April 1, 2014

Evaluation of different crop management and overwintering techniques on winter survival of different Echinacea cultivars


In the past ten years, new cultivars of coneflowers are offered on the market, offering new orange color varieties very different from the traditional pink Echinacea purpurea. In Quebec and Canada, Echinacea has become one of the perennial species with the greatest economic importance in the nursery and may represent up to 15 % of the production of ornamental perennials.

Many of these new cultivars are hybrids between E. purpurea and E. paradoxa, a species found in the central section of the southern United States (Texas, Missouri, Arkansas). This ornamental gain has been at the expense of a loss of their resistance to cold. Indeed, significant winter losses are reported in most Canadian nurseries and the northern United States, so that some growers have become accustomed to start the production of the plant in February, in heated greenhouses, in order to sell the plants in bloom in the spring and avoid having to overwinter them. It may ensue, however, that the consumer will inherit the problem, a large number of plants may not survive the first winter when planted in the landscape.

In an attempt to reduce winter losses occurring among both growers and consumers, this project investigated different production techniques that allow the plant to acclimate to winter conditions. Indeed, the ability of a plant to survive the winter depends on several factors.

Acclimatization is the primary factor affecting the cold tolerance of plants (Perry, 2009). This acclimation is primarily dependent on climatic conditions to which the plant will be submitted before the onset of winter (Perry and Herrick, 1996) but also the physiological stage it has reached and preparation of the plant during the growing season growth. Since in perennials, the vegetative structures ensuring winter survival are the roots and crown (Islands and Agnew, 1993; Iles et al, 1995, Kim and Anderson, 2006), all approaches and cultural practices to promote the optimal development of the vegetative structures, accumulation of sugars and other compounds acting as cryoprotectants, allow a better winter survival. This is what is called "conditioning", which aims to optimize the vigor of the plant and prepare for adverse conditions. Among the practices that condition the plant, fertilization and pruning are essential (Dinesen et al, 1997, Perry, 2009). Time of fertilizer application during the season and the amount applied are other factors that may have an effect on winter survival. Indeed, some studies have shown that fertilization performed later in the season can help plants survive better in the winter and a better recovery was observed in spring (Perry, 2009). Moreover, pruning is another strategy considered to promote tillering plants and crown bulking.
The method used to protect plants against cold is also essential for their survival (Iles et al., 1993). Several studies have demonstrated the need to cover container-grown perennials with insulating fabrics to minimize temperature variations and reduce losses (Still et al, 1989, Perry, 1990a and 1990b; Iles et al, 1993). Many Quebec producers protect their container-grown perennials by covering them with a combination of one or more insulating blankets and a canvas of white polyethylene. For more sensitive plants to cold, such as coneflowers, the majority of producers store them in unheated plastic greenhouses in addition to insulating blankets. Despite these precautions, growers observe significant losses for some cultivars. An interesting avenue for overwintering new Echinacea cultivars and more cold-sensitive perennial cultivars would be to store the plants in plastic greenhouses with temperatures varying between -15° C and 5° C to maintain a constant temperature in pots around -2° C under the insulation blankets . This practice would be easily applicable by nursery producers since the majority of them already have greenhouses.

The goal of this project was to identify and apply different production methods to improve winter survival of orange-colored cultivars of coneflowers.
Materials and methods
From January 2012 to December 2013, seedlings of three cultivars of orange coneflowers 'Tiki Torch', 'Tomato Soup' and 'Tangerine Dream' and a control, the purple coneflower E. purpurea were grown in different experimental devices comprising three fertilization regimes, four treatments of flower pruning, two dates of potting plants and four methods of overwintering in two regions of the province of Quebec. Two nurseries, Fleurs Rustiques and Québec Multiplants, as well as Laval University were partners in this project.
The experimental plot and the irrigation system at Laval University (June 29th 2012)

Fertilization treatments
F1 : Continuous fertilization (≈200 ppm of 20-8-20 up to mid-October (as do the growers);
F2 : Continuous fertilization (≈200 ppm of 20-8-20) up to beginning of August;
F3 : Continuous fertilization (≈200 ppm de 20-8-20 up to beginning of August, following by (≈100 ppm of 20-8-20) up to mid-October.

