Strawberry Production, Growth and Development in Northern Climates

Please note that this electronic prepublication galley may contain typographical errors and may be missing artwork, such as charts, photographs, etc. Pagination in this version will differ from the published version. Strawberry Production, Growth and Development in Northern Climates Anita Sønsteby Saila Karhu ABSTRACT. This paper focuses on the cropping systems in northern European countries such as Norway and Finland. Winter injury commonly occurs in strawberry fields in northern climates and inadequate snow cover further aggravates the problem. During the last decade there has been an increase in programmed out-of-season strawberry production. Greenhouse producers aim for year-round production, whereas those using polyethylene tunnels extend the main production season to include May/June and September/October. The dominant strawberry cultivars in Nordic countries are Senga Sengana for the processing industry and the Dutch cultivar Korona is preferred for direct fresh market consumption due to its good flavor and high yields. Recently, Honeoye has gained considerable interest among Danish and Swedish growers due to its high productivity and its relatively low susceptibility to grey mold. In Finland, the Norwegian cultivar Jonsok is one of the most popular cultivars due to its high tolerance to winter injury and grey mold. [Article copies available for a fee from The Haworth Document Delivery Service: 1-800-HAWORTH. E-mail address: <docdelivery@haworthpress.com> Website: <http://www.haworthpress.com> 2005 by The Haworth Press, Inc. All rights reserved.] Anita Sønsteby is a Researcher at Apelsvoll Research Centre Division Kise, The Norwegian Crop Research Institute, N-2350 Nes Hedmark, Norway (E-mail: anita. sonsteby@planteforsk.no). Saila Karhu is a Researcher at Agrifood Research Finland, Horticulture, FIN-21500 Piikkio, Finland. International Journal of Fruit Science, Vol. 5(1) 2005 http://www.haworthpress.com/web/ijfs 2005 by The Haworth Press, Inc. All rights reserved. Digital Object Identifier: 10.1300/J492v05n01_10 105

106 INTERNATIONAL JOURNAL OF FRUIT SCIENCE KEYWORDS. Production systems, sequential planting of cold-stored plants, freezing resistance, Fragaria ananassa INTRODUCTION The strawberry industry in northern Europe faces many challenges including increased competition from southern production countries and a restrictive legal attitude toward chemical use in plant protection. Locally produced strawberries are in high demand in Nordic countries because consumers appreciate the flavor and low amounts of pesticide used in production. Northern growing conditions can be described as having hard winters with variable snow cover, springs with frost at bloom time, short growing seasons with long day conditions extending far into autumns that in extreme cases may be either very short and cold or long and warm. These conditions require a lot of physiological flexibility and adaptability from strawberry cultivars grown in the north and demand some special cultivation practices for successful strawberry farming. PRODUCTION SYSTEMS The strawberry is the major small fruit species in the Nordic countries. Table 1 shows the acreage, number of growers and average strawberry yields (Davik et al., 2002). Most of the production is sold for direct consumption, and less than 25% of the fruit goes to the processing industry. TABLE 1. Acreage, number of growers, and average strawberry yields in the Nordic countries. (After Davik et al. 2002). Hectares Number of growers Yield (tons/ha)* Denmark 1000 500 9 Finland 4500 2800 4 Norway 1600 1300 5 Sweden 2700 800 5 *Fruit sold through official channels.

Anita Sønsteby and Saila Karhu 107 The traditional production in the Scandinavian countries lasts from the middle of June until the middle of August. Some attempts to extend the season are being undertaken, especially with the use of floating row covers or stored plants, but to a small extent. Growing of everbearing cultivars has not been successful so far. Thus, almost all strawberry growing is perennial production with short day cultivars in open field, and the fields are normally kept for 3-5 years. However, during the last decade there has been a consistent increase in programmed out-of-season strawberry production in the Nordic countries. Greenhouse producers aim for year-round production, whereas those using polyethylene tunnels extend the main production season to include May/June and September/October. The dominant strawberry cultivars in Nordic countries are presented in Table 2 (Davik et al., 2002). Senga Sengana has been the most important cultivar in Norway, and it still is the preferred cultivar of the processing industry. For direct consumption, the Dutch cultivar Korona has been preferred due to its good flavo and high yields. Recently, Honeoye has gained considerable interest among Danish and Swedish growers due to its high productivity as well as its relatively low susceptibility to grey mold (Daugaard, 1999). In Finland, the Norwegian cultivar Jonsok is one of the most popular cultivars due to its high tol- TABLE 2. The most important cultivars in the Nordic countries. Percentage of acreage for various strawberry cultivars in each country. (After Davik et.al. 2002). Cultivar Denmark Finland Norway Sweden Bounty 10 5 Dania Elsanta 27 Honeoye 35 10 10 30 Jonsok 50 Korona 15 45 40 Polka 8 10 15 Senga Sengana 15 20 Zephyr 15

