Wednesday, May 29, 2013

Dogwood misidentification


Due to yesterday's rainstorms, my plans for the day were cancelled.  During a break in the rain I took a trip to a local park that I had not visited yet this spring for wildflowers.  My first stop in this park is a Shrub Swamp located directly off a paved trail.  Less than 10 foot off the trail I located this clump of dogwood.  There are several species of dogwood located in Mid-Michigan and they can be difficult to identify at times.  I took several pictures and moved on - I assumed the shrub was a Silky Dogwood (Cornus amomum) and expected to confirm this identification once I got back to the office.


Dogwood Shrub

Dogwood leaves and flower

Closeup of leaves and flower


Once I got back to the office I started looking at the photos and pulled out The Shrub Identification Book by George W. D. Symonds (William Morrow & Company, 1963).  This book had become my go to guide for identifying shrubs.  I expected to quickly confirm my initial identification as a Silky Dogwood.  The first thing I looked at was the arrangement of leaves on the twigs - opposite.  This didn't help, Silky, Gray (C. racemosa) and Red-Osier Dogwood (C. sericea) all have opposite leaves.

If the shrub had been bearing fruit it would have been easier to confirm.  Red-Osier and Gray Dogwood have white fruit.  Silky Dogwood has blue fruit with a white blush when ripe.

Red-Osier Dogwood berries and twigs

Silky Dogwood berries, leaves, and twigs


I looked at the flower arrangement next.  The flowers were in a flat-topped white cluster.  Both Silky and Red-Osier Dogwood have flowers arranged this way.  This eliminated Gray Dogwood as a possibility.  Its flowers are arranged in a cone-shaped cluster. 

I looked back at the leaves again.  There were five paired veins on most of the leaves in the pictures (some had 4 veins, others had six).  This did not help either.  Both Silky Dogwood and Red Osier can have 4 to six paired veins on each leaf.  One source that I looked at said that Red-Osier has more prominent veins than Silk Dogwood- this was not much help because I was looking at a single shrub.

I had based my initial identification on twig color.  Some of the twigs were red, others were green.  I knew that Red-Osier usually has deep red twigs.  It did not look right for Red-Osier.  Symonds also said that the twigs of Silky Dogwood are often bright red (or maroon or green).  I was fairly confident that this was Silky Dogwood.

Then I noticed  there was one definitive way to decide between the Silky and Red-Osier species - pith colorRed-Osier has white pith and Silky has brown pith.  Did I happen to notice the pith color?  Of course not, the pith is in the center of the twigs.  I didn't think that I would need to look inside the stem to identify the species.

Back to the swamp!

I went back to the park and decided to collect several samples of twigs, not only from the unknown Dogwood but also from two plants that I knew the species.  The first was a Silky Dogwood.

Silky Dogwood twigs, leaves, and flower buds

Silky Dogwood - closeup of leaves and flower bud

Next was a Red-Osier Dogwood.

Red-Osier Dogwood

Red-Osier Dogwood - closeup of flowers and leaves

I also grabbed a sample from the original plant and took all three back to the office for closer examination.

Three samples - Silky (upper left), Red-Osier (bottom left), unknown (upper right)
First I cut into the known Silky Dogwood and the known Red-Osier Dogwood.  The Silky had a brown pith and the Red-Osier had a white pith both as expected.

Silky Dogwood - note brown pith in twig.  Red-Osier (not shown) has white pith.

What about the unknown sample?

Twig from unknown Dogwood
Note the white pith

It had a white pith!  It was a Red-Osier Dogwood (Cornus sericea).  I was certain up to this point that it was a Silky Dogwood.  It did not look like a Red-Osier, but the pith gave the final verdict.  I learned something new today.

3 comments:

  1. I would not trust pith color alone. Check a modern key eg in Weakley 2015 at UNC

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    1. I can't find the key that you refer to - a link would be helpful. However, Michigan Flora also uses pith color to determine between Red-osier and Silky dogwoods. http://michiganflora.net/genus.aspx?id=Cornus

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  2. I was trying to jog my memory from the time I had spring flora and plant systematics classes with Dr. Ron Kapp at Alma College in mid-80's, and the description of your exercise reminded me that the brown pith is the quick diagnostic for silky dogwood. Thanks for sharing!

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