Flower pruning treatments
T1 : No flower pruning (control);
T2 : Pruning of flower when the buds are visible;
T3 : Pruning of flower at the opening of the flower bud;
T4 : Pruning of flower once wilted.

Dates of potting treatments
  • Potting in February
  • Potting in June
For each of the three production treatments of this project, we tested four overwintering techniques:
  • Overwintering in cold storage at -2 °C
  • Outside under geotextile membrane and white polythene (commonly used in nurseries)
  • Overwintering in an unheated tunnel under geotextile cover,
  • Overwintering in a tunnel heated when the temperature reaches -15° C and ventilated when it reaches 5° C;
  • The site at Laval University had one additional treatment consisting of a cold storage room kept at -2° C.
Results and discussion
If we consider our results as a whole, some very interesting effects can be observed between the treatments applied. As expected, we find that the type species, Echinacea purpurea presented very good winter survival, while significant losses were obtained for the other three cultivars. These losses are desirable, because no conclusion could be drawn if all cultivars had reached a high survival rate. The treatments provided were intended to force the implementation of plant dormancy to enable them to better withstand the winter.

It is very important to note that the survival rate of plants tested, following the 2012-2013 winter were unusually high. Indeed, it is the same for all ornamental nursery production. The causes of this phenomenon are still difficult to explain but we believe fall conditions in 2012 were important factors. The normal rate losses allocated to coneflowers from hybridization with E. paradoxa are of the order of 15 % to 50 % depending on cultivars and winters.

The results of the fertilization treatments at both sites clearly show that to reduce hasty fertilization (full stop in August) greatly improves winter survival of the most sensitive cultivars (Figure 1). For E. purpurea, well adapted to our climate, it is instead preferable to apply a conventional fall fertilization regime (continuous fertilization until mid-October). In its natural habitat, E. paradoxa receives very little water and minerals from the end of the summer, allowing it to gradually enter into a dormant state necessary for its winter survival (mild winter). If abundantly fertilized in August and September, no signal reaches the plant telling it to prepare its entry into dormancy and the plant will die not because of freezing, but because of a complete depletion of energy reserves. This response seems to be maintained in the new hybrids obtained from E. paradoxa. Unlike woody species where the first mechanisms of dormancy are initiated by a decrease in photoperiod, most perennials will only react to drops in temperature and change in photoperiod. In the case of Echinacea, it is easy to see that they do not meet any of these factors, since even in October, they keep producing new buds that will perish in the following weeks entailing an unnecessary expenditure of energy by the plant. Our experiment demonstrated that it is possible to "force" this setting of dormancy by a significant decrease in fertilization in late summer.
Figure 1: Winter survival of cold-sensitive cultivar is improved by reducing fertilizer early.
In the flower pruning treatments, it was logical to think that cutting early flowering stems would help the plants save energy, which could then contribute to their winter survival . However, our results indicate that the pruning of the flower stems had little impact on the subsequent winter survival. Early stem size has helped to foster the development of axillary buds on the crown, promoting tillering of the plant and the formation of a wider crown. However, tillering does not improve survival in the winter, but it didn’t hurt. This can be explained by the fact that these plants were fertilized according to a conventional program used by producers. We therefore conclude that the pruning of flower stems is a good practice to obtain a larger (and therefore a better quality of plant) crown, but it has no impact on the implementation dormancy or survival of winter sensitive Echinacea cultivars.