108 INTERNATIONAL JOURNAL OF FRUIT SCIENCE erance to winter injury and grey mold (Altonen and Kemppainen, 1997). NORTHERN ENVIRONMENTAL FACTORS CONTROLLING STRAWBERRY DEVELOPMENT Photoperiod and Temperature Interactions Commercial production of strawberry takes place in its northernmost limits up to about 69 30 N under cool summer temperatures and extreme photoperiodic conditions. This wide geographical distribution of the strawberry growing gives a prolonged season, but the variable climate causes large variations in yield among years and regions. Heide (1977) investigated temperature and day-length effects on flower initiation in strawberry cultivars adapted to marginal growing areas in the north. He concluded that three cultivars that flower early in spring in Norway (69 39 N) initiated flower buds in all photoperiods including continuous light at temperatures of 12 and 18 C. The later ripening cultivars Senga Sengana and Abundance did not initiate flower buds in 24 h photoperiods at any of these temperatures (Figure 1). Heide (1977) concluded that at high latitudes temperature is as important as photoperiod in controlling flowering in the strawberry. The interaction of photoperiod and temperature has been confirmed also with other strawberry cultivars commonly grown in the north (Sønsteby and Nes, 1998). Bjurman (1975) studied the influence of day-length and temperature conditions on vegetative and generative characters in four cultivars at five localities between 55 and 65 north latitude in Sweden. The number of flower trusses, flowers per truss and berries showed a clear tendency to decrease from south to north, irrespective of the vegetative development. Bjurman s data give strong indications that the temperature conditions during the autumn are critical for the following year s yield. This was confirmed in a study done by Døving and Måge (2001), where data from commercial strawberry production in two districts of Norway were compared with meteorological data to study methods for predicting fruit yield in strawberries. They found that temperature during flower induction and flower differentiation was more important for yield levels than temperature during flowering and ripening. Relatively low temperatures in August and high temperatures in September appeared to give high yields in the following season.

Anita Sønsteby and Saila Karhu 109 FIGURE 1. Flowering of four strawberry cultivars as observed at 24 C and continuous light following 5 weeks of treatment at various photoperiods and temperatures (After Heide, 1977). 0 Zephyr Jonsok 12 5 12 18 24 18 24 0 0 Senga Sengana Abundance 12 5 18 24 0 10 12 14 16 24 10 12 14 16 24 The components that contribute to the total yield of strawberries are influenced by specific environmental conditions. Branch crown formation and inflorescence initiation are both promoted by the inductive short photoperiods of autumn, with crown development being affected earlier. In Korona short day photoperiods up to 15 h increased crown branch formation as compared to 18 h photoperiod at 18/16 C (day/ night) temperature. Floral induction failed in many plants in the15 h photoperiod but was successful in 12 and 13.5 h photoperiods (Konsin et al., 2001). The number of crowns and flowers as well as yield increased and runner formation decreased by lengthening the short-day

