Supplemental chill, also known as cold conditioning, takes place after harvest of the transplants, which have gone dormant because of their exposure to the decreasing daylength and lower temperatures of the nursery fields of Northern California where they are grown. Postharvest supplemental chill occurs in a constant near freezing temperature, in the dark and when the transplant has no to very few leaves left on it.
What supplemental chill is actually doing is breaking (reversing) plant dormancy, which sets into motion a series of metabolic events in the plant resulting in a promotion of vegetative growth and inhibition of new inflorescence formation. Petioles grow longer, leaf blades get bigger and more runners are formed as dormancy is broken through supplemental chill. All of this is consistent with the industry understanding that a longer period of supplemental chill results in more plant vigor, again meaning more vegetative growth and less fruiting. The challenge for the berry grower is to strike a balance between the vigor of vegetative growth and the fruiting which is greatly desired.
Growers already know this, but berry cultivars vary greatly in their sensitivity to the dormancy breaking supplemental chill. Generally speaking, short day strawberry varieties need very little – something on the order of one to three days - to break dormancy and in fact most become tremendously vegetative when chilled in excess over the recommended few days. In contrast, day neutral varieties need substantially more days of chill, most often in the range of one to two weeks, to develop the normal balance of vigor and fruiting following planting. Since longer periods of chill are associated with greater vegetative vigor, organic growers tend to chill their plants longer before planting, in the range of 30% longer, so as to enable the plant to handle less hospitable soil environments.
As we creep towards strawberry planting season on the Central Coast, it is always good to review where we stand currently on nursery cold conditioning, and with this in mind, think about adjustments to the post harvest supplementary chill based on what sort of winter we might be expecting.
Those of you who know me understand my faith in hedging my bets and planning for contingencies. I favor decisions with a high probability of success and decent outcomes over swing for the fences with blow your socks off results but missing and failing most of the time. Short supplemental chill times with early planting dates to get the beat on the competition are not my thing since this strategy puts good plant vigor at risk, especially if this 2017-18 winter is warm.
Chill accumulation for this year looks good at the nurseries up at MacDoel. Using Lassen Canyon Nursery's chill accumulation chart (also appreciate the comps to previous years- very useful), shows that, after a warm start in September, hours have really ramped up and currently as of October 5 we are at 337 total hours according to my Utah model calculations (which subtracts chill during warm weather episodes, and discounts temperatures under freezing - look elsewhere on this blog for how I am doing this). That compares very well with previous years, and is in fact ahead of many of them.
Further, looking forward to what sort of winter we are to expect, let's go to the NOAA weather maps. Quite simply, for all of our strawberry production areas, it is as of now 40% probable that we get temperatures above normal this winter (being December, January and February). This is up from 33% a few weeks ago, so I am sensing a trend which seems to confirm where we are headed.
In conclusion, with solid field chill already in the can but a good probability of a warmer than normal winter in the offing, I would again this year favor just a little bit more supplemental chill than customary.
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).