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.

Beyond Fern Fronds

Fig. 1   Fern fronds

A fern that has a flower is not a true fern and anything that resembles a flower on a fern is not a true flower. Ferns are not flowering plants but they are capable of reproduction through spores. The purpose of this article is to point out where this flowerless-reproduction takes place.

New fronds on ferns start out plain green (Fig. 1).  While some fronds are sterile, some are fertile for reproduction which can be differentiated by inspecting the under-side of the fronds as they begin to develop sori - the reproductive structure in ferns (Fig. 2).   

The sori  start out as rows of two dimentional dots (Fig. 2) and as they mature they turn increasingly three-dimensional resulting in an embossed appearance on the fronds. At this point, the sori are enclosed in a thin flap of tissue called indusium.  

Fig. 2.  Sori (singular: sorus) on the underside of  fertile fern fronds.

The indusium at first covers, the entire sorus (Fig.2).  However, as the sori mature, the indusia slowly recede to unveil the growing sporangia (Fig. 3) - each sorus containing clusters of sporangia. Eventually, the and finally detaches from the sorus leaving a velvety house of spores (Fig.4). 

Fig. 3   Indusium slowly revealing the sporangia as they mature.

Sporangium  (Fig. 4) is a receptacle in which asexual spores are developed.  Ferns are flowerless and seedless plants - therefore, they do not undergo sexual fertilization where a male cell unites with a female cell to form a new embryo.  Instead, they have sporangia that are like tiny packages containing multitudes of spores - all potentially capable of cell division to produce a multi-cellular organism - a new plant.  Minute as they are, spores are designed to survive and dispersed - they remain viable for a long time until favorable conditions are met.

Fig. 4    Sporangia all ready for dispersal.

Now that we know about fertile fern fronds that give rise to aggregates of sori that hold the sporangia which are the vessels that contain the spores, we can say that the fronds are not ordinary leaves.  Unlike the leaves of flowering plants which function mainly as sites for photosynthesis, the fern fronds perform dual functions - photosynthetic and reproductive. 

Hibernaculum

Brown garden snails (Cantareus aspersus/Cornu aspersum)  in a hibernaculum - Irvine, California.

Hi.ber.nac.u.lum

Origin:  Latin word hibernare
Literal meaning:   winter residence
 Practical meaning:  a protected space that animals use as a shelter
for hybernation during prolonged periods of inclement weather.

Gardeners, what do you do when you see a population of snails in a hibernaculum?  

Garlic Chives (Kutchay)

Garlic Chives (Allium tuberosum)

As of yesterday, this is how my garlic chives looked like.  They were planted maybe three or four years ago still in the same pot.  They are ready to be divided but I have been delinquent.  Anyway, it looks very good at the moment.  

Garlic chives are different from the common chives (Allium schoenoprasum) by appearance and flavor.  Sometimes they are referred to as Chinese chives or flat chives but in Ilocano (my dialect) they are called Kutchay.   Unlike the chives, kutchay has a strong garlicky flavor which is an important material in my partly-Asian kitchen.  It is good in fried rice, omelets, soups, meats, and noodles.  The leaves as well as the young flowers make a good substitute for leeks, green onions, and sometimes cooked by it self.

Garlic chive is a perennial plant that is easy to grow.  I prefer to start with seedlings purchased local nurseries. Once established, it will continue to spread and comes back year after year.  In fact it is a self-seeding plant thus can become a weed if flowers are not cut off before the seeds get dispersed naturally.  I like to harvest them as needed starting from one end of the row or one side of the pot and moving on.  This way there will always be new leaves to use in the kitchen since regular cutting or harvesting encourages new growth and even spreading.  To harvest, cut the leaves 1-2 inches from the base. 

Annual application of compost early in spring will provide the needed nutrients.  When grown in pots, apply complete or nitrogen fertilizer at a very low rate to replace the nutrients that get washed off during watering. 

Divide and replant when the plants get to crowded (maybe every three to five years). 

Important definition:

Self-seeding plants. (n.) plants with seeds that spread their seeds naturally and without special attention given, the seeds will grow.

Start a kitchen garden...plant herbs.

Mint in the Garden

Peppermint (Mentha pepireta)

Mint must be one of the easiest herbs to grow.  As a perennial plant, mint does not have to be replaced every season.  In fact depending on your location it remains green throughout the year.  I grow mint for adding to drinks, making cucumber sandwiches, yogurt sauce for Moroccan Lamb Kabobs, and garnishing.  

Some Common Mint species:
Peppermint (Mentha piperita) has dark green leaves with a reddish stem and lavender flowers.

Chocolate Mint is a peppermint cultivar which had bronze-green leaves. 

Spearmint (Mentha spicata) has lighter green, pointy leaves and pink flowers.
Apple Mint (Mentha suaveolens) has light green foliage.
Pineapple Mint (Mentha suaveolens 'Variegata’) has green leaves that are banded with white.

It has been my observation that mint undergoes two distinct growth phases in a given year. Understanding the events that happen during these phases helps a gardener grow mint the proper way.

1.  The first phase is the above-ground vegetative growth which happens in early spring (see above picture). The stems will then prepare to flower in the summer.  Frequent cutting will encourage branching and delay flowering. As the plant flowers, the vegetative growth comes to a halt to give way to reproductive development.

2.  After flowering, stolons, which are modified stems, begin to grow laterally underground covering a lot of distance in a very short period of time.  Eventually, these stolons will emerge from the ground and start to grow the next set of stems covering a much larger area.  This is the reason why mint need to be contained.  It is invasive and can become a weed.

Clone your mint
Dig into the soil at the base of your mint plant and look for stolons.   Pull it out and cut into short sections making sure that there are at least three nodes in each section.  I prefer the nodes that already have little buds and roots on them.  Then plant the sections horizontally and cover with soil.  Keep it watered but not too wet.  In no time you will see new green leaves emerge. 

Important definitions:

Propagate  (v.) to clone or grow a new plant from a vegetative part of an existing plant.

Stolon  (n.) a modified stem growing underground.  Distinguishable from the roots by the following: larger in diameter; white color; presence of nodes.

What is Horticulture

The term horticulture comes from two Latin words namely:  hortus (garden) and colere (to cultivate or tend).  Simply horticulture means "tending a garden" or just gardening. 

As a field of study, horticulture is the science, art, and business of growing and marketing fruits, vegetables, flowers, and ornamental plants.  The crops studied in horticulture are those that require more intense and constant care.  These include but not limited to ornamental plants, cash crops and hobby plants.  Therefore a crop is considered horticultural depending on the purpose that they are grown. For example, wheat (Triticum aestivum L.)  is a low-input crop and is traditionally grown for their grains on big acres of fields - it is considered an agronomic crop.  When wheat is grown as wheatgrass for their wheat juice (like the green stuff you get from Jamba Juice) then it is considered  a horticultural crop. 

Horticulture involves two main types of plants:  Plants that are grown for food are called edibles while plants grown for aesthetics are ornamentals.  There are several number of sciences under these two categories which I will discuss as we deal with them in the future.