110 INTERNATIONAL JOURNAL OF FRUIT SCIENCE treatment duration, but the tested short photoperiods did not cause full dormancy of plants at the temperature used even in treatments lasting 49 days, without a longer treatment period or a lower temperature. The plant responses to the environmental inductions in autumn of crown branching, flower induction and dormancy development, thus proceed in this order but are in part concomitant. Enough time and functionally effective low temperatures are needed in addition to the shortening of day length for the physiological and structural development to allow for good over-wintering and yield of plants the following season. In many of the important strawberry areas in Scandinavia, an early winter cuts short the initiation of floral buds more frequently than in the south, and the floral buds will either not develop to the stage where they would produce complete flowers the following spring, or the flower differentiation will be delayed, giving a late ripening of the strawberries. Climatic changes toward milder autumns, may benefit the strawberry production in these areas. Use of non-woven polypropylene row covers or fleece in northern climates has shown to enhance growth processes that are limited by suboptimal environmental conditions. The increased temperatures under the floating cover stimulate increased flower bud formation during the short day induction cycles in the autumn and spring under northern conditions (Pietilä et al., 2002). Additionally, the covers may provide a protective environment where leaves remain functional for longer periods and crown and root development may be stimulated. Over Wintering Winter injury commonly occurs in strawberry fields in northern climates, and inadequate snow cover further aggravates the problem. Freezing injury of plants is one of the main factors that influence strawberry fruit size and yield in the north (Nestby at al., 2000). Freezing injury can be observed as necrosis in the crown tissues. In most cases, the plants will recover from the injury, but it has been shown that a yield loss of up to 20% occurs before the damage is manifested as crown necrosis (Daugaard, 1998; Nestby et al., 2000). Cultivars that are able to tolerate low temperatures have been selected to be grown under northern conditions, but their freezing tolerance is rather limited, however, Palonen and Lindén (2001) observed that crowns of cold hardened strawberry plants can survive 8 C without injury, but severe injury was found to occur at 12 C. During the

Anita Sønsteby and Saila Karhu 111 same time, the plants over wintered under snow cover without any visual damage, although air temperatures fell below 20 C. To increase and stabilize fruit yield it is necessary to avoid low winter temperatures, and also reduce the amplitudes of shifting temperatures around 0 C. This has led to an increased use of various kinds of winter covers by the growers. These both decrease freezing injury in plants with a limited snow cover during winter (Aflatuni et al., 1997) and reduce injury in flower primordia and thus the number of misshapen fruits, allowing the plants to produce larger fruits (Nestby et al., 2000). Warm temperatures can interfere with cold acclimation processes in autumn. Row covers can delay the hardening off process and decrease the winter tolerance of plants if covering is done too early in autumn. On the other hand, good plant establishment and growth during the warm growth season accelerates the development of winter tolerance (Nestby et al., 2000). Thus, cultural practices and growth conditions during the previous summer can affect both over wintering and yield in the northern perennial strawberry production areas. REFERENCES Aaltonen, M. and R. Kemppainen. 1997. Variations in strawberry yields great around the artic circle. Acta. Hort. 439:393-396. Aflatuni, A., R. Kemppainen, A. Heinonen, and T. Hakonen. 1997. The effects of a non-woven cover in combination with different soil mulches in strawberry cultivation. Agr. Food Sci. Finland 6:371-380. Bjurman, B. 1975. Environmental influence on the vegetative and generative development of the strawberry plant. Swedish J. Agr. Res. 5:163-173. Daugaard, H. 1998. Winter hardiness and plant vigor of 24 strawberry cultivars grown in Denmark. Fruit Var. J. 52:154-157. Daugaard, H. 1999. Strawberry cultivars for the fresh market in Denmark. Fruit Var. J. 53:69-72. Davik, J., H. Daugaard, and B. Svensson. 2002. Strawberry production in the Nordic countries. Adv. Strawberry Prod. 19:13-18. Døving, A. and F. Måge. 2001. Prediction of strawberry fruit yield. Acta. Agr. Scand., Sect. B, Soil and Plant Sci. 51:35-42. Heide, O.M. 1977. Photoperiod and temperature in growth and flowering of strawberry. Physiol. Plant. 40:21-26. Konsin, M., I. Voipio, and P. Palonen. 2001. Influence of photoperiod and duration of short-day treatment on vegetative growth and flowering of strawberry (Fragaria ananassa Duch.). J. Hort. Sci. Biotech. 76:77-82. Nestby, R., R. Bjørgum, A. Nes, T. Wikdahl, and B. Hageberg. 2000. Winter cover affecting freezing injury in strawberries in a coastal and continental climate. J. Hort. Sci. Biotech. 75:119-125.

112 INTERNATIONAL JOURNAL OF FRUIT SCIENCE Palonen, P. and L. Lindén. 2001. Winter hardiness of micropropagated and conventionally propagated strawberry plants. J. Hort. Sci. Biotech. 76:685-690. Pietilä, M., A. Aflatuni, A. Kauppi, and K. Karp. 2002. The significance of autumn and overwinter cover for strawberry. Acta Hort. 567:527-530. Sønsteby, A. and A. Nes. 1998. Short days and temperature effects on growth and floweringinstrawberry(fragaria ananassa Duch.). J. Hort. Sci. Biotech. 73:730-736.