Chestnuts and Their Dependence on Growing Degree Days

Growing chestnuts in the Western United States used to be an almost sure thing with a nice crop of nuts coming in from the orchard every fall after the first 4 or 5 years. In 2010 the game changed because of crop failure with chestnut producers on the West Coast. Is crop failure for chestnut producers predictable given that crop failure usually happens due to an event or events that cause the trees to fail to mature a large portion of the crop? Since hind sight is 20/20 then the year 2010 can provide a good vision of what can cause crop failure in chestnuts.



Marival chestnut tree in full bloom showing male flowers - July, 2010

The growing season for 2010 was one of the worst in the Pacific Northwest as well as some crops in California because of a cooler than normal growing season. To best understand why a "cooler than normal" growing season have such a big impact on chestnut production we must apply even more history. In the valleys between the Pacific Ocean and the Cascade Mountains in Washington state, the average growing degree days (GDD) is about 1550 (see Note 1 for formula for calculating GDD). That is a nice big number, but the number by itself tells us almost nothing. Comparing some crops grown in these valleys with their respective GDDs will help provide a perspective founded in actual measurements.

Crop Growing Degree Days

Early Peaches 800
Potatoes 1000 to 1100
Snap Beans 1100
Honeycrisp Apple 1300
Chestnuts 1500 (estimated)
Hazelnuts 1600
Madeleine Angevine Grapes 1600
Sugar Beets 1800


Actually, the sugar beets are not grown in the western valleys of Washington but they are presented here to show a crop that has a GDD in excess of what the growing season provides. When calculating the GDD there is a crop specific fudge factor applied. The fudge factor is actually the minimum degrees a crop requires to be able to grow. For corn and many other crops the minimum degrees is 55 degrees F. Chestnuts appear to require a minimum day temperature somewhere between 55 and 60 degrees F. For the month of April, 2011 the average daily high temperature in Seattle was 56 degrees with only one of those days above 60 degrees F. To calculate the growing degree days for the month of April, 2011 we would come up with:

Number of days where the temperature was below 58 degrees: 29
Number of days where the temperature was at or above 58 degrees: 1


So for the one day where the temperature was above (actually 66 degrees) the minimum growing degrees our calculation looks like this:

66 - 58 = 8 GDD

So for this month we have an accumulated GDD of 8. A typical April would have 4 or 5 time this value. Now we have a little background on how the daily temperature effects the growth of chestnuts, we can now look back into 2010 to see if specific parts of the growing season had too few GDDs for that period. There are some differences between cultivars and between American, Chinese, Japanese, and European chestnut trees and their respective GDD base GDD temps. Chinese chestnut trees appear to have a base of about 55 degrees, Japanese cultivars around 57 degrees, American and European share 58 degrees. Until ongoing research into the degree basis is complete these should be considered guesses.

Data collected by an on farm weather station provided daily high temperatures starting May 1, 2010 and continuing until September 24, 2010. The data collection was stopped on September 24th because the daily high temps would not likely exceed 58 degrees and the first chestnuts started dropping. There are two very important values that come from the data set. The first is the average daily GDDs for each month and the cumulative GDDs for the growing season. Here are the values for 2010 growing season:

Month	Avg Daily	Cumulative
May	6.0	185.6
June	11.0	515.1
July	20.4	1146.4
August	21.4	1789.8
September	12.4	2100.7

The data indicates the warm growing days did not start until some time in July. Reviewing the daily high temps reveals the fact that temps did not get above 70 consistently until after July 5, 2010. Using the data presented, the accumulated GDDs reached the minimum for chestnuts some time in August., but yet the chestnut production was only 10 to 15 percent of expected. Records from the harvest of the chestnuts in the fall of 2010 has entries of some of the chestnuts still being green but not a enough to be considered an anomaly. These facts would present enough evidence for the growing season having enough GDDs to mature the crop. So what happened to the 85 to 90 percent of the crop that was never delivered from the trees?

History and good records helps to isolate the possible offending conditions. Other conditions, other than weather, can have an effect on the production levels of chestnut trees. These conditions would not be found uniformly across all chestnut orchards in the Pacific Northwest states, such as poor fertility of the soil or excessive or restricted water supplies to the trees. Disease can cause production loss in rare cases but in the situation of chestnut trees most diseases are long term issues developing over a number of years and the trees also look in poor health. The probability of disease being the offender is not likely for this and other reasons. Since most other conditions, aside from weather conditions, have been set aside as not probable offenders, the issue of the weather is still outstanding.



Colossal chestnut tree showing female flowers - July, 2010

Since weather is still the most likely offender in causing the chestnut crop failure in the Pacific Northwest in 2010 and the cumulative GDDs were met, the weather information will need to be examined more closely to see if a specific aspect of the weather was an issue. Since water availability was rejected as a potential cause the rainfall for the growing season will not be considered as part of the weather issue. What is left is looking at specific critical periods of the growing season for chestnuts and the associated weather during those critical periods. Lets start with the seasons of the year and summarize the conditions:

Winter 2010 . Normal to slightly colder than normal, normal snowfall, normal overall precipitation
Spring 2010 . Colder than normal, normal to slightly more than normal precipitation
Summer 2010 . Started cooler than normal but quickly fell into normal, normal precipitation
Fall 2010 . Normal to slightly warmer than normal, normal precipitation

In prior years chestnuts did just fine in normal years. In our season review we see only spring was outside of normal for temps. With this information in hand the details of the spring can be compared with the critical times for chestnuts. There are two basic needs for chestnuts to be able to set and produce chestnuts in the spring time. The first is the development of the male and female flowers. The second is the pollination period. The pollination period is best understood so it is a good starting point. The bloom period for chestnuts in 2010 was July 13 through August 4. The weather was warm and dry, providing the best possible conditions for successful chestnut pollination. During this period the accumulative GDD was 454, or about 23 percent of the seasons total GDDs. During this period every day was above 70 degrees F, with the 70 degrees F possibly being the critical threshold for pollination to be successful. With the weather being normal, delivering warm sunny days during the pollination period, this is not likely going to be the offending period. Left to examine is the spring period, the period where the chestnut trees develop both the male and the female the flowers.



Bisalta #3 chestnut tree female flower - July, 2010

The development of the flowers are on the current years growth for chestnuts. As the new growth extends from the buds from the prior year.s growth the flowers become visible at the loaf nodes on the new shoot. This is the growth that occurred during the colder than normal weather in 2010 and possibly triggering imperfect female flower development. Records from field observations in 2010 documented very large burrs developed on the trees. At harvest time the contents of the burrs were 90 percent nuts lacking kernels, the other 10 percent of the nuts had a high percentage of poorly developed kernels. Also, the burrs had 3 to 8 blank nuts inside indicating the female flowers were unable to receive the male flower's pollen. Once female flowers start developing (about 190 GDD using base of 50 degrees) on the new branches, the flowers can be harmed by temps below 28 degrees F.

Growing degree days is an important part of plant development during the growing season. Most research focused on GDD is around the accumulative GDD a crop needs to successfully mature to harvest. With chestnuts and possibly other crops the GDD has to be delivered across the entire growing season and shortages during critical development periods can possibly result in significant crop failure.

Note 1:

Growing Degree Days are calculated with the following -

Daily GDD = ((Tmax -Tmin) ÷ - Tbase

Tmax = the daily maximum air temperature
Tmin = the daily minimum air temperature
Tbase = the GDD base temperature for the organism being monitored