It is August, nearly September, and I am setting up for my forcing programs. The final touches: checking on coolers and equipment and stocking up on potting media and containers. An unexpected phone call from a supplier; seeing the number on the display, I was getting a little anxious, wondering about which cultivar would not be available. I prepared for discussing possible substitutes and consequently the extra work of making adjustments to the production schedule.

As it turned out this call was not going to be about my orders. I was asked, “Do you know how to grow hydrangea and turn them blue?” My reply: “Excuse me – hydrangea?” The supplier’s answer: “Yes – hydrangea. A brand new breeding line consisting of high-performing cultivars is being released by an American company. I think you have heard of Plants Nouveau during the OFA short course?” I was very surprised, yet pleased. In my opinion hydrangea is underused while there is a potential market looking for a wider selection of high quality crops. Until now, it was next to impossible to get properly prepared rooted cuttings for a blue hydrangea program in the U.S. Perhaps this is why high quality pot hydrangeas and well-grown cut flowers are rarely seen in the standard assortments. I thought “It’s about time to get a potentially high margin crop back into production.”

Soon thereafter was a request from the ASCFG for a paper with an emphasis on blue hortensia for cut flower production. Now I am trying to organize my culture notes, summarizing as much information as possible since so little has been written about hortensia in the last few years. In my opinion, it is not enough to throw a great crop back into the trade without some additional information. To growers, including myself, and especially for consumers, it is of little interest if one cannot identify with the product. Sure, hortensia are a great-looking crop if grown well, but how do I grow it? Why bother going through the trouble of changing my assortment and production schedule? Why take on this risk?  Interestingly, the questions of growers and consumers are similar. A consumer might wonder “What am I supposed to do with this flower – how can it be used, what will it do for me?”  Below, you may find a few answers – basic background information on hydrangea as well as a simplified production protocol for how to grow your own blue hydrangea crop.

What are hortensias?
 
The florists’ hydrangea. Some of you may know this plant by the name of mophead hydrangeas, or big-leaf hydrangea; the scientific name is Hydrangea macrophylla. The international market designation is “hortensia”. To be consistent and to avoid potential mixup with other important horticultural crops such as oakleaf hydrangea (Hydrangea quercifolia) and panicled hydrangea (Hydrangea paniculata), I prefer the name hortensia, since it is specific to the breed and the name is grand. This is important since unlike the other species of hydrangeas, hortensia is designed for intensive forcing programs that include both pot and cut flower production.

Learning how to grow hortensia was one of the major assignments of my apprentice-ship many years ago. Hydrangea at that time was part of the core assortment of floriculture crops – it was essential to study and to successfully grow hortensia. An apprenticeship takes place in a trade school and commercial greenhouse operations; there apprentices (junior growers) are trained and supervised by guild masters.  Hortensia was a challenging crop, yet it became one of my favorites to grow. Hortensia is what I would call a model crop which requires thorough knowledge of production methods. An apprentice learns how small differences in soil pH affect flower color. Water management and crop timing are very important factors that require attention to minute detail by the junior grower. Unlike bedding plants, hydrangeas need a significant time investment and they are not forgiving. If an apprentice makes a mistake it will show. Bad nutrition, water management, or insufficient vernalization will leave their mark. For instance, a muddy blotchy color of the flower is the result of bad pH management as well as mistakes in fertilization.

The characteristic of a high-quality hydrangea flower is the clear coloration (blue) which is perfectly contrasted by the dark green foliage. Depending on the product line the time investment may take up to 15 months, sometimes more, to grow a market-ready product. Single stem standards and large patio containers certainly fall into this long-term category. Pot production on average requires 9 to 12 months production time. Perhaps the largest time investment is developing a cut flower program. I would estimate 3 years from propagation to a plant size that supports profitable cut flower harvests. More detail on culture and production planning will be discussed in the production chapter below.

The Relevance of Hortensia

I had been growing hydrangea frequently through-out the years, eventually it dropped out of the product assortment. This was mainly due to problems with obtaining high quality propagation material.  Writing about this crop brings back memories and it reminds me, as strange as it may seem, of “old world” cooking. This is because the heavy clay pots were moved from the cold frame to the forcing house in the winter, from December through January. It is the season we used to get Powidltatschkerln (the Austrian spelling is Powidltascherln) for lunch, a Bohemian specialty unfortunately completely unknown  in the U.S. I only can recommend ordering some when you happen to be in Europe (Austria or Czech Republic); they are a definite must when visiting – they are so good. Similarly with hortensia, few flowers achieve this timeless splendor and show. Seeing well-grown hortensia is just as pleasing as enjoying some Powidltatschkerln – a delight to the senses. Both food and flowers play an important role in defining cultures.

