Native Range and Climate

Chinese chestnuts (Castanea mollissima) are native to East Asia.  The exact region of origin is unknown, but it has been extensively cultivated throughout most of China, Korea, and Taiwan.  They should be able to tolerate a wide range of climates with little trouble, from northern Florida to southern Maine, and all the way to the west coast, provided they are planted in the appropriate soil type.  The primary exception may be the western prairies, such as eastern Colorado, western Kansas, and everything north and south of it.  Wild temperature fluctuations frequently experienced during late winter and early spring may prove to be too harsh on new growth most years.

While certain varieties of the other common chestnut species can thrive in a fairly large range as well, Chinese chestnuts generally exceed their boundaries.  They are consistently the most disease resistant, require the least amount of chill hours to bloom, and have a greater tolerance of cold winters and late frosts than European and Japanese chestnuts.  While the non-dormant terminal buds of Chinese chestnuts are occasionally damaged at the following locations, they appear to avoid it often enough that a few commercial orchards are grown in states like Iowa, Missouri, Ohio, and northern Florida.  It is "implied" that their buds are most vulnerable around the time late blooming peaches are flowering, but this may not be the most accurate comparison.

American chestnuts (Castanea dentata) are primarily native to the Appalachian region of the United States, from southern Maine to central Mississippi.  The area receives an average of 3" to 5" of rainfall per month, and the average temperature on the hottest day of the year ranges between 80F (26.7C) to 90F (32.2C).  Since they evolved in the eastern half of the United States, they leaf out later than the other common chestnut species, which helps their non-dormant buds avoid frost damage.^[1][2]

Based on limited information, it appears the terminal buds of the Ozark chinquapin (Castanea ozarkensis), a species closely related to the American chestnut — native primarily to the northwestern half of Arkansas — break out roughly a week after late blooming peaches are around full bloom in southern Missouri.  Until I am able to find more reliable information, I would assume bud break generally happens around a similar time for the American chestnut and the other (southeastern) American native, the Allegheny chinquapin (Castanea pumila).

European chestnuts (Castanea sativa) are native to scattered parts of Southern Europe, the coastal area of northern Turkey, and the Greater Caucasus Mountains.  Most of the area experiences dry Mediterranean summers, while the Greater Caucasus receives around 2" of rainfall per month.  Additionally, it has naturalized throughout France, parts of Central Europe, and to some extent, England.  The average temperature on the hottest day of the year ranges between 65F to 90F (18.4C to 32.2C), but it is generally 75F to 85F (23.9C to 29.4C).

Japanese chestnuts (Castanea crenata), often called Korean chestnuts, are native to Japan and South Korea.  Much of the region receives 3" to 6" of rainfall per month during spring and summer, but a significant area can experience 10" or more for a few months in the middle of the grow season.  The average temperature on the hottest day of the years ranges between 75F to 90F (23.9C to 32.2C), but it's generally near 85F (29.4C).

Bloom and Pollination

Chestnuts produce male and female catkins on the same tree, but they are self-sterile.  Pollen production begins very late in the season, roughly 2.5 months after bud break, and the flowers become receptive a few days later.  They last for one or two weeks.  Pollen from a tree that produces larger nuts may increase the nut size of the tree that is being pollinated (metaxenia).^[1] This is not typical for fruits and nuts. 

Some varieties may bloom heavily a second time during the grow season, but if the second crop is not thinned out (which is generally not done due to the large size of the tree), it may reduce the nut size of the primary crop.^[3] In addition, the removal of secondary burrs may allow the tree to produce a larger primary crop the following year.  Alternatively, varieties that produce a large number of burrs during the secondary bloom may produce a respectable crop if damage from a late frost prevented the occurrence of the primary bloom.^[3]

Unfortunately, secondary bloom is rarely discussed, and many of those mentioned happen to be fairly uncommon varieties.  For example, Peach, Qing, Simpson, Carr, AU Homestead, and Gideon produced few secondary burrs, according to a three year trial, but Armstrong, AU Cropper, Carolina, Crane, Eaton, Ford's Tall, Jersey Gem, Orrin, Mossbarger, Revival, and Willamette produced many secondary burrs.^[3]

Disease

Chestnut blight (Cryphonectria parasitica) can be a very serious disease.  Species that shared a similar native range as the fungus, such as the Chinese and Japanese chestnut, naturally developed resistance to the blight.  While there is variation within both species, the Chinese chestnut generally shows a higher degree of resistance.^[4]

European chestnuts are somewhat susceptible to blight and will eventually die from the disease in certain environments.  Hybrids between C. sativa and an Asian species regularly display some degree of resistance, but oftentimes, it is not enough for them to survive long term.

