green on top and silvery-white underneath. They have three, five or seven pointedlobes and are 4–12 in (10.2–30.5 cm) long and almost as wide. They vaguely resemble larger versions of oak leaves, similar toQuercus species with lobed foliage. Plants in shade have larger leaves than those grown in sun. The leaves turn rich shades of red, bronze and purple in autumn that persist in winter accompanying the persistent dried flower-heads.
Hydrangea quercifolia flowers are borne in erectpanicles 6–12 in (15.2–30.5 cm) tall and 3–5 in (7.6–12.7 cm) wide at branch tips. Flowers age in colour from creamy white, aging to pink and by autumn and winter are a dry, papery rusty-brown.
Unlikebigleaf hydrangea (Hydrangea macrophylla), flower color does not vary withsoil pH. Flower color does, however, vary throughout each growing season. The flowers are a pale green as they emerge and open to a bright white, ageing to either pink or brown (depending on the cultivar/seedling).[4]
Hydrangea quercifolia andHydrangea paniculata are the only hydrangeas with cone-shaped flower clusters (i.e.panicles); all the others have their flowers in ball-shaped or flat-topped clusters, calledumbels.[5]
Like many otherHydrangea species, oakleaf hydrangeainflorescences are composed of two flower types; showy flowers with enlarged petal-likesepals and inconspicuous flowers with non-showy petals and sepals.[4] The showy flowers, whose function is to attract pollinators, typically obscure the inconspicuous flowers to some degree.
Although faint,Hydrangea quercifolia flowers have a pleasing scent. Thefloral scent in oakleaf hydrangea is stronger than that of the other commonly cultivatedHydrangea species.[4] Presumably the function of the floral scent is to attract pollinators, although this has not yet been studied. The fragrance can be described as being most similar toSyringa reticulata (Japanese tree lilac), although not as strong.
The fruit ofHydrangea quercifolia aredry dehiscent capsules which are green followingpollination and ripen to brown as they mature. Thestigmas remain on the fruit and dry to form horn-shaped appendages atop the capsule with an opening between them to allow the seeds todisperse.[4]
Hydrangea quercifoliaseeds are quite small, approximately 0.6 mm long[6] (and with a thousand seed weight of 0.0356 g[7]) allowing several to develop within each capsule. Seed color varies from light tan to dark brown with longitudinal striations. Seed shape is ellipsoidal to ovate.Seed storage behavior is considered to beorthodox in nature,[7] allowing them to store for relatively long periods of time at cold temperatures. The seeds typicallygerminate readily in 7–14 days as long as they are not allowed to dry out after sowing.
Hydrangea quercifolia is native to the south eastern United States (Alabama, Florida, Georgia (w.), Louisiana (e.), Mississippi, North Carolina, South Carolina, Tennessee), but naturalized in other parts of the US and widely cultivated elsewhere.
In nature,Hydrangea quercifolia is found in a variety ofhabitats but almost always on well drained, shady slopes. These include riverbanks, bluffs and rocky outcroppings. The tendency of the species to grow oncalcareous soils or cliffs as well as deepalluvial soil suggests it has a wide tolerance of soil substrates as long as they are well drained.[8][4]
Hydrangea quercifolia was noted by 18th-century botanistWilliam Bartram in his botanizing exploration from the Carolinas to the Florida panhandle in the 1770s.[9] His first documentation of the plant was located along the "brook called Sweet Water" in what is now south-central Georgia.[10] It was slow to enter British and American gardens. In Britain it flowers less profusely and even has a reputation for being tender.[11]
Hydrangea quercifolia is cultivated as anornamental plant in gardens and parks. Though frequently seen as an isolated subject in gardens, it is at its best in a natural or landscaped woodland habitat against the backdrop of larger shrubs and trees. It prefers partial to almost full shade, with morning sun and afternoon shade as optimal. It will tolerate drought, but may not flower. In the UK thecultivarsSnowflake='Brido'[12] andSnow Queen='Flemygea'[13] have gained theRoyal Horticultural Society'sAward of Garden Merit.[14]
Propagation is via cutting or division; short sections of clustered stems with some root attached will make a small shrub in a nursery row.
Fresh or dry, the blossoms ofHydrangea quercifolia are attractive as cut flowers.
Oakleaf hydrangea is typically stated to be hardy toUSDA Zone 5-9,[15][2] although the true cold hardiness varies for each cultivar and throughout thespecies range.[16][17] Plants from more northern sources are typicallymore cold hardy in midwinter than plants from southern sources; moreover, plants native to Florida are substantially less cold hardy than those from other southern sources.[16] Cold hardiness also varies throughout winter, with early loss of cold hardiness being the main limiting factor to cultivation in northern environments. The cultivar 'Ruby Slippers' maintains its cold hardiness well into April, while 'Sikes Dwarf' (despite being very cold hardy in midwinter) loses its cold hardiness very rapidly in late winter. Out of nine cultivars tested,Snow Queen='Flemygea' was consistently the least cold hardy throughout winter.[16]
There are several notable diseases that infectHydrangea quercifolia.Root rot diseases can be fatal[18] whilefoliar diseases tend to be only cosmetic.[19] Root rot diseases includePhytophthora,Pythium andFusarium, which are allfungal diseases.[18][20]Management techniques to decreasesincidence of root rot include proper watering (allowing the soil to dry between waterings), removal of diseased plant parts, and fungicides if needed. Compared to otherHydrangea species, oakleaf hydrangea is more susceptible toFusarium root rot; however, disease severity varies among cultivars with 'John Wayne' andSnowflake='Brido' being the most tolerant.[20] The primary foliar disease in oakleaf hydrangea isXanthomonas leaf spot, which is abacterial disease.[19] Management techniques to decrease leaf spot severity include avoiding overhead irrigation and plantingresistant cultivars. In a recent study, the cultivars 'Alice' andSnow Queen='Flemygea' had the lowest disease severity while 'Queen of Hearts' had the highest severity.[16] Some wild plants that are native to Florida have even lower disease severity than any cultivar and could serve as an important seed source for breeding disease resistance.[16]
Oakleaf hydrangea is adiploid species with 2n=2x=36chromosomes.[21]Hydrangea quercifolia has one of thesmallest genomes in the genus (1.95 - 2.17 pg 2C DNA).[22][23] In nature, thegenetic diversity ofHydrangea quercifolia isstructured geographically in six 'clusters' of genetically similarpopulations.[24] These genetic clusters are weakly differentiated but each one has unique genetics nonetheless. In addition to neutral genetic diversity (13.5% of the diversity), there is evidence of genomicselection in response to climatic variables (11.3% of genetic diversity) of which,precipitation seems to play a major role.[24]
The native range ofHydrangea quercifolia appears to be contracting, despite highabundance in the central portion of the range.[4] In a recent survey of the species, 23% ofpreviously recordedpopulations no longer existed and many of the remaining populations were at high risk ofextirpation.[24] Causes of local extirpation was attributed tohabitat loss or degradation often times due toinvasive species, logging or commercial development. Oakleaf hydrangea was noted as not tolerating highcompetition and is therefore highly susceptible to the negative impacts of invasive species. The researchers noted that these effects could be mitigated byprotecting and managing land with substantial populations, especially on the species' range edge. To add to the conservation concern, many of the populations on the edge of the native range haveunique genetics (especially in Louisiana and Florida) making them especially important to protect as a source of valuablegenetic diversity.[24]
Hydrangea quercifolia was declared the official state wildflower ofAlabama in 1999.[25]
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