Most of you may know hydrangeas are grown as potted plants for sales from Valentine’s Day though Easter. In Europe a few additional occasions, such as weddings and holidays include massive hydrangea sales, many of which take place in May; the Pentecost or Whitsun holiday; and Corpus Christi.  Infrequently, in the last few years, I have seen white cultivars in the Christmas assortment. In various parts of this world, feast days are associated with special dishes and flowers, foremost hydrangeas.  Although pot crops are an important segment of sales, weddings and religious festivals demand huge amounts of cut flowers. Therefore, the main uses of hortensia based on volume are cut flowers as well as pot plants.

Hortensia: A New Cash Crop?

The account above tells about feast days and celebrations, expressions of culture and lifestyle.  The American culture and lifestyle have changed and keep on changing. Trends are fast; products that fit this development become extremely popular and profitable, e.g. Apple products, or Google and Facebook.  Where is the opportunity for the floriculture industry? Is there going to be a floriculture iPad? Why are flowers, especially cut flowers, losing out? There is opportunity; hortensia is not yet a commodity, postharvest traits of cut hortensia are acceptable but have limitations. Often it is the locally-grown material that is of much better quality than flowers shipped halfway around the planet. This characteristic especially should provide opportunity for small and medium size operations serving a regional market – a market traditionally underserved with top quality local flowers. You have the opportunity to grow a great product, there is a market and demand, and it is up to you to develop a message with which to reach, engage, and most importantly – a message consumers can identify with – the perception about the product (brand) needs to fit the current lifestyle. Just a quick review – a logo is not a brand – your brand is how your customers and your competition perceive your company – it’s up to you to answer this question for yourself: Is your assortment current, attractive and good quality?  Are you helping your costumers solve questions and problems in regard to home decoration, garden design, gifts, and holiday decorations?  Are you selling your customer (pushing) a product only; or are you actively trying to improve  your costumer’s perceived lifestyle? Look at your assortment – and perhaps hortensia is a product that some of your customers will perceive just to be the right thing – but it is up to you to position it.

Background Information for the Grower

Hortensia is a group of cultivated varieties of the genus Hydrangea. There is conflicting information regarding the number of named cultivars, which is estimated by some to range in the hundreds. Formerly this genus was included in the Saxifragaceae; today they have been assigned to their own family, the Hydrangeaceae. The genus Hydrangea includes some 70 species, many of which are native to East Asia, and can be found in countries such as Japan, China, and Korea.

Hydrangea macrophylla is native to Japan and was cultivated for some time prior to its first description by Carl Peter Thunberg in the 1700s. Thunberg discovered these early cultivars near Nagasaki on the Japanese island of Kyushu. Due to some confusion during categorization, these plants were classified with the genus Hydrangea and not as initially proposed hortensia. Since the discovered materials were cultivars and not species, an interesting debate occurred.   

Systematics is a fascinating topic and I can recommend reading the stories of early explorers – they are not boring scientific papers. Most are written in prose and are, at times, very witty. Between 1790 and 1800 some plants were brought to Europe where they were used as the initial breeding stock. Much of the early breeding occurred in France where plants  were grown in botanical gardens and by some plant collectors. By the late 19th century hortensia found its way into the horticulture trade.

In a relatively short time, hundreds of new cultivars were bred in floriculture hotspots such as Belgium, the Netherlands, Germany, and France. These new western cultivars were eventually brought back to Japan, where they are called Seiyou-ajisai, and are popular additions to the original assortment.  Western, as well as the original Japanese cultivars, will grow into attractive, several-feet tall shrubs in the garden. Surprisingly, potted plants for the house or the patio which may be later transplanted into the garden are little used in the marketing of pot crops. For example, which average consumer would know to use hortensia as an indoor plant in the winter, to transplant it into the garden after the frost, and eventually harvest cut flowers from this plant for indoor decoration for years to come? I think enlightening consumers about the various uses of flowers may eventually generate interest and lead to additional sales.