American chestnuts are highly susceptible and were decimated by blight in the early 1900's, but decades of backcrossing, along with the mollissima genes responsible for blight resistance, has created highly resistant trees that are over 93% American^[2] — although they are still quite rare.  Other operations are also taking place, such as genetic modification with a wheat gene responsible for fungal defense^[5][6] as well as intercrossing pure American chestnuts that have been found to be less susceptible to blight.^[7]

The European chestnut almost suffered a similar fate as the American chestnut, but the lower degree of susceptibility of C. sativa, much like the few American chestnuts that survived and retained their original form, generally shared another significant quality.  Their death sentence was delayed long enough for a mycovirus to infect the blight, causing it to become hypovirulent.^[8] This allowed the infected trees to recover.

In North America, treatment of individual cankers has proven to work, but the hypovirus struggles to spread on its own.  The extended absence of the hypovirus, along with the American chestnut's higher degree of susceptibility, has allowed the blight to thrive and become genetically more diverse than the blight present in Europe.^[8]

Varieties that are susceptible are best grown in a dry climate, since rain and humidity aid the development and spread of the blight (particularly during spring and summer).^[9][10] While chestnut trees tolerate dry climates fairly well, water stress will reduce nut size and yield.  Furthermore, soil pH often found in such a climate is often too high for chestnuts to survive.  One possible solution is the use of certain oak species as a rootstock for alkaline soils, such as Quercus trojana and Quercus coccifera, since they have been used with some success.^[11]

Chestnut trees have a fairly low level of tolerance for clay and other poorly drained soils.  Excessively wet soils will also encourage phytopthora root rot (Phytophthora cinnamomi) on European and American varieties.  Phytophthora cambivora is also a problem in Europe, but in the United States, it is rarely mentioned outside of the west coast.  P. cinnamomi is believed to have eradicated American chestnuts from the lower elevations of the southeast well before the arrival of the chestnut blight, but it did not have a significant effect on the mountainous areas, since the pathogen struggles to thrive in the environment.

Pests

The chestnut gall wasp (Dryocosmus kuriphilus) can be a serious problem in regions without natural predators, since insecticides have proven to be ineffective.^[12][13] Not only do they inhibit shoot growth — reducing leaf and fruit production — but their abandoned galls serve as an entry point for chestnut blight.^[14] However, the gall wasp's primary predator, the Torymus sinensis wasp, has been introduced to infested regions throughout the world, and they have become well established.

Several native parasitoids have also learned to exploit the unintentional introduction of the chestnut gall wasp, such as Ormyrus labotus, which can out-compete T. sinensis in more vegetatively diverse locations, provided there are a significant number of oak trees nearby.^[15] This isn't necessarily positive news, since the results of this particular study indicated that only 70% of the galls were parasitized (50% by O. labotus and 20% by T. sinensis) rather than ~95% when T. sinensis was the dominant predator.  It is believed that labotus suppresses the population of sinensis by parasitizing their larva in addition to those from the gall wasp.  Fortunately, outside of forest settings, O. labotus does not appear to be common and have been seen to only parasitize 0 to 25% of the galls.  This observation will likely be consistent, considering they are a multi-generational species and chestnut trees aren't enough to sustain them throughout the season.^[15]

Some varieties are considered to be resistant to the chestnut gall wasp, and in-spite of the uncertainty some appear to have around this claim, it could very well be the case.  Within the first decade after their introduction in 1941, the Japanese discovered multiple C. crenata varieties that were resistant.^[13] One of these varieties, Ginyose, was discovered to produce seedlings that were resistant nearly 100% of the time, even when crossed with a non-resistant variety.  The others produced resistant seedlings roughly 75% of the time.  By 1959, their breeding program named and released three additional resistant varieties selected from a group of over 3,500 seedlings.  They were Tanzawa (Otomune x Taishowase), Ibuki (Ginyose x Toyotamawase), and Tsukuba (Ganna x Hayadama).  Ishizuchi (Ganne x Kasaharawase) was released in 1968.^[13]

