Strawberry plant responses to environmental conditions are a complex topic. The following is what we consider as the current general understanding based on available literature. Responses to specific cultivars need to be examined rather than simply applying this general information. Especially those cultivars that are bred for and cultivated in open fields, there is little information regarding responses to individual environmental factors (and their interactions). Greenhouse controlled environments allow independent control of photoperiod, light intensity and temperature among other factors, and thereby potentially control the plant growth and development to maximize the crop productivity and fruit quality.
Air temperature. Three temperatures to consider for any controlled environment applications are daytime, nighttime and 24-h average temperatures. Daytime temperature is selected for photosynthesis, nighttime temperature for fruit quality, and 24-h average temperature is for overall growth and development including flowering.
Optimum temperature of strawberries photosynthesis is reportedly 20°C (68°F) (Kimura, 2008) and many cultivars seem to exhibit comparably high photosynthetic rate between 15-27°C (59-80°F) temperature (Kimura, 2008; Morgan, 2006). However, temperature set point during the day affects not just photosynthesis but also flowering. Our recommendation is to keep the daytime temperature between 20-24°C (68-75°F) and to achieve the optimum 24-h average temperature around 18°C (64°F), by combining with a lower night temperature.
Generally, fruit size increases at lower temperature. We also know that fruit flavor is improved (sweeter) when the fruit develops at lower night temperature. We recommend nighttime temperature at 10-12°C (50-54°F). A lower night temperature than 10°C is acceptable but, unless the daytime temperature compensates, lower 24-h average temperatures reduce the overall grow and productivity. When night temperature increases and exceed 16-18°C (60-65°F), titratable acidity increases, reducing the sugar-acid ratio.
Root and crown zone temperature.
Growers and researchers also need to notice that growing systems would affect optimum temperature setting. Root zone temperature in hydroponic/soilless culture usually has greater diurnal temperature oscillation compared with the soil-based culture system when the same greenhouse/high tunnel system is used. Root-zone temperature would directly affect the crown temperature where flowers and leaves are developed, while fruit temperature is more directly affected by air temperature when fruits are hanging in the air.
Optimum crown temperature is reportedly at 18°C (M. Okimura, personal communication) and localized temperature control by introducing temperature controlled water inside the tubing placed around the crowns showed some success in enhancing production in soilless cultivation. We have examined the crown temperature control in collaboration with Dr. M. Okimura at Kyushu Agriculture Experiment Station in Japan and achieved ~10% yield increase during our winter production seasons between 2009 and 2011 (unpublished data). [Photo in right shows such a water circulation system developed in Kyushu Ag Experiment Station to maintain crown temperature relatively in an optimum range]
DLI – Daily light integral. Strawberry is cultivated in off-season winter greenhouses with varied light environment. Strawberry plants can produce fruits under low light environment but the productivity goes down almost proportionally with lower light. DLI (daily light integral) can provide good prediction of growth and production potential for greenhouse crops and growers are recommended to find the DLIs over the seasons they intend to grow strawberries. We recommend 12 mol/m2/day DLI to target as minimum level for good productivity and consider the optimum between 20-25 mol/m2/d. Under DLI exceeding 30 mol/m2/d, strawberry plants tend to be stressed (shading is required in that case). Interactive DLI map was developed recently by two scientists (Joanne Logan and James Faust) and useful for finding monthly DLI outdoors. We usually recommend a conservative 50% transmission of your greenhouse to find indoor DLI from outdoor DLI. Multiple factors including structural shading, overhead equipment, reflection per sunlight beam angle, and age of glazing affect the daily transmission of photosynthetical active radiation (400-700 nm). A new well designed greenhouse can have as high as 70% transmission.
Of interest, strawberry plants show relatively steep diurnal decline of net photosynthetic rate (Inaba, 2007), suggesting that morning hours are the critical time for promoting photosynthesis. The cause is likely the unbalanced sink-source relationship (Garcia and Kubota, 2017), which may lead to a new climate control strategy for maximize productivity.
Day length. Length of day and night affects flowering and the dormancy status (rosette-type morphology). Night interruption lighting is a unique practice done in Europe and Asia to keep the plants more vigorous under winter light condition. More information about this practice is found under ‘Lighting‘ section of this website.
Relative humidity. Strawberry plants are sensitive to dry climate. Therefore use of fogging is recommended during the day (Morgan, 2006) if possible. When they are grown in low relative humidity, particularly low night time humidity, tipburn and calyx burn may be pronounced (Kroggel and Kubota, 2017). A minimum of 3 hours of high humidity (95%) for 2-3 consecutive nights (followed by 2 nights per week without humidification) is needed to avoid tipburn for sensitive cultivars (Kroggel and Kubota, 2017). As long as this requirement is met, the rest of the night time outside of that 3 hour period should be at much lower humidity (to avoid fungal diseases). Daytime humidity is between 40-60%. See that ‘tipburn’ page for more information.
CO2 concentration. 
Since strawberry plants are grown in closed greenhouse/high tunnels in winter, CO2 enrichment has been practiced for enhancing the photosynthesis under low light conditions when greenhouse vents are closed. Literature suggests that expected yield increase by CO2 enrichment is about 15-20% for fruiting crops including strawberry. A typical range of CO2 concentrations applied in greenhouse is 1,000 -1,500 ppm. The source of CO2 can be natural gas used for combustion or liquid CO2, which is typically more expensive. However, compared with lighting costs, CO2 enrichment is more economical option as the cost to achieve the same yield increase is lower for CO2 than for supplemental lighting. One of challenges of using CO2 is that system needs to be closed (i.e., no or little infiltration), as large amount of CO2 will be wasted when injected under large infiltration. Another challenge is that the response to CO2 enrichment is positive but small (10-15% yield increase for fruiting crops such as strawberry and tomato). This means that it can only replace a few moles of DLI by CO2 enrichment as alternative to supplemental lighting. [Photo on the right is a small scale CO2 burner used in high tunnels in Japan).
References
Garcia, K. and C. Kubota. 2017. Physiology of strawberry plants under controlled environment: Diurnal change in leaf net photosynthetic rate. Acta Horticulturae 1156:445-452.
Garcia, K. and C. Kubota. 2017. Flowering responses of North American strawberry cultivars. Acta Horticulturae 1156:483-490.
Inaba, Y. 2008. Studies on the horticultural characteristics of strawberry for the development of year-round production and the release and the extension of a new cultivar adapted for It. Ph.D. dissertation submitted to Tokyo University of Agriculture and Technology. 93pp.
Kimura, M. 2008. Vegetative growth and reproductive growth, p.73-96. In: Encyclopedia in Vegetable Crops Horticulture – Strawberry, 2nd Edition. Nobunkyo, Tokyo. 692pp.
Kroggel, M. and C. Kubota. 2017. Controlled environment strategies for tipburn management in greenhouse strawberry production. Acta Horticulturae 1156:529-536.
Morgan, L. 2006. Hydroponic strawberry production. A technical guide to hydroponic production of strawberries. Suntec NZ, Tokomaru, NZ. 117pp.