A Botanical Profile of Hortensia

Plants are deciduous. The opposite, dark green leaves are large and to some degree serrated. The typical florist’s hydrangea produces a large inflorescence on the end of relatively long stems. The  spherical bloom is massive and may have a diameter of 5 to 10 inches. The flower head can be described as an umbelliform cyme consisting of numerous small florets. Most interestingly to growers, a modification of the florets resulted in sepals being the dominant part of the flower and it is the sepals that are responsible for the showinesss of the flowers. The petals, on the other hand are small, insignificant, and frequently completely missing. The blooms are white, pink, deep rose nearly red, and blue. Unlike petals, the size of sepals may be modified through cultural practices. In other words, a grower can specifically modify the pigmentation of the flower (except white cultivars) as well as increase the size of the sepals with improved cultural practices.

General Environmental Requirements

The native habitat of hortensia is wooded areas near the ocean, where it is an understory shrub protected from the elements such as too much direct sunlight and wind. The climate is mild, perhaps with a touch of northern temperate. Winters are cool, with temperature remaining just above freezing. The summers are warm yet some level of precipitation continues and the relative humidity is high, essential for hydrangea. As the name may suggest, they require a constant supply of water. The linguistic root of the prefix Hydr(o) is the Greek uäùñ (hýdor) the name of water. Another anecdote I read some time ago, I am not so sure about.  Hydra is a Greek mythological figure with many heads.  Indeed, hortensia has a lot of flower heads per plant.  In my mind, this is about as far as this allegory should go.  Hydra was of such unworldly abhorrence that anyone looking into her face would instantly turn to stone.  Well, one can only wonder how these parallels arise.

I encourage my production students to be inquisitive enough to discover the origin of their crop plants. The botanical name, and information about the native habitat provide valuable hints for what might work in the greenhouse; the differences of cultivars withstanding, it reveals clues to cultural requirements and the temperament of the plant. In the case of hydrangea, the very specific environment hortensia evolved in became imprinted into the species, even into the current cultivars. Hydrology, soil composition, and temperature range of the natural environment define hortensia. Modern cultivars thrive when these environmental factors are met.

For example, flower initiation occurs in the late summer or early autumn as soon as average day temperatures range 55°F to 60°F – the optimal temperature is 59°F.  Unlike temperature, photoperiod does not seem to be an obligatory factor; while some questions remain, for the trade this debate is of little applicable interest. The bud remains dormant throughout the winter and requires temperatures just above freezing to complete vernalization. It will produce a flower the following spring. Frost and untimely pruning destroys the flower buds and the plant will fail to bloom the following spring. Some remontant (ever-blooming) strains will set new flowers in the spring but are generally not well suited for forcing.  Cultural practices and the greenhouse environment should approximate these demands. For example, a good potting media should exhibit similar properties as forest soil: plenty of organic matter, good water retention to retain moisture, yet well draining to prevent water-logging. The plants prefer acidic media to facilitate iron uptake, while pH affects flower pigmentation. This information is obtained simply by learning about the natural habitat as well its history and cultural context. I believe it to be the foundation of appreciating a crop, the privilege of a grower.


PRODUCTION OF BLUE HORTENSIA

Propagation

Rooted cuttings should be bought from reputable suppliers only, particularly if your goal is to produce blue hydrangea. Finding such a supplier can be challenging: sometimes the material is badly conditioned, unsuitable for blue hydrangea production; other times cultivars are mixed up, or a cultivar is unavailable (sold out). Since obtaining high-quality propagation material can be tough, some operations maintain their own stock in-house. Having stock certainly has advantages such as not being affected by availability of certain cultivars or quality issues. However, more and more of the new cultivars are protected by breeder rights and patents. Taking cuttings from such material is prohibited. Check thoroughly before establishing your own stock plants and make sure to work with free market cultivars only. Violations of breeder rights and/or plant patents can result in serious legal trouble.

