Tom's Blog

Saturday, August 2, 2014

Why aren't compass plants flowering this year?

Black Earth Rettenmund Prairie is a great site for compass plant (Silphium laciniatum), but this year only about 10% of the plants are flowering. Why? Even though they are non-flowering, compass plants have nice fresh foliage, indicating that they are alive and well. See update below

I have been unable to find any work in the literature on this phenomenon in S. laciniatum, although there is work with other species. In some species, all (or virtually all) of a single species in a population will flower, and none at all the next year. In some cases, this cycle will be repeated again, and perhaps again and again. So the question is: how do plants know when other plants (of the same species) are flowering?

At Rettenmund, this is the first year I have noticed that flowering has not occurred. Certainly, for the past four or five years we have had good flowering. Why this year?

Note added 8-11-2014. I just learned yesterday from Mark Martin at Goose Pond Sanctuary that their compass plants are not flowering this year either. This makes me think the phenomenon must be weather-related.

The compass plant consists of two parts, the greatly dissected basal leaves, and the thick, robust flower stalk that often grows quite tall and carries the flowers. My information is that the flower stalk arises (separately from the leaves) from the thick basal crown. Thus whether or not a plant will flower depends upon differentiation of flower primordia in the crown. Although I have found no data on this, I assume that the trigger which starts flower stalk differentiation occurs the previous fall. If a stalk primordium does not develop, then no flowering occurs.

A single plant can form more than one flower stalk, although they are all connected underground via the root stock and crown. See photo below (The crown must be quite large.)

A multi-stemmed (9) compass plant in the middle of Toby's Prairie. Note that all the significant foliage is basal.
Although very little flowering is occurring at Rettenmund, we have normal compass plant flowering at Pleasant Valley Conservancy. Thus, it seems unlikely that we can blame this on bad fall weather, or bad winter weather, or bad spring weather, since Rettenmund and Pleasant Valley are only 4 miles apart.

At the moment, all I can conclude is that compass plants at Rettenmund are having an "off" year. We'll have to wait to see what next year develops.


Blogger Peter said...

Are they forming a stalk without flowers, or not getting a stalk at all?

August 3, 2014 at 5:26 PM  

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