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.

Fallen Stars: Effects of High Temperature on Pomegranate Fruit Set

Fig. 1     Pomegranate (Punica granatum L. 'Wonderful') flowers on the ground.

A star is a promise of illumination - like a flower is an early promise of a fruit to come. This season, my pomegranate tree (Punica granatum L. 'Wonderful') growing in a large pot is producing numerous bright colored flowers.  However, this week the flowers are dropping like falling stars (Fig. 1) at a very high rate.   The only factor that I can attribute this to is the sudden rise in temperature - from the 70s to high 90s (F).

Fig. 2     Pomegranate at an early stage.

Temperature and humidity both play an important role in fruitfulness of orchard trees.  As a general rule, lower temperatures - without the frost - is most conducive to better flower initiation, successful pollination, and fruit set.  On the other hand, high temperatures (90+ degrees F) and low humidity can result in the abscission of flower buds and fruitlets (Fig. 1) - leading to unfruitfulness.  In our area, winter and early spring temperatures may be favorable but summer temperatures can be detrimental - hastening the deterioration of the embryo sac and the premature senescence of the ovules. 

Based on last year's observations,  Pomegranate 'Wonderful' blooms continuously from spring to fall (Fig.3).  Some of the earlier flowers managed to set already (Fig. 2).  I am assuming that there are more fruits yet to set when the temperatures relent once more.  But we'll see...after all, this is only the second blooming year for this tree.

 

Fig. 3    Pomegranate (Punica granatum 'Wonderful') in bloom.

 

High temperatures inhibit fruit set in a number of plants.  The reason for this was demonstrated in a study on flower development and fruit set in apricot (Prunus armeniaca).  Results showed that warmer temperatures hastened that external development of flowers without significant advances in pistil development.  For fertilization to take place, the pollen and the pistil need to be synchronized.  Under warm temperatures the flower can unfurl its petals that attract pollinators but the ovules are immature.  I hate to use this analogy but it is like a girl acting like a grown-up prior to puberty.  In fruit trees, this situation does not end up in a successful fruit set.

Abscission of flowers and fruitlets is a natural mechanism for the plant to retain enough fruits that is equivalent to its capability to support under the existing environmental conditions.

Beating the Heat

Daylily (Hemerocallis L.)

Yesterday

Maximum Temp:  102 ºF 

Minimum  Temp:    66 ºF

Realtive Humidity:   20%

Today

Maximum Temp: 99 ºF

Minimum Temp:  64 ºF

Relative Humidity: 17%

It is that time of the year again when being in the garden is almost like being in a furnace. 

Plants love the sun.  Plants are plants because of the sun and the heat energy they derive from it.  To most plants summer is heaven.  In addition to the abundance of sunshine, temperature also increases the rate of photosynthesis or production.  That's what plants love to do.  But anything good especially in excess always comes with a price.  Intense sunlight which is accompanied by high temperatures jeopardizes the availability of water to the plant.

Ground water dissipates from the ground through evaporation while absorbed water escapes the plants through transpiration at a high rate.  Ideally, natural weather conditions would provide replacement of lost water through precipitation.  My garden is not so ideally located when it comes to rainfall.  Affected by the Mediterranean climate, California's subtropical environment is characterized by a dry summer.   The example of weather condition above shows how dry it is here by the relative humidity of lower than 20%.  Gardening here requires some techniques to beat the heat during the dry months.

Adjust watering schedule. 

1.  Water less frequently but deeply to encourage the roots to grow deeper. 

2.  Water during the cooler times of the day/night to avoid loss by evaporation.

3.  Drip irrigate to avoid runoff especially where the soil is on a slope.

4.  Check irrigation pipes for leaks and breakage.

Mulch.

Mulching is the simply the process of spreading a protective layer of material on top of the soil.  It is an effective way to conserve soil moisture, reduce soil temperature, and prevent weed growth.  There are several materials available at your local nursery or home improvement center.  When using organic materials (those that are derived from plant and animal by-products) mulching eventually improves the water-holding capacity of the soil.