Purchasing rooted cuttings may be the least complicated option for starting your hydrangea cut flower program. Rooted cutting liners should be available for delivery in late autumn; weather permitting winter shipment is okay too. Make sure your supplier is not sending plants when outside temperatures are too low – your liners may arrive frozen. Upon arrival unpack and place liners into your greenhouse.  Transplant swiftly since the plants need space and crowded conditions encourage gray mold and mildew.  I like to transplant rooted cuttings into a moderate size pot before going into the final pot size. Especially in the winter, plants may be a bit more sluggish and if the pot size is too large, the media may stay too wet for too long. If you choose to plant into a 6-inch pot or 8-inch pot, grow them on until the young plant is fully established and the pot completely rooted through, at which time transplant into the final container.  For cut flower greenhouse production, pot one plant per 3 to 5 gallon container, depending on how vigorous your cultivars are. If the climatic conditions of your area allow field production skip the last step and plant the well established plants outdoors.

Potting Media – Greenhouse Production
 
In contrast to most standard pot crops, cut flower plants frequently will stay in containers for several years. Selecting the right growing media is of paramount importance. You must select a media that does not shrink over time. As indicated above, hortensia require plenty of water without being submitted to waterlogged conditions. Potting media amendments may include coarse and stable materials such as haydite, pine-bark compost, and coarse sand. Peat moss should be used sparingly due to the fine particle size. In many of my long-term crops, including hortensia, I incorporate some percentage of topsoil into the potting mix. Frequently, I test a few formulas before settling, assessing recipes that work best under my growing conditions. The media should receive an initial starter charge and the pH needs to be adjusted if necessary. The final pH for blue cut flower production needs to be adjusted to 5.0 to 5.5. A pH over 6.0 inhibits the uptake of aluminum ions; the flower will not turn completely blue and often result in a muddy, blotched wine color.

Turning Hortensia Blue – Greenhouse Production

Choosing the proper cultivars is the most important aspect of setting up a cut flower program.  The following are considerations for identifying suitable cultivars:
• Consult with your supplier and identify cultivars suitable for greenhouse cut flower production – they need to be sufficiently vigorous.
• White cultivars cannot be turned blue.
• Not all pink- and rose-colored cultivars will produce a clear blue, even under perfect conditions.
• Trial cultivars in small numbers – not all cultivars perform equally in different climatic zones.

Pigmented hydrangea flower either pink, dark rose, or some cultivars nearly red. Depending on  cultivar and the intensity of the pigmentations, flowers may turn blue; some cultivars produce an almost violet/purple. Changing the color from pink to blue depends on the pH and concentration of aluminum ions present in the media. Nonetheless, not all pigmented cultivars make great blue flowers. Consult with your supplier to identify the best cultivars for your area. 

Once you have identified suitable cultivars for blue cut flower production, adjust the pH and apply alum. Make sure to maintain the proper pH throughout the growing cycle. Adjust the pH with lime or alum to a value between 4.8 and 5.5. Alternatively, aluminum sulfate works just as well and can be used to correct the pH down. When growing blue flowers, the pH should never exceed 5.5. On the other hand, reducing the pH below 4.8 may damage plants and growth is impaired. At a pH over 6.5 not only will flowers not turn blue, but iron becomes unavailable; consequently the foliage may become pale and chlorotic.

In short, both aluminum potassium sulfate (alum), the chemical abbreviation is KAl(SO4)2, and aluminum sulfate, Al2(SO4)3, are essential tools in manipulating the flower color of hortensia. They are used to control pH and to add sufficient quantities of aluminum ions to the media. Since the optimal concentration is based on pot size and cultivars, consult with your supplier and extension agent for appropriate rates. In addition to the initial treatment, you need to apply alum or aluminum sulfate in September and November, and again in March and May. A missing and/or untimely application of alum frequently results in a muddy blue color. Follow the schedule and increase the amount of alum in situations where irrigation water is alkaline (containing high amounts of calcium). Under some circumstances it may be advisable to use rain water or chemically softened water to limit the calcium accumulation in containers.

Managing Water and Light

Proper water management is essential for growing excellent hortensia. The availability of water affects sepal size, which can be manipulated by providing the optimal amount of water. However, the threshold between optimal and too much water is slim. At best, excessive water will lead to blind stems and flower bud abortion. Most commonly Pythium blight will take hold in your greenhouse. To prevent fungal disease, watering with drip irrigation is preferred – avoid overhead irrigation. Keep irrigation zones small to allow custom watering schedules. Make sure your containers are free-draining and have plenty of drainage holes. Elevating the pots onto a low table can help. Alternatively, placing pots onto a gravel bed helps drain extra water away. Make sure the bed includes a drainage system as well – a French drain will do the job. Many hydrangea cultivars are highly susceptible to root rot, and Pythium especially thrives in waterlogged conditions. Free-standing water and splashing may quickly spread this pathogen throughout your greenhouse.