For a plant to go dormant faster in late summer, one might think that all needs to be done is to start its cultivation earlier in the spring. Our experiment on the age of the plants shows rather that to lengthen the growing season will simply give largest plants in the fall, and sometimes too many roots. These plants do not present a better winter survival rate because again the plant receives no input signal to go dormant and will die of exhaustion of its reserves. For E. purpurea, improved survival has been observed even on younger plants. Although the fact of putting the plant in cultivation earlier allows producers to sell in the first year, it is the consumer who will pay the note since it is very likely that the plant will die in the first winter when planted in the landscape.
If it may be difficult to force the implementation of a dormant plant by technical methods, another possibility that can enable them to survive the winter is the use of an adequate overwintering technique. For each of the three production treatments of this project, we tested the four overwintering techniques listed above. These techniques did not significantly improve winter survival. This latter allowed us to achieve a major finding. While overwintering in cold storage gave the best survival rate, this rate remained relatively low, indicating that the plants cannot survive a temperature of -2 °C which usually is the ideal storage temperature for dormant perennials or woody plants since it limits the plant respiration without deep freeze while preventing mold growth. This finding allows us to make an assumption, namely that it is not the cold that kills coneflowers (up to a certain level), but rather an inadequate physiological condition of plant prior to overwintering. This is exactly what was observed in the fertilization treatment, where we concluded that dormancy was favored by an early decrease in fertilization.

If a plant is unable to tolerate a temperature of -2 °C, no overwintering technique will offer a high survival rate, except a lighted and heated greenhouse. However, it has often been found that specimens of cold sensitive Echinacea cultivars that have managed to survive their first winter will survive the following winters without problem. What difference is there between a plant in its first year and between a plant in its second year? Visually, a plant can reach a good size after only one year of cultivation and have an abundant root system. Another factor that has to be considered is the technique of in vitro propagation of these cultivars. All these new cultivars that cannot be propagated by seed are micropropagated by in vitro laboratory in culture media rich in growth hormones (Harbage, 2001). The plants are propagated by axillary budding from an original mother plant obtained several months or years before. These plants have not known dormancy for several years and are sold to nurserymen once acclimated. We hypothesize that since these plants were kept actively growing for several cycles on cytokinins-rich media, they no longer respond to dormancy input signals brought by autumn temperatures and reduced photoperiod. But, once a first winter (for those who survived), the life cycle is put in order and the plant survive the following winter without problems if snow depth is sufficient. To validate this hypothesis, one should compare the winter survival of plants produced in vitro compared to seedlings of similar ages and obtained by dividing the crown of a plant of the same cultivar having already survived a winter.
The experimental design at the end of the season at Laval University (October 18th 2012)

Our results indicate that a significant gain of winter survival can be achieved if fertilization is completely stopped by early August to promote the stopping of growth and conditioning plant dormancy. Early flower pruning encourage better tillering of the plant and provides more roots for some cultivars, but has little impact on winter survival. The potting of plants in early spring allows obtaining a plant with a larger crown in the fall, but our results show that it does not improve winter survival. Among the four overwintering techniques used, a greenhouse heated to -15
° C and ventilated at 5° C offers more security against winter temperature deviations in areas where snow is scarce.

Our work has enabled us to establish a theory that one of the causes of mortality of orange-coloured Echinaceas after their first winter may be related to the method of multiplication of these coneflowers. Coneflowers from in vitro propagation could be in an inadequate physiological condition at the time of acclimatization to winter. This situation could potentially confuse the establishment of mechanisms for the implementation of dormancy. Further work will be needed to demonstrate this hypothesis and to propose improvements to the technique of propagation allowing the plant to respond to climate signals indicating the onset of winter.

Part of this project was financed through regional Industry Councils of Quebec and Ontario, whom carry out the Canadian Agricultural Adaptation Program (CAAP) acting on behalf of Agriculture and Agri-Food Canada Agriculture and Agri-Food (AAC) has committed to work with the industry’s partners. The opinions expressed herein are those of the applicant and are not to be considered as automatically shared by the ACC.
IQDHO wishes to thank the company Fleurs Rustiques, Quebec Multiplants and Terra Nova for their helpful participation in this project. Special thanks to Marie-Pierre Lamy, Eric Dugal and Jacques-Andre Rioux from Laval University for their dynamism and support throughout the project. Also thanks to Marie-Claude Limoges and Julie Bilodeau and all of the IQDHO team for their contribution to this project.

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