Why is a Flower Bed Like a Pandemic?

 

Pink Evening Primrose (Oenothera speciosa) flower.
Rowan County, North Carolina.

We have a small flower bed at the corner of our garage.  It is entirely enclosed by the driveway, a curving sidewalk and the garage.  In fall we plant pansies (Viola sp.) that flower through the winter and spring.  In spring, Pink Evening Primrose, Oenothera speciosa puts on a flowering show.  Early summer brings Bee Balm (Monarda didyma) with its purple flowers and the red, pink and white blooms of the exotic Impatiens balsamina.   So, this small flower bed blooms throughout the year.

 

Our flower bed with Pink Evening Primrose and Pansies. in flower.
Rowan County, North Carolina.

  

Time lapse video of Primrose flowers, mid-April to mid-June

Rowan County, North Carolina. 

I have always been impressed at how the number of Pink Evening Primrose flowers increase, reach a peak then fall off.  This spring I counted the Primrose flowers and photographed the bed nearly every day for almost two months.  By the middle of June, the blooms were tailing off and I graphed the number of flowers for each day. 

 

Graph of Pink Evening Primrose flowers per day 
from mid-April to mid-June.

The graph shows a rapid increase in the number of flowers with the peak flower number coming in the middle of May.  Around the peak, more than 300 flowers were open each day in this small bed.  The number of flowers started a gradual decrease as we got into June.  Flower numbers fluctuated from day-to-day after the peak. This was probably because of the natural variation in flower bud production by the Primrose and may have also been influenced by environmental factors like day length and rain fall.

I was struck by how this graph of flower numbers over time resembled the pattern of cases in an epidemic.  When a new disease enters a human population the number of cases rapidly increases.  The rate of increase depends on many factors like transmissibility of the infectious agent, the density of the human population and preventative measures. The numbers eventually reach a maximum then decline. 

Idealized graph of an epidemic.  
https://www2.palomar.edu/anthro/medical/med_3.htm

The ongoing COVID-19 tragedy is a more complicated situation.  A pandemic is a world-wide, long-term epidemic.  COVID-19 started in China in late 2019 and reached the United States and many other countries in the winter of 2020.  By March 2020, US case numbers began to increase.  The case numbers peaked in April and began a gradual decline as preventative measures like closing locations where people congregate, working from home and mask wearing was required.  During the summer of 2020 cases began to increase again as people traveled for business and pleasure.  The numbers began to decline again in late summer but as the weather cooled, people gathered inside and traveled for the holidays.  This caused another spike in the number of COVID-19 cases that reached their peak in January 2021.  Two vaccines were released to the American public in early 2021 and the number of COVID-19 cases began to rapidly decline.  This decrease has continued into the summer of 2021.

Number of COVID-19 cases in the United States, February 2020-June 2021.
New York Times. 

Both Primrose flowers and COVID-19 cases showed a similar pattern. Pink Evening Primroses in the flower bed next to our garage had a rapid increase in number, peaked, then as the days got longer flower production tapered off.  The United States population has suffered through several peaks of COVID-19.  Millions were sickened, hundreds of thousands died and while the virus is still circulating, it is now on the decline.  This welcome decrease in cases was caused by a massive public health response, natural immunity in recovered COVID-19 patients and the vaccination of millions of susceptible people. In many countries the pandemic continues to rage as their people await effective public health responses.  

So, why is a flower bed like a pandemic?  Well, it is not, except in a superficial way.  The dynamics of biological systems allow for exponential growth.  At some point, the growth is limited by intrinsic or environmental factors and the numbers decline. The real connection is that both the virus and the flower are governed by similar biological laws.  

Flying Seeds

 

Goat's Beard (Tragopogon dubius) head with many fruits. 

These flying fruits disperse seeds over long distances.  

Rowan County, North Carolina. 

 

Plants disperse their seeds in many ways. Seed dispersal is vital for plants to colonize new areas and so new plants are not in direct competition with their parent.  In flowering plants, the seed is part of the fruit, and the main function of the fruit is dispersal of seeds. Some seeds are wrapped in a tasty fruit and are carried away by animals and deposited in new locations.  Other fruits are hitchhikers that cling to the fur of animals or the bootlaces of unsuspecting hikers.  Still others have buoyant fruits and float away on ocean currents.  And some fruits fly.  

Fruits of Box Elder (Acer negundo). Like all
maples, Box Elder makes winged fruits called samaras. 
Rowan County, North Carolina. 

A single samara of Box Elder.  The seed is at 
the bottom and the wing is at the top of the picture.
Rowan County, North Carolina. 

Slow-motion video of Red Maple (Acer rubrum) flying samaras.

Rowan County, North Carolina. 

Flying seeds have two main strategies for dispersal, wings or parachutes.  Maples (Acer sp.) make a fruit called a samara with the seed at the end of a single, asymmetrical wing. When the fruit falls from a maple tree it helicopters down and on a breezy day it can travel long distances.  The wings of maple samaras are airfoils like those of airplanes.  The wing shape produces lift and allows the seed to remain aloft for a long time.

Winged seed of Loblolly Pine (Pinus taeda).
Rowan County, North Carolina. 

Pines (Pinus sp.) do not make fruits since they are Gymnosperms.  Gymnosperms have naked seeds borne on cones and are not part of fruits.  Pine seeds are very similar in appearance to maple fruits and fly in a similar manner.  Ash trees (Fraxinus sp.) also make samaras.  Ash fruits have a seed and an airfoil wing and they too are good flyers.  Long distance dispersal means these trees are often the first to colonize new open areas.  

Samaras of American Ash (Fraxinus americana).
Rowan County, North Carolina. 

Close-up of an American Ash samara.
Rowan County, North Carolina. 

Parachute fruits are made by some members of the sunflower family, the Asteraceae.  The best known of these is the Common Dandelion (Taraxicum officinale).  Everyone has picked a fruiting head of Dandelion and blown the fruits into the wind.  Dandelions are natives of Eurasia and have spread themselves across North America by means of these flying seeds.  Another member of the sunflower family that makes these flying fruits is Goat’s Beard (Tragopogon dubius).  This introduction from Europe is much taller than the Common Dandelion and makes large fruiting heads with abundant flying seeds.  The fruits of both these plants are called achenes.  They have the seed at the bottom, a stalk called the beak and fine cellulose fibers at the top, the pappus.  The pappus acts as the parachute and allows the fruit to travel long distances. 

Fruiting head of Common Dandelion (Taraxicum officinale).  Each achene
is made up of a seed, beak and pappus.
Rowan County, North Carolina.

 

Close up of a Common Dandelion achene.  The seed is at the bottom,
the pappus is at the top and they are connected by the beak.  
Rowan County, North Carolina. 

Bull Thistle (Cirsium vulgare) is yet another Asteraceae with parachuting fruits.  Bull Thistle grows to several feet in height, has spiky leaves and makes purple flowers.  Unlike Common Dandelion, Bull Thistle fruits lack a beak but fly just as well. 

Bull Thistle (Cirsium vulgare) leaves.
Rowan County, North Carolina.

Bull Thistle flower.
Rowan County, North Carolina.

Bull Thistle achene being released from a fruiting head.
Rowan County, North Carolina. 

Video of Bull Thistle achenes flying from a mature flower.

Rowan County, North Carolina.

On windy spring days the air can be filled with flying maple samaras or a summer walk through a meadow can bring clouds of parachute borne seeds of Dandelions.  All these seeds are trying for the same thing.  A fresh start for the plant in a new place.