Although hydrangea requires medium-high light levels, too much direct sun will damage foliage and flowers. Supplemental lighting may improve early growth of the crop during the low light winter months. As soon as days get longer and the intensity of the sun is increasing, provide shading. Protect flowers from wind drafts and direct sun. For most areas a 50% shade cloth will do the job. Direct sun, overhead irrigation, and (wind) draft are the most common mistakes, causing (sun or wind) burn and fungal infections of flowers and foliage.

Temperature

High-performing forcing cultivars do not tolerate frost. Freezing temperatures may damage the flower buds. Maintain a range of 35F to 40F during vernalization. Hortensia need about 1,000 hours or about 6 weeks of cold treatment to complete vernalization. This is a rough estimate and works for many cultivars; however, the actual duration depends  on cultivar. Your supplier ought to give you cultivar-specific instructions. Forcing can start in February allowing harvest of the first flowers for the early season cutting dates. Raise the greenhouse temperature to 65F during the day;  drop the night temperature to 60F. This temperature range will initiate shoot growth. It will take about one week to see the first signs of development. The flower bud will enlarge and start to break open. Once the first small foliage becomes visible, drop the night temperature to 55F (the day temperature remains at 65F). After about 3 weeks, a short shoot should have grown; now you can lower the daytime temperature to 55F. This will conserve energy as well as improve quality of the crop. Forcing at low temperature increases the strength of the stem. It is a good practice to remove excess and weak stems. Try to limit the number of stems to 10 to 15 per plant, removing the weakest stems first.

By bringing on the flowers slowly and limiting the number of stems per plant, the size and the quality of flowers are enhanced. A zero DIF environment promotes stem elongation; a greenhouse temperature in which the day temperature is the same as the night temperature. Lower forcing temperatures are possible if you need to hold your crop for a later cutting date. If you invest in several compartments, temperature can be used effectively to schedule harvest over a longer period, essentially spreading out harvest from April through July. Hortensia should not be pruned after August since flower bud development for the next year will begin then. Depending on cultivar, flower harvest needs to be stopped earlier to allow sufficiently strong stems to develop in the subsequent year. This is cultivar-dependent and your supplier ought to give you guidance what to expect from a specific variety.

A good relationship with your supplier and experts who have experience growing hortensia is very beneficial. As indicated above, over the years an enormous number of cultivars were released into the market. Some are very specific in purpose and are designed to perform only as a potted hortensia crop.  Others may work only for outdoor or field production since they may not tolerate confinement in containers. Your supplier and perhaps your extension agent might be good resources for selecting cultivars.

Space and Production Requirements

On average, plants are spaced to one or sometimes two plants per five square foot, depending on cultivar. Make sure to include ample space for access pathways. A good estimate is a width of two feet.  The flower bed should be 4 to 5 feet wide; dimensions need to be adjusted according to the vigor of different cultivars. Netting should be installed to support the large flowers. Standard wire netting is sufficient or any 8×8 or 10×10 netting will work. On average, a well- established hortensia plant produces 10 to 15 stems per season.

Harvest and Postharvest Guidelines

Limit harvesting to the morning when it is still cool; essentially you should be following the cutting rose protocol. Adding postharvest flower preservatives (Chrysal) to the water works well. During peak season expect to harvest fresh flowers 2 to 3 times a week. When cutting flowers make sure you leave 4 to 6 inches of the stem. This will serve as the foundation for the next year’s shoots. Make sure these stems will not be cut again, especially not after August, otherwise you can expect a lot of blind stems the following year.

Common Diseases and Pests of Hortensia

Diseases and pests affecting hortensia include spider mite, aphids, mildew, gray mold and Pythium. Good water management and sanitation are your first line of defense against Pythium, gray mold, and mildew. Since a number of cultivars are extremely susceptible, you should work with your supplier to identify culivars that perform best in you climate. Some new cultivars are more resistant to these diseases. Frequent applications of fungicide for Pythium and Botrytis may still be necessary. If you had problems with Botrytis the year before, make sure all dead foliage has been removed from pots, tables, and floors. A preventative application of fungicide is strongly stuggested before starting to force the crop. Contact your extension specialist for which treatments are available and legal in your area.