Propagation of Hydrangea

by Keith E. Howe

In general, propagation evolves around three basic aspects: the art of propagation is the techniques and manipulations used to reach the desired goal; the science of propagation is the general knowledge of the plant's anatomy, physiology, genetics, etc.; and knowledge of individual genera or species and what can be expected from the type of plant being propagated. There are several additional reading sources listed at the end that cover the first two aspects of propagation (art and science) in great detail. I will deal with the third aspect of propagation -- understanding the anomalies of hydrangea and how they apply to their successful propagation. Most of the discussion will deal with human efforts to increase hydrangea and will address both sexual and asexual aspects of propagation.

Sexual Propagation

Growing hydrangea from seed is easy, rewarding, and advisable if we are going to preserve the genetic diversity of the species. Seeds mature two to three months after the fertile flowers' senescence. They may be sown immediately. Dry storage is not necessary. Viability, however, is not diminished by it. Germination takes 10 and 14 days. The seed of H. quercifolia has an absolute photo requirement for germination(1). Norman Deno of the University of Pennsylvania demonstrated that neither cold treatment at 40°F. for three months nor six months of dry storage at 70° F. had any effect on germination. Seeds germinate 95% in light at 70° F. while 0% germinate with the absence of light. H. anomala petiolaris seed require 30-60 days of cold treatment (40°F.) to overcome their dormancy(2). Sow the seeds in flats and place them out of doors or collect the seed, place in damp perlite or other similar material, and store in refrigerator for 30 to 60 days prior to sowing. For a number of reasons -- including limited seed material, lack of proper greenhouse environment or space, or desire for rapid clonal multiplication -- seeds may be germinated in vitro. The author separates the seed from extraneous material using a dissecting microscope, sterilizes them for 15 minutes in a solution of 0.5% sodium hypo chlorate (10% solution of household bleach) and places them on the surface of MS media (3) supplemented with 30g/L sucrose, and 6.5 g/L agar. The pH needs to be adjusted to 5.5-6.0. The tissue culture vessels are placed in 65° F. environment, under a 100 foot candle light source on a 12 hour on/off cycle. The seeds germinate in 10-14 days and depending on grower's purpose may either be placed on a multiplication media or planted out in three to four weeks.

Asexual Propagation

Most asexual propagation is accomplished by taking vegetative cuttings although several of the species respond well to layering. Hydrangea macrophylla ssp. macrophylla is so easy to root that it is not necessary to make the cutting at the node. Roots initiate in greater numbers at the node but develop in sufficient numbers elsewhere. Three types of cuttings can be made from leafy vegetative stock material. The tip cutting is taken from the terminal 3-5" portion of the stem. The one to three node cutting should have some of the leaves removed from the lower nodes to prevent them from coming into contact with the rooting medium. This will also reduce the space required in the propagation bed. Do not trim individual leaves as this provides pathogens with easy access The lower nodes of the stem can be made into butterfly cuttings. This type of cutting provides a propagule with two eyes that will initially develop into a plant with two leaders. To produce a butterfly cutting begin from the base of the shoot, making a horizontal cut just above a pair of buds and an internodal cut 2-3" below the base of the leaf petioles. It is advisable to leave a short length of stem above the buds to reduce damage to the bud tip when making the horizontal cut. The same lower nodes on the stem may be trimmed immediately above and below the node and the stem split providing two single eye cuttings The single node or "leaf bud" cutting is particularly useful when stock material is limited or when the material is so large as to be cumbersome using other techniques. The tip cutting will root more rapidly than either of the other two methods. The butterfly cutting takes 10-20% longer to root and while they will produce a plant with two blooms the first year, the two shoots do not always develop at the same rate and do not always produce a bloom on both shoots. The leaf bud cutting takes the longest period to root, and because of size is the most sensitive to stress. They, however, have a large number of nodes concentrated at the base of the single eye cuttings and with pinching will develop into a many branched uniform plant. Except for a six to eight week period (mid November to mid January) while bud set is occurring and dormancy requirement being met, hydrangea may easily be propagated. They root from hardwood, soft wood and semi-ripe wood. Cuttings taken from soft wood root more rapidly, while those taken from hardwood prior to the leaves emerging in the spring, suffer the least from transpiration problems.

Cuttings root readily under intermittent mist at 65-70ƒF. and take 3-4 weeks for good roots to develop depending on species or variety, time of year, and type of cutting. The use of a rooting hormone treatment such as 1000 ppm IBA or NAA in talc contributes to a more substantial root system and the fungicide present in most commercial applications provides a useful safeguard against these pathogens. The author has found that cuttings of H. macrophylla macrophylla taken during the height of the growing season while the leaves are rapidly expanding appear to root just as well with or without a rooting hormone treatment.

