Fig. 1:  Tripped flowers on a raceme.  What do they really mean?

          If you want to try to measure the amount of pollination that is taking place in an alfalfa field, it is natural to plan to count numbers of open flowers, tripped flowers, and perhaps buds and wilted flowers on a sample of racemes.  Here is something to think about if you plan to make these measurements.

         Your counts will be a snapshot in time of the condition of the raceme.  Counts per se give you no information about the rate at which flowers are opening or being tripped.  Those two rates may be very different.  

Flowers that are not tripped will remain open for about a week.  Flowers that are tripped wilt within an hour or so.  Temperature plays a role in these rates, but the difference is great.  

So consider, the number of tripped flowers at a given moment in time tells you how many flowers were tripped in the past hour or so.  If they were tripped before then, they are wilted by now.  On the other hand, the number of open flowers tells you how many flowers opened sometime in the past week, but were not tripped. 

Because tripped flowers measures such a short period of time, it's important to know the rate at which flowers opened in that time period to judge whether pollination is occurring rapidly or slowly.    

One way to know this would be to make counts throughout the day after similar periods of time, say every 2 or 3 hours.  Count and mark the flowers on a raceme that opened, and flowers that were tripped, in each time period .   Or perhaps you could prepare racemes before a measurement period so that all except buds are removed.  Then wait 2 hours, and count the number of open, tripped and wilted flowers. Temperature, time of day, and variety are likely to affect the rate of flower opening.

Fig. 3 - Flower development.  The phenology of alfalfa flowering, starting with buds (bottom) through open flower, wilted flower, and seed pod development.  Bottom right is a raceme with all flowers removed; pedicels along the raceme indicate where a flower was attached.  Top right is a raceme with maturing curls. 

The rate of development in each flower stage is different.  Unless you know the rates, it is difficult to interpret the meaning of the numbers of each stage at a moment in time.

If however, you count open flowers at the end of the day, you are counting flowers at a time when no more flowers are opening, and no more pollination is taking place.  You are seeing what is left over after a of a day of pollination.  Hence, counts made at the end of the day tell you something about how well the pollination has been going that day.  (But they don't quantify the amount of pollination or the amount of bloom that day).  Don't expect to see tripped flowers once the bees have stopped foraging.  Flowers that are still open will be available for the bees first thing in the morning.  If pollination is heavy relative to the amount of bloom, only buds and wilted flowers will be left on the raceme. 

I hope that this helps alfalfa seed growers and researchers understand why sampling in the evening works. It should also help researchers of alfalfa pollination design experiments.

How many bees should I introduce to my field? A rule of thumb.

Alfalfa is an indeterminate bloomer, like many crops.  That means that it continues to bloom as long as conditions are appropriate and the fruits do not mature.  Read about the implications of indeterminate blooming in my essay Squash, Beans, and Deadheading Flowers.