production perennials

Isotoma plants were chilled at 45°F for zero, 2.5, five, 7.5, Some Perennials 10, 12.5 and 15 weeks, and then grown under 16-hour-long days for nine Like It Cold weeks

by ART CAMERON, BETH FAUSEY,  treatment.” It is somewhat of a confus- Vernalization is relatively specific to SONALI PADHYE and ERIK RUNKLE ing term in the sense that the definition biennials and herbaceous perennials. comes from “vernal” (meaning spring), Still, not all herbaceous perennials have ERBACEOUS perennials com- and hence the literal translation is “mak- a vernalization requirement and the lack prise a diverse group of plants ing spring.” This phenomenon was origi- of this requirement does not necessar- from a variety of habitats nally described for food crops such as ily relate to cold hardiness. For instance, around the world and, of and rye. For instance, and Campanula carpatica, which is hardy at course,H many make great garden plants. spring wheat are very closely related, least to USDA hardiness Zone 3 (-40°F), While retail sales of perennials in but winter wheat must be planted in the will flower readily without a vernalization continue to increase nationally, growers fall or it will not flower. Spring wheat, treatment provided it is given long-day can be challenged to develop and utilize as the name implies, can be planted in photoperiods, while Isotoma axillaris, production protocols that will consis- the spring and readily in the first hardy only to zone 7 (0°F) has an obligate tently result in scheduled flowering for season. The only difference is that winter vernalization requirement. many of these popular plants. wheat has an obligate vernalization re- In fact, one of the challenges in our For the past several years, we have quirement (must be exposed to cold or research program over the past decade studied the flowering requirements for it will not flower) whereas spring wheat has been to establish which perennials a wide range of herbaceous perennials, does not. Early researchers learned that have a vernalization requirement and and it is clear that the most important if they artificially provided cold to young which do not. Sometimes, the require- environmental cues for growth and flow- plants or even imbibed seeds of winter ment is obligate (absolutely required, ering are photoperiod, vernalization and wheat, they could, in essence, make plants will not flower without cold) but light quantity. In this series of articles, we spring, since the plants then would pro- often is facultative (merely beneficial, will focus on the vernalization or “chilling ceed to flower requirement” needed for flowering of when planted. many, but certainly not all, herbaceous Current re- perennials. We will attempt to summarize search has some of the new and old information on shown that a vernalization and describe how growers single gene can adapt and utilize this information in can account for various climates. the presence or absence of Vernalization a vernalization The textbook definition of vernaliza- requirement. Left: Improperly vernalized columbines. Right: campanula ‘Birch ’ tion is “the acquisition or acceleration of stock plants after several months in greenhouse. These plants rarely flower. the ability to flower following a chilling

38 October 2005 • Greenhouse Grower signals at the molecular level. There is production still a great deal to be discovered before perennials we will understand why different plants require different lengths of exposure to plants benefit from but do not require cold or even have apparently different a cold treatment). Other factors such as temperature optima for the chilling re- the amount of light that a plant receives quirement. in a day can also interact with the chilling treatment or even override the need for Seasonal Flowering Of Herbaceous chilling and affect the final flowering of a Perennials And Vernalization specific plant. Other plants, such as achil- Response lea ‘Moonshine’ flower only for a specific Periodic or episodic flowering are duration in the garden and even dead- terms that are used to describe flower- heading does not cause a repeat bloom. ing habits of many herbaceous peren- Yet cuttings taken from these plants nials. It is this trait that contributes to readily flower once rooted and there is the continual change in a well-designed no evidence that the block in flowering is perennial bed throughout the growing Stachys ‘Hummelo’ before and after 15 related to vernalization. season. Early blooming perennials such weeks at 41°F. From left to right: nine-hour as Phlox subulata (creeping phlox), iberis short days, 16-hour long days and 16-hour long day with extra supplemental lighting. Control Of Flowering In Nature (candytuft), Euphorbia epithymoides So exactly why do plants control flow- (cushion spurge) and Anemone pulsatilla ering and why would they have a vernal- (Pasque flower) are among the many that ization requirement? It is best explained brighten gardens early in the spring but baceous perennials and have attempted by the fact that survival in the wild often then remain vegetative the rest of the to determine for each the vernalization depends on precise timing of flower- summer, rarely if ever producing another and photoperiodic requirements for ing, which must be coordinated with flower. These are typically species that re- flowering. Generally, we start with rooted pollinator activity and general weather quire vernalization. Then come the many cuttings or seedlings. We then grow one conditions. In many cases, this involves late spring flowering perennials such group under nine-hour short days, 16- the focusing of resources because flow- as coreopsis ‘Sunray,’ campanula ‘Birch hour long days or 16-hour long days with ering and subsequent seed production Hybrid,’ Aquilegia spp. (columbines), substantial supplemental light. A second are expensive processes, requiring plenty Oenothera fruticosa (evening primoses), group of the same plants are put at 41°F of energy. Many herbaceous perennials astilbe and Lavandula angustifolia (lav- for 15 weeks. After this period, the plants have evolved specific mechanisms to ender). Many of these species have a are potted and grown under the three focus flowering at a specific time of the vernalization requirement and perhaps growing environments. This relatively year, some in the early spring while oth- even an added long-day photoperiod simple approach permits us to study the ers are in the summer or even late fall. requirement. Most late summer and flowering response to vernalization, pho- The most important environmental cues fall-blooming species such as rudbeckia, toperiod and light quantity. that govern precise flowering are usu- echinacea (coneflower) and even mums Major Response Categories For ally thought to include photoperiod and do not typically respond to vernalization Herbaceous Perennials vernalization. A vernalization require- but are usually strongly photoperiodic. In the tables online (see “Continued ment could prevent flowering late in the Online”), we have listed the herbaceous season, and thus prevent a reduction of Empirical Determination Of perennials that require or at least benefit stored energy reserves needed for over- Vernalization Requirements from a vernalization or chilling treatment. wintering. We have now studied nearly 500 her- We have found that most fall into just a One important aspect of vernalization is that herbaceous perennials do not enter or generally even leave the chill- ing period with a flower bud. The shift of the growing from vegetative to floral actually can occur days or even weeks after exposure to the chilling temperatures. Thus, one interesting char- acteristic of the vernalization process is that the plant retains a “memory” of the cold and only later uses this information to guide in the timing of flower initia- tion. The nature of this “memory” is still under investigation by researchers, but it Lychnis ‘Jenny’ before and after 15 weeks at 41°F. From left to right: nine-hour short days, 16- hour long days and 16-hour long days with extra supplemental lighting. is known that it involves specific genetic

