Alders Fix Nitrogen

Smooth alder or hazel alder (Alnus serrulata)
Photos by Georgia Native Plant Society

Hazel alder or smooth alder (Alnus serrulata) is blooming now with its showy yellow catkins. The woody female flower heads look like little pine cones. It's a tall multi-stemmed shrub to small tree in the birch family (Betulaceae).

This alder is native to freshwater and brackish water wetlands from Texas to Maine and into North Florida. It plays an important role in these wetlands because it fixes nitrogen in its root nodules using Frankia, a genus of anaerobic, filamentous bacteria in the Actinomycetes order. 

When soil is wet all or most of the time it is anaerobic (without air) and in these conditions, the normal soil microbes cannot survive and most plants cannot obtain enough nutrients from this soil. The alders' symbiotic relationship with this weird bacteria provides the nutrients they need to survive and they do so without much competition from other plants.

Alder root nodules

I first learned about the alder's root nodules when I was a botany grad student at the University of Maryland. My thesis project was comparing the widespread smooth alder with seaside alder (A. maritima), a fall-blooming alder, which occurs only on Maryland's eastern shore of  the Chesapeake Bay along with the smooth alder, but usually further out in the water, in one spot in north Georgia, and along the Red River in Oklahoma where there was no smooth alder at all.

One thought was that the smooth alder had better, more efficient root nodules. So I dug up roots in Maryland where the two species grew together, brought them into the lab and ran them through a gas chromatograph. I did this 3 or 4 times at different times of the year. There was no discernable difference. 

I also took a trip with 3 other women botanists to Oklahoma. It was combined with a trip to New Orleans for a biological conference. After the conference we hooked up with a botanist at the University of Oklahoma who knew where the alders were growing. We prepared samples and brought back about 20 live plants. We wrapped them in a sleeping bag and tied it to the roof rack of the red Volvo we were traveling in. Every time we stopped for gas, we watered the sleeping bag, which caused quite a stir. Someone called the cops that these 4 crazy women traveling alone in the late 1970s might have a body tied to their car.  Also, I learned that "chicken fried" on the menu in the South is not chicken. 

Seaside alders (Alnus maritima) growing on
the Eastern Shore of the Chesapeake Bay
in Maryland.

Seaside alder (Alnus maritima) has larger and smoother "cones" than the smooth alder.
Weirdly, it blooms in the fall, not the spring.

Digging up alder roots to study the root nodules 
was muddy field work. I often brought my young
children with me.

Seaside alder (Alnus maritima) growing on
the Red River tributaries in Oklahoma.

My fellow grad students and I pressed Oklahoma alders for the Univ. of MD 
herbarium to make sure that these specimens would be available for future study.

Seaside alder plants brought back
from Oklahoma.

The plants survived the trip in the sleeping bag and were planted on the campus. They were also shared with other universities and botanical gardens. A few from the Maryland population were planted the botanic garden in Annapolis, near where I used to live and last time I checked they were huge and had multiplied around the shoreline of a pond there. 

I proposed in my thesis that the Actinomycetes symbionts in the alder roots were bacteria, not fungi, which was the scientific consensus in those days because of their filamentous structure. The professors on my committee strongly disagreed with me on that, but over the years since then, the scientific community now agrees with me. 

I published my findings in a peer-reviewed botanical journal. Several times over the years, botanists who were doing further research on these plants have contacted me. My normal recommendation was that I would meet them at that garden in Annapolis so they could see the plants in person. I know that a least a couple of them also drove over to the Eastern Shore to see them in the wild. I told them where I had found the best populations. 

Thanks for coming with me on this journey back to one of my past lives.

Okay, back to Florida gardening... So, if you have a wet area in North Florida, smooth alders would be an excellent choice. Here is a link to its FNPS plant profile, which includes a FANN link to native plant vendors who have it in stock. 

Also, wax myrtles (Morella cerifera) fix nitrogen with the same class of bacteria, which I mentioned in this article. I also highly recommend this plant for an easy-to-care-for screening plant or as a part of a hedgerow.

Green Gardening Matters,
Ginny Stibolt