Strawberries: Understanding the Effects of Photoperiod and Temperature

Have you ever wondered why some strawberries bear fruits earlier than others?  Have you ever wondered why some strawberries bear fruits longer than other ones?  Or for that matter, have you ever wondered why some plants flower only at specific times of the year?  

Definitions

Before we answer these questions, here are three terms we need to define for ease of understanding:

1. Photoperiod or Daylength is the number of hours of daylight during a 24-hr period.

2. Flower Initiation - is the beginning of reproductive phase in plants which is triggered by biochemical and hormonal changes in the plant.  For us gardeners, we would see this phenomenon later when visual indicators are met - flowers are out.  

3. Remotancy is the ability of a plant to bloom repeatedly throughout the growing season.  This is an important characteristics for flowering well as fruiting perennials because it prolongs the productivity of the plants. 

Photoperiod and Flowering

It has long been established in plant science that flower initiation is influenced by photoperiod. Extensive research has been done to show chemical and hormonal changes that correspond to varying photoperiods.  Shifts in hormone levels induced by photoperiod either lead to initiation or inhibition of flowering depending on the genetics of plants.  Angiosperms - flowering plant species - have been classified into three categories based on their response to daylength as follows:

     Short-day (SD) plants are those that bloom only when the daylength is less than twelve hours.

     Long-day (LD) plants are those that bloom only when the daylength is more than twelve hours.

     Day-Neutral (DN) plants are those that can bloom regardless of the daylength.

Strawberry Varieties and their Response to Photoperiod

Strawberry is one example of a plant that is responsive to photoperiod. which plays a significant role in its flower initiation as well as remotancy.  Strawberry varieties are classified according to their flowering response to this factor.  

June-bearing varieties - SD strawberries.   These are recommended for canning purposes because they yield a heavy crop in the summer.  They are the varieties that are generally grown commercially. June-bearing varieties bloom in spring when the days the less than 12 hours and warm enough for plants to grow at a reasonable rate.  In the fall when the days get below 12 hours of daylight, they undergo flower initiation again.

Ever-bearing varieties are LD strawberries.  These varieties start to bloom late in April when the daylength exceeds 12 hours and all the way to autumn when daylength shortens to under 12 hours again. Ever-bearing strawberries are recommended for a kitchen garden where fresh strawberries every now and then is desired.

Day-Neutral strawberries are those that are not significantly affected by photoperiod.  They bloom regardless of the photoperiod as long as other factors are favorable for growth.

Temperature and Fruit Setting 
Can we then say that strawberries will always bear fruit when the right photoperiod is achieved?  The answer is no.  The story is not that simple. The effect photoperiod on flowering assumes optimum temperatures.  After flower initiation occurs, temperature comes in and plays a significant role in the development of the flowers into a fruit.

In our area (Zone 9), June-bearing varieties, can start to show flowers as early as February but such flowers fail to develop into fruits because the temperatures are too cold for fruiting.  Similarly in the fall flowering is triggered again by the shorter photoperiod but such flowers fail to develop due to low temperatures.  The reason for the characteristic heavy crop from June-bearing strawberries is the favorable temperatures in the spring to early summer for fruit formation.

For ever-bearing varieties, we would expect that once the photoperiod is long enough that they would continue to produce fruits all summer long.  Not again.  Because they bloom later in the season, high temperatures (Zone 9) limit fruit development. Studies have shown that although flower initiation occurs under high temperatures on ever-bearing varieties, the percentage of fruit formed over  the number of flowers is significantly reduced (1). This is the very reason for the characteristic continuous fruiting with relatively low yields on ever-bearing varieties.  In spite of their remotancy, only few flowers successfully develop into fruits.

Photoperiod determines flower initiation; temperature determines the development of the strawberry fruit, but the behavior of the plant is determined by genetics.