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.

Bok Choy: A Fast Food

Bok Choy or pechay (Brassica rapa)

Some of pechay made their way to kitchen today.

Exactly one month from the time of sowing, the bok-choy in my garden is ready for harvesting.  If there is such thing as "fast food" in the garden, bok choy (a.k.a. pechay) is one of them.  

Bok Choy is a very common vegetable in the South East Asian countries. It is one of the most common vegetables grown in backyard gardens.  I grew up eating a lot of them. :)  For some reason (nostalgia) I seem to always buy a packet of seeds every year and sometimes I also forget to plant them.   I remember the first time I tried to plant this in my California garden.  It was summer and I was pleased to see the young plants emerge so quickly.  However, shortly after they put on their first pair of true-leaves they "bolted" (flower).  Bok Choy is a long-day plant - a lesson I had to learn.   

As of today the daylength in California is 14 hrs and 33 minutes.  So far no sign of bolting has been observed.  I wonder what the critical day length is for bok choy.

Cilantro: Flowering Too Soon in Spring

Cilantro (Coriandrum sativum)

These cilantro plants were just seedlings two months ago... and guess what?  They are already flowering.  As I said before, it is always a bad news when any crop that is grown for their leaves starts to flower because it means that growth begins to shift from foliage development to the seed production.  Well, for cilantro it's not all bad news.  So you stop getting the leaves for your salsa but soon you will have coriander seeds for your other recipes.  The seeds coming from cilantro are the same as coriander.  

I am always fascinated by photoperiodism in plants because I grew up and did all my previous research work in the Philippines where the difference between the shortest day and longest day of the year is only about one hour.  I remember my Plant Breeder-friend had to extend the daylength by providing light to the potatoes so that they would flower.   Without some kind of daylength manipulation some plants grown along the equator will not flower because of the lack of the much needed flowering stimulus - the critical daylength. 

Have you ever been frustrated with some plants in your yard because they are flowering sooner than you expected?

Photoperiodism

Photoperiod is the duration of daylight during a given day.  Photoperiodism is the ability of any organism to detect and respond to changes in daylength.  In plants this phenomenon is expressed mostly in flowering.  Plants are classified into three categories depending on the number of hours of light they require before flower initiation.

Short-Day Plants - plants that flower only when the daylengths are less than 12 hours. 
Common examples of short day plants are poinsettias, chrysanthemum, Christmas cactus, some varieties of strawberries.

Long-Day Plants - plants that flower only when the daylengths are longer than 12 hours.  Among the plants that fall under this classification are radishes, lettuce, spinach, potatoes, sedum, and beets.

Day-Neutral Plants - plants that do not have a particular response to varying daylength.  They flower regardless a long as the rest of the growing conditions are favorable.  Examples of day-neutral plants are tomato, corn, cucumber, and grapes. 

To know the tendencies of the different horticultural crops is important in achieving desired results for the home gardener as well as the commercial grower. 

My backyard observation of photoperiodism.
Last spring I planted cilantro and parsley.  During this period the daylengths were on the rise.  I observed that bolting occurred early on during the growth the plants.  For these plants which are grown particularly for their leaves, bolting or flowering is anti-productive.  Once flowering begins, growth is diverted to the reproductive parts of the plants and the vegetative growth ceases.  I tried to deter flowering by adding more nitrogen fertilizer but that did not work. 

Early this fall, I planted another batch of cilantro and parsley, again.  The same cultural management was applied.  To my surprise, the plants continue to grow lots of leaves, and there is no sign of flowering at all. 

With these observations, I can say that both cilantro and Italian parsley are long-day plants.  They will not flower until the days get longer than 12 hours.  

Commercial Application of Photoperiodism
Photoperiod or daylength can be artificially altered in order to get a certain response from photoperiodic plants.  Poinsettias for example are short-day plants which require 14 hours of continuous darkness for at least one week before they begin to flower in which the leaves change colors.  Under natural conditions, this would happen only in late December.  Why then do we see bright colored poinsettias for sale even before Thanksgiving?  It is because all the poinsettias you see were grown in greenhouses where the lengths of days can be shortened by merely pushing a button.  For the plants in an artificially-lit greenhouse, turning off the lights means that the night has begun.

The cilantro and the parsley that we buy from the grocery store in the summer, on the other hand, are most likely grown in greenhouses where the dark period is long.  Remember, long-day plants flower when the days are long.  Herbs like cilantro and parsley are grown for their leaves.  One can trick these plants to keep producing, by artificially shortening the daylength.