Shortly after their release, land consisting of chestnut trees dramatically increased, quadrupling from 10,000 to 40,000 hectares within 10 years.^[13] However, around this time (1970), a new strain of gall wasp adapted to the trait that previously granted resistance and began to spread throughout Japan once again.^[13] Fortunately, the Japanese discovered and introduced the predatory torymid wasp from China in 1975, with larger populations selected and introduced again in 1979 and 1981.^[12] In 1983, 43% of the shoots at their research site were infected, but by 1986, that number had dropped to 14%, far below the tolerable threshold of 30%.  Two years later, it was below 5%, and by 1992, less than 1% of the shoots were infected by the gall wasp,^[12] causing interest in resistant varieties to rapidly decline.

Chinquapins from the United States (Castanea pumila and Castanea ozarkensis) are also believed to be resistant to the gall wasp, but, perhaps, to a much greater degree.  In 1995, chinquapin x Asian chestnut hybrids, pollinated by American chestnuts, were planted in North Carolina to determine if the seedlings would carry over gall resistance from their chinquapin ancestry.^[16] By 2006, 53 out of 93 were still alive, 36 of which were C. dentata x (C. pumila x C. crenata) and the other 17 of which were C. dentata x (C. ozarkensis x C. mollissima).  14 out of the 25 C. dentata x (C. pumila x C. crenata) and 7 out of the 9 C. dentata x (C. ozarkensis x C. mollissima) that showed little to no blight infection had few or no galls present.  Trees that were afflicted with a more severe case of blight were excluding due to the stress possibly making them more attractive to the gall wasp; however, some of them were free of galls as well.^[16]

The most significant pest is the Chestnut weevil.  In the US, they primarily consist of two species, the small chestnut weevil and the large chestnut weevil.  The small chestnut weevil (Curculio sayi) is the most common of the two.  They begin to lay their eggs 3 weeks before the nuts ripen (or after the burrs open).  Eggs hatch after 8 to 10 days and the larva feed for 2 or 3 weeks, but cooler temperatures can dramatically delay their development.^[17] The larva exit the nut after the nut falls to the ground.  Adult weevils also feed on the nuts.  The damage (caused by the adults) is insignificant, but it creates an entry point for pathogens that may harm the crop later.

Large chestnut weevils (Curculio caryatrypes) begin laying their eggs 1 or 2 weeks before the small chestnut weevil, since their longer snout allows them to gain access to the nut before the burr opens.  Their eggs hatch in only 5 to 7 days.  These advantages allow most of their larva to exit the nut before it falls to the ground, causing them to be more difficult to control, but, as mentioned, they are not as common as the small chestnut weevil.

Sanitation must be practiced if you wish to control the small chestnut weevil.  Fallen nuts should be collected within a day or two, before the larva exit the nut.  Afterwards, they should immediately be placed in a hot water bath heated to 120F (49C) for at least 20 minutes.^[17] This will kill the eggs and larva without damaging the nut.  If done properly, most of the affected nuts should contain unhatched eggs or small and unnoticeable larva.  If 10% or more of the crop becomes affected, commercial operations should consider using an insecticide;^[17] however, this is not practical in a non-commercial setting, since the tree will eventually become quite large.  Traps can be used for monitoring, but they are generally not effective on their own.

Grafts and Seedlings

Delayed graft failure — as well as immediate failure — can be a problem for chestnuts, but it may take a few years to occur.  In addition, graft failure appears to be more common near their northern range as well as on transplanted trees.  However, using seedlings from the variety you wish to graft as a rootstock may increase compatibility.^[18]

In one study, grafting within one's species had the highest rate of success, while grafting hybrids had a lower success rate than grafting two different, non-hybridized species together.^[19] This may not always be the case, though.  A few highly incompatible variety and species combinations were discovered.  For example, Chinese chestnuts grafted to European rootstock, and vise-versa, had little to no success.^[19] In another study, whip and tongue grafts on non-irrigated rootstock had a significantly higher success rate than on irrigated rootstock, but it did not mention long term results.^[20] Overall, this whole situation is very complicated, and none of the studies I've read were very satisfying as they all felt incomplete.

Since they are relatively unreliable, many nurseries no longer sell grafted chestnut trees.  They generally sell seedlings from high quality Chinese varieties, since their nut quality is often quite good.  Seedlings from Japanese x European hybrids are sometimes sold as well, but their quality is more variable.