The prepared cuttings should be placed in a well-drained medium such as #8 crushed quartz (coarse) and watered with intermitted mist. It is helpful to lower light levels to limit transpiration and keep the cuttings turgid. The use of sharp sand (crushed quartz) without any amendments has its pros and cons. Without any amendments the propensity for the cutting material to rot is greatly diminished. Sharp sand alone does not provide any nutrients, however, and cuttings need to be planted up immediately after roots emerge or there is a danger of stunting the growth of the plantlet. Cuttings can be rooted directly into growing medium in small pots to eliminate transplanting later. This practice is complicated in that the growing medium does not drain as well as a sharp sand rooting medium and the stems have a tendency to rot when kept wet enough to prevent the leaves from wilting.

H. arborescens are undoubtedly the easiest of all of the hydrangeas to root. Softwood cutting taken in June-July will root in high percentages in three to four weeks when treated with 1000 ppm IBA as a dip or combined with talc. M. Dirr(3) reports 100% rooting of H. arborescens 'Annabelle' using 5000 ppm of K-IBA on early July cuttings. Hardwood cuttings taken in November or December benefit from auxin treatment and have been successfully grown in open ground, open bench as well as under polytent material. Protection the first winter is an important consideration for H. arborescens and if softwood cuttings are going to be used it is beneficial to take them as early as possible.

H. heteromalla and H. paniculata root equally well when taken from soft, semi-hard or hardwood cuttings. Softwood cuttings of either taken in April-May and treated with 1000 ppm. IBA in talc or solution will root 90-100% in three to four weeks.

Hydrangea anomala ssp. petiolaris is more difficult to root especially after the wood has ripened. Two methods(4) have proven successful in propagating this species. Non-flowering shoots of soft wood have root initials along the stem that serve as aerial roots to enable the vine to cling to a vertical support. These shoots are placed in a well-drained rooting medium and roots will develop at the site of the initials. A second method imitates French or continuous layering in which the stock plant is planted in open ground and the stems are pegged down onto sandy soil. H. anomala ssp. petiolaris does not require any trenching or mounding-up and will produce vertical shoots with roots from the pegged stem.

Next to H. anomala petiolaris in difficulty is H. quercifolia. The Oak leaf hydrangea is very sensitive the first winter after rooting and if at all possible should be left undisturbed in the rooting media during the first winter. Cutting taken in early summer will root 100% if treated with 3000 ppm. IBA while only 10% if untreated. Root cutting taken while the plant is dormant root well if planted vertically under cover.

The French or continuous layering method(5) has been used quite successfully in Europe to propagate Hydrangea aspera ssp. aspera, Hydrangea paniculata as well as Hydrangea quercifolia. This method requires an open sandy site. The stock plants are pruned to the ground in mid-winter (February). The new shoots that come from this root stock, when 4-6 inches in length, are pegged against the ground. They are then covered with a thin layer of sand. New vertical stems will break from each node of the pegged shoot and roots will form in the sand. This process is repeated for two to five years resulting in a bed of hundreds of rooted propagules.

The possibility of year-round production without the cost associated with maintaining large numbers of stock plants in a controlled environment exists. Micro propagation can provide means whereby continuous production, and maintenance of stock material coexists in a micro environment. Barnhill-Jones(6), Stoltz(7) and Bailey(8) have all described in vitro propagation of hydrangea. As with other vegetative propagation methods, hydrangea proved to be easily grown in vitro. The three investigators used three different media: Gamborg's B5, Murashige and Skoog's basal media and Lloyd and McCown's woody plant media. Cytokinins used included 2iP, BA, and Kinetin. Shoot multiplication rates ranged from 2.5 shoots per subculture to in excess of 6. In all experiments salable plants were produced in less than one year.

REFERENCES


  1. Deno, Norman C., 1991, Seed Germination Theory and Practice. Pennsylvania State University
  2. Pope, Derrick R., Proceedings of International Plant Propagators Society, 24:194-195.
  3. Dirr, Michael, & Charles W. Heuser, Jr., 1987. The Reference Manual of Woody Plant Propagation: From Seed to Tissue Culture. P.133. Varsity Press, Inc., Athens, Georgia.
  4. Pope, Derrick R., Proceeding of International Plant Propagators Society, 14:253-254.
  5. Macdonald, Bruce. 1986. Practical Woody Plant Propagation for Nursery Growers. pp.423-426 Timber Press, Portland, Oregon.
  6. Jones, Jeanne Barnhill. 1979. Commercial use of tissue culture for the production of disease-free plants, p.441-452. In W. R. Sharp, P.O. Larsen, E.F. Paddock, and V. Raghaven (editors), Plant cell and tissue culture. Ohio State Univ. Press, Columbus
  7. Stoltz, Leonard P. 1984. In vitro propagation and growth of hydrangea. HortScience 19:717-719.
  8. Bailey, Douglas A., Gary Seckinger, and P. Allen Hammer. 1986. In Vitro Propagation of Florists' Hydrangea. HortScience 21(3):525-526.

ADDITIONAL READING SOURCES


The above information is only meant to represent my "truth". I would be more than happy to entertain any and all disagreements that you might have. Please e-mail me. Thanks, Keith.

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