The Inbetweeners: Hybrid American Chestnuts

When I first started my nursery, I was always trying to find new seed sources to grow trees. A lot of my early sources were people known to me by reading Organic Gardening Magazine and Mother Earth News. I read these periodicals at the library in the magazine section.  (I’m old.)  One of these was a source for American hybrid chestnut seed called the ‘Douglas’ hybrids. This was a seedling discovery found by late Earl Douglas in New York who had planted the Manchurian and American chestnuts within wind pollinating distance of each other. He soon noticed the in-between progeny of the Manchurian chestnut and American chestnut and then let them fruit. His American trees eventually died, and his hybrid trees continued onward showing resistance to chestnut blight. Chestnut blight is not controllable. Nothing is left standing using the all-American chestnut, so I was very excited to find this seed source. Here is his pamphlet reproduced here.

For many years like clockwork, I grew the Douglas hybrid chestnuts all from this source along with two cultivars called “1” and “1A” that were a generation or two away from the originals.  I found a few other seed sources that people discovered called the “Simpson” and “Gellatly” hybrids. These were a great blend of three species including the European chestnut. (See the above picture.)  I was able to create a thousand or so hybrid trees every year for my nursery to sell to the public. Eventually I made selections from the seed beds to plant in my outback as I continued selling the hybrid seedlings in the nursery trade. I had the idea of creating a seed orchard of hybrid chestnut hoping for the ‘new and improved’ version to grow at my farm. I chose trees based on fast growth and good branch structure with strong central leaders. The sales of the in-betweeners were a challenge, but it was consistent, putting it in the top ten plants sold at my farm. People did wonder, “Is it one thing or another?” and “Does it matter if they are all mixed up?”  Today it is a different retail environment, and the hybrid is understood and desired as many other seed strains are available for cultivation.  Along with that is also the pure and illustrious thinking of the holy species where people shun the crosses because they have a species bias. This falls into the native only camp. Each species cross is different. Most are for nut production only and use specific Chinese cultivars. At that time I wanted a taller timber tree much like the American chestnut free of blight. Others who wanted this same outcome took it to extremes and created non-profit organizations and companies devoted to it. I did not have the resources for that so I did what is done in the oak world and created hybrid swarms. In this scenario, you keep the healthiest plants with the best forms. Nut production is not the priority but if it comes along with it, that is good.

The mini repository of all things Castanea allowed me to see this great diversity with the in-between hybrid populations. It was not an American look alike contest. I was focused on a more timber like chestnut like the American but with a healthy vascular system like the Chinese.  I found many unique attributes to growing all of these inbetweeners. There were plants in my seed beds barely two months old with seed burs on top of the plant as well as highly vigorous 6-foot-tall trees all in one year’s growth. One tree as it matured produced an average of 9 nuts per burr. This wide variation was a hint of the very fluid nature of the hybrid chestnuts. What I didn’t understand at the time was the remarkable chestnut blight disease and its great destructive power. Because it was not found on my farm due to my treeless isolation, I was living a dream chestnut life.  Eventually the blight blew in and found the perfect host tree: my hybrid American chestnuts. Trying to navigate it or prevent it is impossible.  I gave up. I was completely overwhelmed. The whole planting was affected to various degrees. Most of the mature trees died within a 2-4 year time frame.

This super laden hybrid American chestnut pictured below with artichokes in the foreground was a causality. After this massive crop, it quietly returned to the soil.

I began harvesting the wood and had much of it milled into fine lumber.  I tried making selections. The problem was I just couldn’t tell if my choices were good or bad in terms of blight resistance, so I let everything go and encouraged natural seeding from both the new and old trees in the plantings. When the tree reached 1-2 inch in caliper then the seedling trees would immediately show the damage or lack thereof.

I didn’t mow or remove brush or fallen limbs. A lot of blackberries grew in those locations as the grass faded away.  Soon you could see seedlings of apples, pears, cornelian cherries, walnuts, multiflora rose and shellbark hickories seeding in these areas with the fading chestnuts. Once I did that, the solution came forward in ever greater numbers of strong growing, highly resistant trees with very vigorous strong timber like growth. Some of the trees that are now gracing my farm are the result of this population expansion into the hybrid swarm that produces an intermediate “species” . Some natural crosses proved very successful. One of these was the Korean chestnut, Castanea crenata crossed with the Douglas hybrids along with the European hybrid crosses. This resistance becomes apparent in successive generations and skips through the normal long wait time needed to see what the negative effects of the disease are. The disease becomes weaker with time and trees stronger. The blight gets a weaker form of blight, and the trees find a way around it through callus formation. The callus formation makes it possible for the trees to continue their life into maturity and fruit continuously even after infection.   

The inbetweeners become the progeny of little fluctuation. It’s a whirled-up species blending and not one or the other. The population is at the point of least excitation and can move forward without the massive causalities of yesteryear. Weak trees fade quickly now and new trees remain relatively free of blight to the point it becomes a minor cosmetic appearance.  The disease is still there as it prods and shapes the new tree species. The inbetweener has left its original form and is now something new.

Pollarding the trunk for use in wood working and timber production is a great agroforestry goal. Coppicing is an option yet each of the hybrid chestnuts used for this purpose was highly variable both in the production of sprouts and the resistance to blight. Some selections got worse over time eventually becoming completely unusable. Here is an excellent one defined by strong sprouting and resistance to blight. This seedling is the best one at my farm for pollarding because it combines fast growth, numerous sprouts at the base of the tree and high resistance to chestnut blight including the new sprouts. This seedling was selected from a group of Douglas hybrid chestnuts that were very precocious in flowering and more shrub like than tree. The ‘sproutiness’ is inherent in this strain and this is one that was selected just for this purpose as a clonal strain. It would have to be rooted and not grafted to reproduce this characteristic. In this case, the death of the main trunk due to blight and over bearing became an avenue for increased coppice. A tree like this in an orchard for nut production would likely be removed.

Letting the population go on its own and ‘create’ the immune chestnut. It is random but becomes dramatically less so with time. Some trees create seedlings which are more immune than the parent.

Evaluating the disease as a means to strengthen the chestnut tree enhancing genetic diversity in a hybrid population. This took roughly a 50 year period to come to some sort of conclusion and a way forward. The ideal situation would be to replicate the seedlings from the strongest and healthiest immune plants pollinated by other immune plants so you could winnow it down even farther while enhancing growth rate as the populations even out over time.

Enjoy, Kenneth Asmus

Enjoy. Kenneth Asmus