Nutrition

Chestnuts are unlike other nuts in that they are low in protein (2-10% vs 9-24%) and fat (1-10% vs 45-70%).  They are also high in carbs (35-45% vs 10-30%), contain a lot of water (35-60% vs 2.7-5.3%), and they are the only nut with a significant amount of vitamin C (25-40mg per 100g).  These qualities cause them to have a fairly short shelf-life if they are not dried or refrigerated properly.

It's difficult to determine, even a rough estimate of, the average nutritional content of the different chestnut species, because some of the results vary considerably.  It does, however, appears that the American chestnut may generally have the highest protein content of them all.

In one study that compared the American chestnut with Chinese and European chestnuts, the American had the highest protein content (4.8%).^[21] The Chinese chestnut was close behind (4.2%), and the European had the least (2%).  All three had a fairly low fat content (~1.5%).  The American and the Chinese chestnut contained less water (44%) than the European chestnut (~55%).^[21]

In one source that compared two American chestnuts with two Chinese chestnuts and one or two European chestnuts (among others of an unknown species), using results from multiple old studies, both of the American chestnuts had a fat and protein content around 10% and a water content of 33-47% (the rest primarily consisted of carbohydrates).^[18] The Chinese chestnuts, one being the 'Eaton' variety, had a protein content of 4.4%, a fat content of less than 2%, and a water content of ~56%.  The European chestnut contained 3.1% protein, 1.6% fat, and 52% water.^[18]

More recent studies have shown that some European and Chinese chestnuts have a much higher protein content.  Seven European varieties from Portugal were tested, and almost all of them had a protein content over 5%, with two being around 7%.^[22] The fat content was between 1.73% to 3.1%, and the water content between 46.3% to 53.3%.^[22] Chinese chestnuts from 4 different regions in China had a protein content between 7.54% to 9.74% and a water content of 46.43% to 49.75%.^[23] Their fat content was not measured.

Nut and Tree Sizes

Chestnut sizes will vary, sometimes greatly, by region and year.  Examples are given in the variety section so you'll know whether you should expect something on the larger or smaller side.

Size can also vary greatly within species, but European chestnuts are the most consistent at producing very large nuts.^[24][25] Japanese chestnuts often produce large nuts as well, but there is more variation within the species.  Chinese chestnuts regularly produce medium-size nuts, and the Americans produce the smallest.  Chinese and American chestnuts are more likely to taste better and peel easier than European and, to a greater extent, Japanese chestnuts,^[24][25] although there are plenty of Euro-Japanese hybrids that supposedly produce high quality nuts.

Chinese and Japanese chestnuts produce smaller trees than European and American chestnuts.^[24][25] They range between 30 to 60 feet in height at maturity.  European chestnuts grow about 20-30 feet larger.  All three grow in a spreading form, but it's more pronounced with the Asian species.  American chestnuts grow very fast in a columnar or timber form.  They can be as tall as 100 feet or more at maturity.^[24][25]

American Chestnut and Chinquapin Foundations

Non-profit organizations are responsible for breeding blight resistance into the American chestnut and another American native, the Ozark chinquapin.  They rely on donations and volunteers to continue their work.  Making a donation and becoming a member is currently the best way to obtain seeds from trees that have some form of blight resistant; however, resistance within the seedlings may vary.

The American Chestnut Foundation focuses on backcrossing American chestnuts with the genes responsible for blight resistance from the Chinese chestnut.  They are over 93% American.  Some of them also carried over mollissima's resistance to phytophthora root rot.  TACF is also working on transgenic American chestnut trees.  They are nearly 100% American, the exception being a wheat gene responsible for excellent blight resistance.

The American Chestnut Cooperators Foundation is attempting to develop blight resistant American chestnut trees without genetic modification or the use of other chestnut species.  However, they do not make it easy to donate or obtain seeds, and I have not seen any updates from them since 2017.

The Ozark Chinquapin Foundation is attempting to improve blight resistance of the Ozark chinquapin (Castanea ozarkensis).  In spite of being native to only a small part of the country, the Ozark chinquapin has significantly more genetic diversity than the American chestnut.  Although it's not entirely clear, this appears to have lead to the discovery of multiple trees with a greater degree of resistance than the surviving American chestnuts.  These trees are now being used by the OCF for cross pollination to selectively breed seedlings with improved resistance.