Uneven Leafing in Walnut Trees

In 2014, walnuts are showing extremely uneven leafing in California. As of late April, we have 2nd leaf Howards with leaves expanded to 10 inches on the north side of the tree and buds just beginning to swell on the south side (see photo to the right). The symptoms seem to be slightly less severe on earlier leafing cultivars and worse on later ones.

The most likely cause of this phenomenon was the relatively warm winter with many sunny days and a lack of fog. Temperature measurements at the bud positions in the orchard shown in the photo showed that on the south to southwest side of the tree, daytime bud temperatures rose to 8-10°C higher than the air temperature and at night they fell 8-10°C lower than the air temperature. On the north side of the tree, daytime high temperatures were approximately equal to air temperature and night time temperatures again fell to 8-10°C lower than the air temperature. This fluctuation from low to high temperatures likely results in a cancelling of chilling hours on the south side of the shoot leading to low chill conditions.

The minimum shoot growth was at an angle about 20 degrees to the west of south and the maximum shoot growth at an angle 20 degrees to the east of north (see figure to the right). This agrees with our temperature measurements around the vertical shoot with the warmest winter shoot temperatures occurring just to the west of south at about 2-3pm.

This phenomena has been observed in other years but not as severely as in 2014. In 2014, the author has observed it in at least 7 counties from San Joaquin County north. It likely occurred farther south as well. It was most obvious on shoots that elongated rapidly during the 2013 growing season. It was observed on second through fifth leaf trees on vertical shoots. The uneven growth occurred on older orchards as well but it would is much less obvious due to the more complex canopy structure. It is uncertain if the sudden cold temperatures in early December played a role in this phenomenon. Historically, we have seen more fog in the Central Valley. On foggy or cloudy days, bud temperature very closely tracks the air temperature. This past winter, there were almost no foggy days. Because bloom is spread out by 3-4 weeks, it is likely that this will lead to variability in nut size and maturation.