Since digging for strawberry plants destined for Salinas and Watsonville started at Macdoel just a few days ago, I thought it would be judicious to have a look at how many chill hours we've accumulated so far and what it means for additions to supplemental chill, especially for our day neutral varieties.
I checked with the Lassen Canyon nursery chill accumulator here: http://lassencanyonnursery.com/cumulative-chilling-hours-and-weather-conditions/ .
Looking at the data for Oct 18 of this year and running my calculations via the Utah model (which subtracts chill hours for temperatures realized above 60oF, see previous posts), we have currently accumulated 325 units of chill. Given that last year's chill accumulation was 164 units and by most commentator's opinion a decent accumulation, 325 accumulated chill units this year is very satisfactory.
So what does this mean for adjustments on supplemental chill? Personally, I think growers may want to take the strong field chill in stride, and now look forward to what sort of winter we are going to have. Looking at the NOAA data, we are probably in for a weak “La Niña” system this year, which according to the “Color Outlook Maps” for temperature, we have something like a 40% chance of having slightly warmer than normal temperatures in November, December and January.
The question then is what sort of adjustment should or needs to be made to supplemental chill. It's actually not an easy question to answer, given the strong field chill. Then again the odds of a slightly warmer than normal winter would give me some reason to err on the side of caution and go a tad longer than customary on the supplemental chill ./span>
So the question now is, so what Mark, we're around 68 chill units accumulated as of September 22 in Macdoel, but what does that actually mean to me as a grower? How does it compare to years past?
Fortunately, we can reference past year's chill accumulation and make just that comparison.
Again, this year up to September 23 (I'm adding in last night's numbers right now), we have 32 hours below 32o F (0.0 chill units/hour x 32 hours), 153 hours between 33o F and 45o F (153 hours 1.0 chill units/hour), 170 hours between 46o F and 60o F (170 hours x 0.5 chill units/hour), and 188 hours over 61o F (140 from 61o F to 80o F and another 48 over 81o F). Negative chill units will be from the 188 hour (which we multiply by a quarter to obtain an approximate range for temperatures from 61o F and 65o F), which gives us (188 x 0.25) 47 multiplied by -0.5 chill units/hour = -24 plus the balance of 141 multiplied by -1 chill units per hour = -141. Adding -141 + -24 + 153 + 85= + 73 chill units accumulated so far.
Looking at the numbers for 2014, which we consider a pretty hot fall in Macdoel, the same calculations as above from 3 below 32o F, 137 hours between 33o F and 45o F, 147 hours between 46o F and 60o F, and 256 hours over 61o F (202 from 61o F to 80o F and another 54 over 81o F) brings us to -13 chill units accumulated by September 23. This squares with observations from the field in September 2014 that the plants were barely chilled at all.
Next, let's have a look at 2013, which has been described to me as a "normal" chill year. Referring to the Lassen historical chill chart, by September 23, we experienced 10 hours below 32o F, 127 hours between 33o F and 45o F, 208 hours between 46o F and 60o F, and 226 hours over 61o F (181 from 61o F to 80o F and another 45 over 81o F) which by using my calculations brings us to +34 chill units accumulated by September 23, 2013.
Conclusion; from the looks of it we are tracking right along and having a normal accumulation of chill in the autumn of 2015.
Strawberries (and caneberries) rely on enough accumulation of chill to develop normally. Insufficient chill in strawberry results in lower vigor and productivity, while excessive chill can result in overly vegetative plants with again lowered productivity. This is an expansive topic, and readers can refer to other articles in this blog to familiarize themselves with the concept further.
The question we take up today is with the current spate of warm temperatures during the day in the strawberry plant production areas in Northern California, are we losing some of the accumulated chill during the night. The answer is an unequivocal “yes”.
Consider the chart below taken from the UC Davis Fruits and Nuts website which comes from the publication “Chilling Accumulation: Its Importance and Estimation” by David H. Byrne and Terry Bacon out of the Department of Horticultural Sciences at Texas A&M University:
The Byrne and Bacon explanation of calculations used by the Utah model:
The Pomology Weather Services program calculations for the Utah model:
1 hour below 34°F = 0.0 chill unit
1 hour 35 - 36°F = 0.5 chill units
1 hour 37 - 48°F = 1.0 chill units
1 hour 49 - 54°F = 0.5 chill units
1 hour 55 - 60°F = 0.0 chill units
1 hour 61 - 65°F = -0.5 chill units
1 hour >65°F = -1.0 chill units
1 hour below 34°F = 0.0 chill unit
1 hour 34.01 - 36°F = 0.5 chill unit
1 hour 36.01 - 48°F = 1.0 chill unit
1 hour 48.01 - 54°F = 0.5 chill unit
1 hour 54.01 - 60°F = 0.0 chill unit
1 hour 60.01 - 65°F = -0.5 chill unit
1 hour >65.01°F = -1.0 chill unit
Let's go to the accumulator of chill hours from Lassen Canyon Nursery to apply this to our situation today:
As of 9/22/2015, it looks like we have 32 hours (or units) below 32o F (0.0 chill units x 32 accumulated), 143 hours between 33o F and 45o F, 166 hours between 46o F and 60o F, and 177 hours over 61o F (130 from 61o F to 80o F and another 47 over 81o F). I'm assuming the interesting part of chill accumulation for Lassen is the 33o F to 45o F range, which we'll multiply by 1 for 143 chill units, then add on the less effective 46o F and 60o F of 83 chill units (166 hours x 0.5) for a total of 223 accumulated hours in Macdoel so far.
However, from this total of 223 hours accumulated we are going account for the negative effect of 177 hours over 61o F. We don't have the advantage of knowing how many hours were in the range of 60o F to 65o F for which the multiplier is only -0.5 chill units, but indulge me and let's put that amount at a quarter of the 177 hours over 61o F (177*0.25 = 44) . Meaning that (44 hours x -0.5 chill units) + (133 x -1.0 chill units) = -155 chill units.
As such, according to the Utah Model, total accumulation this year as of September 23 in Macdoel is 68 chill units (223 units - 155 units).
I was just speaking with a colleague doing some work this weekend up at MacDoel in northern California and he reported really low temperatures last night. Chill is certainly starting to accumulate at the strawberry nurseries, and good thing too since this winter might be a bit warmer than usual. In other words, it's looking good up there.
Remember that chill can be defined as temperatures below 45 degrees F and above 18 degrees F.
Chill accumulator courtesy Lassen Canyon Nurseries:
The link below is to a paper written by Doug Walsh, Frank Zalom, Doug Shaw and my predecessor Norman Welch. It succinctly makes the point that a warm fall following transplanting decreases plant vigor, encourages precocious bloom and can predispose strawberry plants to infestation by twospotted spider mites. Rain can reduce overwintering mite populations, but still warm fall temperatures will decrease plant vigor.
With the understanding that this fall and winter has more than even odds of being an "El Niño" year (http://www.elnino.noaa.gov/), which could result in warmer temperatures and more precipitation, growers and agricultural people might want to be thinking about being a little bit longer than customary on cold conditioning of the day neutrals up here on the Central Coast.