A Thousand Acres of Silphiums

From one of my favorite birds to one of my favorite plants, the shrub known as Ozark Witch Hazel (Hamamelis vernalis).

I have featured and discussed this plant a number of times on this blog and on Flickr over the years, but there were (and still are) a number of things I did not know and hoped to discover about this fascinating shrub of the Ozark Highlands.  As the species name vernalis suggests, these shrubs flower from January through late March, while their sister species, the more easterly Common Witch Hazel (H. virginiana) blooms in the fall.  There may be some overlap in flowering and hybridization might actually occur in certain circumstances, but I was unable to find anything that suggests how frequently this might take place.

Here is an image I made this year.  Notice the snow and ice in the background.

One of the things I wanted to get to the bottom of, either through direct observation or finding in literature is what are the specifics of pollination for a shrub that blooms in the winter months?  Nearly all of the scholarly articles I could find dealt mostly or exclusively with H. virginiana, but I believe that we can assume that most of these results can be used to describe OWH as well.  There are several things to consider.  The first and perhaps most logical consideration is that the plant may predominantly self pollinate or rely on wind-pollination.  A couple of papers suggest that although self-pollination does occur, self-fertilization does not, making the species self-incompatible, so we can eliminate that option.  The potential for being primarily gravity or wind-pollinated does not make sense when considering that the plants put some considerable resources into making colorful flowers with a noticeable and pleasant odor.  The flowers of OWH have all the classic signs entomophily – brightly colored, nectar producing, fragrant, large, sticky pollen, with male and female structures found in close proximity to one another.

The long strap-like petals of these flowers (see photo below) will unfurl on warmer days and odors increase, thus suggesting the strong likelihood of attracting diurnal pollinators.

Assuming there must be an active pollinator that moves the sticky pollen from plant to plant, what are the potential options for such an insect species in the Ozark Highlands?  An interesting article written by Bernd Heinrich of popular science writing fame gave one possibility, at least for the more eastern H. virginiana.  In this paper he recorded that winter-active moths he was researching used that shrub’s flowers as a food source.  This has been picked up by a number of writers on the internet who have jumped to the conclusion that these moths must be the primary pollinators of the North American Witch Hazels.  However, as these primarily warm-colored (preferred by bees) flowers often roll their petals closed and cease odor production at night, the case for nocturnal moths as pollinators should be considered fortuitous at best, and not a reliable vector for pollination by this plant.

I wanted to visit a good stand of blooming OWH on a warm, sunny day in early February or March and see if I could identify and hopefully photograph a pollinator.  No such luck.  We were able to see a few small midge-like flies and a few native bees surrounding the plants, but photographing one while visiting a flower was to be impossible.  Next time I will bring my handy corn syrup in order to coax pollinators to stay a while longer.  To get an answer, it was off to the literature.  In a study focused on Common Witch Hazel, insects of six orders were identified as visiting flowers.  Of these, flies (order Diptera) were most prevalent and comprised 73% of floral visitors and 52% of the identified species, followed next by Hymenoptera (bees, wasps and ants).

Why flower during colder months?  Advantages and disadvantages can be identified with a little thought.  What are the advantages of an entomophilous plant that opens its doors for business in the colder months?  Not many plants will be flowering in such cold conditions, therefore, on warmer days that allow for flying insects to move about and earn a living, there will be little competition and most pollen collected and moved from flower to flower will be of a compatible, intraspecific origin.  In contrast, insects moving about in mid June might be carrying loads of pollen from a number of species, and if these other species provide a higher quantity or superior quality of nectar and pollen, you might find yourself unable to be cross-pollinated.

This strategy is not as clear-cut of an advantage as it may sound, however.  It has been found that the North American Witch Hazel have a pretty poor flower to fruit ratio – with a less than 1% fruit set on average (the average fruit set in angiosperms is close to 25%).  This makes sense.  If you bloom in the winter then there will be generally fewer pollinators, and in particularly long and harsh winters fruit set can and has been documented to fall to zero in Common Witch Hazel.  One thing the Hamamelis do to assure that a minimum number of successful seed are produced is by increasing the number of flowers.  It has been studied and determined that these plants produce more flowers than similarly sized shrubs.  So, although the success rate of each flower is generally poor, when factoring in the number of flowers produced per plant along with the fact that these plants are usually found in dense stands, the total number of seed produced per season is enough to keep the population sustained.

Another of the fascinating natural history notes of the North American Witch Hazels are their means of seed dispersal.  These plants exhibit what is termed “explosive dehiscence”, similar to another Ozark native – the Jewelweed.  The drying fruit capsules split suddenly, ejecting the seed(s) contained within up to three meters.  In the following photo you can see an empty fruit case some time after it has expelled its seeds.  An observer will notice another interesting characteristic in the photo.  These plants hold onto their spent fruits long into the next season, in this case the plant is in full bloom with its next flowers while still holding onto last years spent fruits.

For now, that is about all I have to say and share about the Hamamelis of the Ozark Highlands.  If you find yourself in a sandy stream-bed within the St Francois Mountains during the first quarter of the calendar year, be sure to keep your senses open and prepared for an unexpected blast of spring.

Please note – other than a few easy observations and a little bit of thought, I produced no original work in the written portion of this post.  If I was worth my carbon, I would have cited the source of each work I used within the text, but this is my blog, so I don’t have to.  Instead, here is a list of works I consulted in writing this.

• Anderson, G.J., & Hill, J.D.  (2002).  Many to flower, few to fruit: The reproductive biology of Hamamelis virginiana (Hamamelidaceae).   American Journal of Botany, 89(1): 67-78.
• Bradford, J.L., & Marsh, D.L. (1977).  Comparative studies of the Witch Hazels Hamamelis virginiana and H. vernalis.  Arkansas Academy of Science Proceedings, 31: 29-31.
• De Steven, D. (1982).  Seed production and seed mortality in a temperate forest shrub (Witch-Hazel, Hamamelis virginiana).  Journal of Ecology, 70: 437-443.
• Gapinski, A. (2014). Hamamelidaceae, Part 1: Exploring the witch-hazels of the Arnold Arboretum.  Arnoldia, 72(2): 2-17.
• Yatskievych, G. (2013).  SteyerMark’s Flora of Missouri Volume 3 – Revised Edition.