It was late July, 2019 and a normal Finger Lakes summer. The water temperature in Seneca Lake had morphed from threatening into almost pleasant, a handful of boats were out on the water and, for the first time, we were busy readying Black Squirrel Farms to accept black walnuts for fall processing. Our 2019 plan was to grow and process the black walnuts from our 80+ trees on 590 State Route 14 and collect and process black walnuts from other area growers. We were so focused on what was happening on the ground that it took us until July to notice that this year our trees didn’t seem to be growing black walnuts.
Our trees looked healthy, with green leaves and no obvious widespread insect damage so we ruled out an unidentified infestation or disease as the likely cause of our production deficit. We opened some early-falling walnuts and found partially formed shells and black glistening masses instead of nutmeat. Underdeveloped walnuts falling early can indicate insufficient pollination. While there are many ways pollination can go wrong, most are related to weather and the weather irregularity most likely to cause widespread loss is temperature. Spring temperatures below 26°F will kill emerging buds and destroy catkins and pistillate flowers.
A review of Penn Yan airport weather station data revealed that exposure of our trees to temperatures of 26 F or below could have happened as late as April 29, 2019. This is unusually late for temperatures this cold. Logically, if the April 29 cold snap caused our walnut deficit, other area growers would be similarly affected. To test this, we reached out to regional black walnut growers in late July and early August. The map below compares black walnut yield estimates from these growers with the April 29 temperature lows from NOAA monitoring stations. Without the benefit of sophisticated climate modeling to guide us, we crudely defined two areas probably exposed to temperatures cold enough to damage vulnerable non-dormant black walnut tissue on April 29.
Figure: April 29, 2019 Low Temperatures and 2019 Black Walnut Yield at Selected Sites
It’s not clear that sites within this cold exposure zone on fared significantly worse than sites outside this zone. “Maybe a little” is a fair response to that question. It does appear that sites nearest the largest and the deepest of the Finger Lakes, such as ours, fared demonstrably worse.
Even if it the cold temperatures on April 29 did inhibit 2019 black walnut yield, how that happened is likely not as simple as “our blooms froze”. We couldn’t find many reasons to believe that our trees were blooming on April 29 and we hadn’t been monitoring our trees. The information we found signaled that a mid-May bloom date is most likely for our site. For example, a still-cited 1915 article indicates that black walnut trees flower in early April in the southern portion of their North American range then progressively closer to the beginning of June at northern sites like ours. In contrast and confusingly, Wikipedia suggests that black walnut budbreak travels north to south, governed by a 14-hour day length requirement for leafout. Interestingly, this implies first leaves by the end of April and blooms in mid-May for our site. Some excellent 2002-2006 work done with multiple cultivars in New Franklin, Missouri reveals a preponderance of mean pistillate flower bloom dates in mid-May. Both our site and New Franklin, Missouri are in USDA plant hardiness zone 6a, so this study signals mid-May blooms for our trees by analogy. Certainly, trees experience annual variability in bloom date. While we don’t know how much annual variability is normal for black walnuts, the well-documented long-term bloom history of Washington D.C.’s famous cherry trees shows that while cherry tree peak bloom dates often vary by more than two weeks from one year to the next, peak bloom more than ten days earlier or later than the long-term average is possible but unusual. Earlier blooming in recent decades has been widely noticed for many northern plant species but the New Franklin work is recent enough to have inherently taken this into account. This information taken together makes it seem implausible that our black walnut trees were blooming in late April and therefore had blooms susceptible to the cold snap.
Do black walnut trees near the largest and deepest of the Finger Lakes bloom earlier or later than other Central New York black walnut trees? Could our trees have lost their first set of emerging leaves to cold and then, rather than expend already-depleted starch reserves on a nut crop, used their depleted energy supplies to create another set of leaves? Our takeaway is that until we can observe black walnuts growing, it makes sense to assume that the scale of that year’s black walnut crop is unpredictable. Year to year variability in local and regional black walnut yield is almost certainly driven by more than just the alternate bearing nature of the trees. We are responding to this year’s disappointing yield by deferring some of our 2019 plans until 2020 and moving multi-year black walnut storage capability from our “nice to have” to our “need to have” column. Perhaps the science enabling us to predict black walnut yield earlier than a few months before harvest will emerge but until then we will treat an unpredictably fluctuating crop volume as just another known business challenge.