Date: August 2002

Reblooming Bigleaf Hydrangeas

R. E. Bir and J. L. Conner
NC State University
455 Research Drive
Fletcher, NC 28732

Nature of Work: Most gardening texts state that bigleaf hydrangeas, Hydrangea macrophylla var. macrophylla, H. macrophylla var. normalis, H. macrophylla var. serrata and their hybrids form flower buds on old wood the year before flowering. If those buds are destroyed by pruning or freeze injury then plants will not flower because new flower buds will not be formed then develop and open during the current growing season.

Recent research has demonstrated that bigleaf hydrangea cultivars exist that are truly remontant as well as others that will flower in autumn even if they have already flowered earlier in the year. Speculation exists concerning whether the flowers are from lateral buds that were not removed or freeze damaged or whether new flower buds form during favorable conditions in late summer and early fall then open during an extended period of short days and non-freezing autumn temperatures (1). To the landscape and gardening trade, it does not matter why these hydrangeas rebloom or continue to bloom into autumn. They want to know which ones will be both summer and fall flowering.

The bloom times of cultivars in an existing hydrangea collection (2) at MHCREC, Fletcher, NC, were recorded weekly during the 2001 growing season. Plants that were flowering in July as well as flowering on October 1, 2001 are listed as reblooming in Table 1. Those that flowered during the 2001 season but were not in bloom on October 1 are listed in Table 2. Those that did not flower at all have been reported previously (2).

Results and Discussion: Most plants were killed to the ground by cold weather during the 2000-2001 winter. First killing frost was on October 10, 2000 with the lowest winter temperature recorded at MHCREC, Fletcher being 8 F. on December 20,2000. However, much flower and vegetative bud damage occured in spring due to variable temperatures. For example, high temperatures in excess of 75 F. were recorded every day April 6-12, 2001 followed by 3 nights below freezing the next week, another three days above 75 F. then the last frost was on April 27, 2001. New growth stimulated by warm spring temperatures was killed by temperatures in the twenties.

Table 1. Cultivars of Hydrangea macrophylla that were flowering in July as well as in October 2001 at MHCREC, Fletcher, NC.
CULTIVAR
All Summer BeautyKluis Superba
AltonaLaMarne
Blue BoyLanarth White
Blue DanubeLilacina
Blue PrinceMarechal Foch
Coerulea LaceNikko Blue
Decatur BlueRevelation
Mme. Emile MouillereSouv. Pres. Doumier
EuropaVeitchii
Geisha GirlWhite Wave
Table 2. Cultivars of Hydrangea macrophylla that flowered but did not flower in fall 2001 at MHCREC, Fletcher, NC.
CULTIVAR
Ami PasquierMasja
Beaute VendomoiseNiedersachen
Blue BirdOregon Pride
Cardinal RedOtaksa Monstrosa
FasanParis
Gen. Vicomtesse de VibrayePresioza
Gertrude GlahnRed Star
GoliathRose Supreme
HadsburySeafoam
HamburgShishiva
Heinrich SeidelTeller Red
HolsteinTrophee
Kastlen 

Significance to the Industry: Twenty of the fifty-two cultivars which flowered produced new flowers both in early summer and early fall.

Literature Cited:

  1. Adkins, J. A. 2002. Hydrangea macrophylla: Travels, Trials and Some New Ideas. Proc. NC Nursery Short Course:18-19.
  2. Bir, R. E. and J. L. Conner. 2000. Flowering of Hydrangea macrophylla and serrata Cultivars in USDA Zone 7 Landscapes. Proc. SNA Res. Conf. 45:445-446.

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