For Details Circle No. 37 on Postcard or at www.greenhousegrower.com For Details Circle No. 32 on Postcard 40 October 2005 • Greenhouse Grower The plants in this category have a more probable that they do not require cold. production complex flower induction strategy and Though we have not listed them here, perennials require both chilling and long days to it is somewhat surprising how many flower. These typically flower a bit later in herbaceous perennials tolerate and sur- few major categories: the season, and can usually be chilled in vive severe , but do not require • Cold required – day neutral – rapid the plug tray with satisfactory final plant cold for future flowering. In many cases, flowering after cold. These are day- quality. this is one of the intriguing aspects of neutral perennials that typically flower • Facultative cold requirement studying the flowering behavior of this early in the spring. In fact, most of these – mostly long-day plants. This is an in- diverse and fascinating group of plants. flower so fast, that if they are cooled as teresting category that includes plants small plants (for instance in a plug tray), that do not absolutely require cold to they do not gain adequate size and are Future Articles In  flower. They may flower without cold if This Series really not appropriate for retail sales. given long days or perhaps high light. In subsequent articles in this series, Since they have an obligate chilling However, they may flower even under we will cover some of the details about requirement, they will not bloom again short days after vernalization or might the chilling requirement of herbaceous until they are again exposed to cold. flower faster or more profusely. In perennials and how growers can adapt • Cold required – day neutral – slower general, most perform after a cold treat- this knowledge for improved production to flower. These are the few day-neutral ment. The interaction between length of techniques. We will cover many related perennials that we have discovered are day, quantity of light and vernalization topics including the most effective tem- slow enough to flower such that they requirement is of great interest, and peratures and durations for maximum can gain in size rather appreciably after further research may unearth clues as responses on a plant by plant basis, in- the end of the chilling period. These to how plants monitor and respond to teractions of light quantity and a primer can be chilled in the plug tray and the complex changes in the environment. on some of the underlying mechanisms final plant quality will be satisfactory for • No cold required. You may find that of how plants perceive and even mea- most growers. some of your favorite perennials may sure cold. GG • Cold – required – long-day plants. not appear on any of these lists. It is About the authors: Art Cameron is a professor of horticulture, Beth Fausey is floriculture pro- gram manager with The Ohio State University Extension, Sonali Padhye is a graduate student and Erik Runkle is an assistant floriculture professor. For more information, contact Art Cameron, [email protected].

Campanula ‘Cherry Bells’ before and after 15 weeks cold – short days, long days and long days with supplental lighting. Note that this requires cold and long days to flower.

Continued Online

To see crop tables related to this article, please log on to www. greenhousegrower.com/grower_ tools/html. There you will find:

• Table 1. Major crop categories responsive to vernalization.

Stay tuned to Greenhouse Grower for more perennial vernal- Agastache for comparison. Agastache is a plant that does ization articles in the November not require chilling to flower. and December issues.

For Details Circle No. 41 on Postcard or at www.greenhousegrower.com 42 October 2005 • Greenhouse Grower