Use what is available and free.  In my garden I use dried grass clippings for mulching instead of throwing them away.  It has to be dry before they are used to mulch shallow rooted plants but you can directly use fresh clippings to mulch around trees or in opens spaces.   Coffee grounds also make a good mulch material; I use them for mulching potted plants.  Coffee shops such as Starbucks and Peets Coffee offer coffee grounds for free.   Sometimes they already in bags ready for anyone interested. 

Protect potted plants. 

Potted plants are "caged" and they have limited space to hunt for resources particularly water.  During the hot months of summer, it is best to move them to a place where they can get some shade.  This will reduce water loss through excessive transpiration and evaporation. 

And then fertilize...

Frequent watering depletes the soil from the essential nutrients especially those in pots.  Applying fertilizer on a regular basis benefits the plants.  It is not enough to provide water for the plants.  The nutrients they need have to be available also for continued growth and development.  The potted orange trees in my yard are great examples. After a couple of months into the warmer months, the leaves show signs of yellowing which is a symptom of nitrogen deficiency.  A little bit of complete fertilizer quickly restores the nice green coloration of the leaves. 

Summer gardening is fun as long as we beat the effects of high temperatures.

Peach Leaf Curl

Distorted peach leaf

The Disease.  Leaf curl is a fungal disease that affects peaches.  It is caused by a pathogen called Taphrina deformans.  I guess that the word 'deformans' reflects the nature of damage on the plant instead of the fungus itself. Infected areas of the leaves appear thickened and reddish in color.  They turn into distorted or malformed leaves.

In My Garden.  Every year, this disease shows up on my one and only peach tree.  Right after the first set of leaves appear, infection follows.  When the leaves are all deformed and discolored, the tree seems hopeless and not fit for any serious photosynthesis.  Fortunately, the disease can only survive under cooler and spread with the aid of the rain.   As the temperatures rise, the infected leaves fall off.  In some previous years I also helped the tree by had-picking the infected leaves.  This reduces the chance of the disease spreading on new leaves and thus shortening the overall "leaf curl period".

Currently, the temperature here is about 58º F and raining.  The weather still favors the development of the disease.  However, I noticed that most of the infected leaves have fallen off the tree and the proportion of healthy leaves is now more than the red-distorted ones.

In Your Garden.   If you have this problem on your peach tree, don't despair.  Read and understand the life cycle of Peach Leaf Curl.   Acquaint yourself with your climatic condition in relation to the requirements of the pathogen.  Although the disease can affect the yield and growth of your tree, the potential extent of damage is controlled by nature itself with some preventive measures from you.

Stroll in your garden...get acquainted with your unseen enemies.  For it is much easier to subdue them when you know their secret life.

Color Change in Citrus

Fruits on pot-grown oranges (Citrus sinensis)

Tropical Zone.  Citrus fruits like oranges and lemons start out green and remain in that color till maturity.  I remember, having grown up in the tropics, that all the citrus fruits I've seen were green.  When they are mature and harvested their color are still green different only from their immature counterparts by the shine and gloss on the rind.  By the time they turn orange or yellow, they are already over-ripe and cottony and tasteless.  Commercially grown citrus can be treated with ethylene after harvest to induce uniform and timely yellowing for a more appealing look. 

Temperate Zone.  Citrus plants grown in temperate places with mediterrenian-like climate, low temperatures induce a change in color to deep orange or yellow even before the fruits mature.   The color can be deceptive.  It is important to harvest the fruits when they are fully mature, it is when the sugar and the acidity balance is at its best. 

Lemon (Citrus limonia)

Hidden from our eyes are processes that are constantly happening in the plants.  Shifts in these processes are caused by a number of environmental factors.  The green color in unripe citrus plants is a result of the dominant presence of chlorophyll on the rinds.  The yellow or orange color in ripe citrus fruits is a manifestation of the high concentration of carotenoids.  When temperatures go down to a certain critical level, such as in the winter, the rate of chlorophyll production in the plant slows down or completely stops.     Furthermore, the existing chlorophyll undergoes a process of degradation while the biosynthesis of carotenoids increases, particularly in the fruits.

Stroll in your garden... its good for you and your plants.