blooming pattern: another variable?

Jim McKenney
Sat, 05 Nov 2005 06:44:26 PST
In much of the world, the day we're about to have today would be a summer
day. It's November 5th, yet we have not yet had a killing frost. The garden
is still full of impatiens, scarlet sage, coleus, cuphea and other tender
annuals. The temperature is predicted to be in the mid-70s F today and
thunderstorms are predicted for tomorrow. This in not so-called Indian
Summer; that comes after a killing freeze. This is an extension of summer
into fall, and some might see it as a portent of the Mediterraneanization
(choke on that, spell check!) of our climate.  

And I've got loads of things to do in the garden. So I won't be tricked into
sitting here at the keyboard all day.

But I'm finding this topic of fall blooming bulbs fascinating. 

I my earlier post I largely ignored the role of pollinators. But the more I
think about it, the more important the role of pollinators seems to be. 

Harold pointed out two advantages of autumn bloom: less competition for
pollinators and a reduced likelihood of rain damaging the pollen. These
factors would indeed prove advantageous to plants which are already by
nature autumn flowering. Alberto pointed out that autumn flowering positions
the ripening seed to take advantage of the long growing season ahead. Again,
this undoubted advantage would work to favor plants which are already by
nature autumn blooming. But contrast that with the situation in Californian
themdiaceous plants which Mary Sue pointed out. 

Paul focused on the significance of the seasonal activity of pollinators,
and really hit the nail on the head: his explanation accounts for the very
existence of seasonal flowering in some plants.

Paul's explanation makes sense in a climate where pollinators and plants are
active throughout a long season. Paul said that if an ancestral plant
population were active all year, but its pollinator came to be active only
part of the year, eventually the plant flowering season would shift to
reflect the period of activity of the pollinator. If the pollinator's major
season of activity were autumn, then you would get autumn blooming plants. 

Pollinators are probably the major cause of fall blooming plants.

Let's take it a step further. Paul's scenario would account for the
emergence of a seasonal flowering flora anywhere. I'm particularly
interested in what happens in areas where relatively mild winter areas
adjoin areas with severe winters. The Mediterranean basin is an example of
such an area. What makes the flora of this area interesting is that it
includes taxa with members which are fall blooming and late-winter and
spring blooming, and taxa which are fall blooming and have obvious relatives
which are spring blooming. 

Because plants which bloom in the spring provide pollen and nutriment to
pollinators, it makes sense that the competition of pollinators for the
earliest pollen sources would cause the plant populations they pollinate to
become progressively earlier in their flowering time. The pollinators select
plants for early bloom, and they thus cause plant populations to bloom

In areas with severe winter cold, this process of selection for early bloom
can only go so far: the plants will not be selected for bloom any earlier
than the pollinators can be active. If the pollinators are not active during
the winter, then the plants are not selected for winter bloom. 

But in areas with comparatively mild winters, the opportunity exists to
allow pollinator activity all winter. As a result, an ancestral
spring-blooming stock could be selected for progressively earlier and
earlier bloom until the plants in question bloom earlier and earlier in the
spring, earlier yet into the winter and finally earlier yet into the fall.
It's in this sense that pollinators can cause plants to become fall

In a climate with harsh summer conditions, this process of earlier and
earlier bloom would stop at the autumn blooming phase (if the pollinator is
not active during the harsh summer, then there would be no selective
pressures to continue this process even earlier into summer).

No one thinks it's unusual when asters or chrysanthemums bloom in the fall.
Maybe the reason this does not seem unusual is that the plants themselves
have been in place growing all spring and summer in the time leading up to

With our autumn blooming bulbs, one of the things which makes their bloom so
remarkable is that it seems to come from nowhere. They just suddenly pop up
out of the ground. The other thing which makes it remarkable is that these
autumn blooming bulbs have obvious relatives which bloom in the spring.  

Many bulbous taxa seem to have been pushed to three extremes: fall blooming
at the beginning of a fall-winter-spring growing cycle, late-winter/early
spring bloom in areas with severe winters,  and summer blooming at the end
of the growing season (the themidaceous/Calochortus cycle) in some dry
summer areas. Each provides advantages to the taxa in question: reduced
competition for pollinators, specialized pollinator relationships and
enhanced seedling survival. 

Jim McKenney
Montgomery County, Maryland, USA, USDA zone 7, where I'm off to plant lilies
and peonies and to forget about prophylls and themidaceous things (that word
themidaceous would have drawn a total blank from me two years ago).   

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