By looking at the map you can see that I traveled both east and north from my home base in Chetopa, KS. What the map doesn't show you is how the seasonal climate differs between these pecan orchards. In the Table below I have listed the annual average cooling degree days for each location as well the the average number of frost free days (above 32 F), and the average number of freeze free days above 28 degrees F.
Cooling degree days are calculated by the National Weather Service (NWS) as a measure of summer heat used to estimate the energy demand needed for refrigeration and cooling. But cooling degree days also can be used to estimate the amount of heat a pecan tree receives during the summer. Pecan is a heat loving crop and even the earliest ripening pecan cultivars require significant summer heat to fill out kernel and ripen (see pecans and climate).
The freeze free days above 32 degrees is the most common measure of length of growing season and a number most gardeners are familiar with. But, unlike common annual garden plants, the green tissues on a pecan tree do not freeze at 32 degrees. Pecan leaves and shucks freeze at about 26 degrees which makes the freeze free days above 28 degrees (data reported by the NWS) a closer approximation of average length of the pecan growing season.
As you might expect, locations farther north had a shorter growing season than more southerly locations. The two most southerly locations (Chetopa and New Madrid) have the most summer heat. However, there seems to be east-west heat gradient among the 4 other locations that all lie approximately on the same northern latitude. Paola, KS is the hottest during the summer while Carlyle, IL is the coolest.
At each location, I checked on the development of two of our most popular pecan cultivars -- Pawnee and Kanza. Lets look at the photos, starting with Kanza.
huge aphid outbreak.
The one orchard I did not find Kanza fully shuck-split was located in a bottom-land orchard near Paola, KS. From the table above, you can see that Paola has the shortest growing season of all the sites I visited but this location gets plenty of summer heat. What's interesting is that just a few miles away and on top of a limestone ridge, Kanza nuts were fully split open.
Now here's a valuable lesson. Macro-climatic conditions (as measured by the NWS) may be useful for predicting cultivar adaptation on a general level but micro-environmental conditions will determine the exact timing of pecan bud-break and nut shuck-split at any specific location.
In the Spring and Fall, cold air flows like water down hill and accumulates in the valleys and low areas. This natural downhill movement of cold air is the reason we have areas in the landscape called "frost pockets".
The cumulative impact of cold air movement into the bottom-land site near Paola means that things warm up later in the spring and they cool off sooner in the Fall than the nearby upland site. As a consequence, a location where cold air settles has a slightly shorter the length of growing season and a reduced number of cooling degree days. This movement of cold air also explains why the upland Kanza nuts were ripe last week while the bottomland Kanza nut were still a few days away from splitting shuck.
Locations with a shorter and cooler climate should ripen pecans later in the year.
In this case, Pawnee nut development at New Franklin was delayed in comparison to Columbia by two factors--position in the landscape and soil type. The HARC orchard lays at the foot of the river hills in the Missouri river flood plain--a perfect spot for cold air to settle. Pawnee trees at this location are also growing in a heavy clay soil. This type of "gumbo" soil warms so slowly in the Spring that bud-break can be delayed pushing back the entire nut development process.
When I arrived back in Kansas, I found that stage of Pawnee ripening mimicked the observations I made for Kanza at the same two orchards. Pawnee growing at the upland site in Kansas was split much like Columbia, MO and Carlyle, IL. At the bottom-land site in Paola the shucks were still tight just like the nuts I saw at HARC.
In comparing Pawnee and Kanza across all locations this year, the ripening dates for these two cultivars were nearly similar at the four most northerly locations. In Chetopa and New Madrid, Pawnee ripened significantly earlier than Kanza. But every season is a little bit different. If I were to repeat this trip next Fall, I would probably find slightly different patterns of pecan ripening.