I had a conversation quite recently with Doug Shaw, UC plant breeder, concerning the concept of chill in the day neutral varieties and the desire of some Central Coast growers to reduce the recommended amounts significantly, even all the way down to zero days of chill:

In the way of review, please recall that chill requirement in strawberry in California is made up of two essential parts.  One part is what the plant accumulates in the field before being harvested, and the other is accumulation of chill after harvest and the plant is in storage.  There is a big difference between the two and one does not supplant the other.  In-field chill takes place when the plant is still in the soil, out in the open and still has all its leaves.   Supplemental chill takes place after harvest of the plant and occurs in a constant near freezing temperature, in the dark and the plant has none to very few leaves left (Figure 1 below).

Accumulation of chill, especially supplemental chill, in the strawberry transplant makes it stronger and better able to survive the stress of plant harvest, transplant and the growing season beyond.  

As some of the chill sensitivity has been bred out of the modern UC day neutral varieties such as Albion, Monterey and especially San Andreas, it is indeed possible to grow these varieties with less than the recommended amounts (10-18 days) of supplemental chill.   However, those who choose to reduce chill below these amounts should recognize the amount of risk they are taking and that this is not an exercise for those still using training wheels.   Should growing conditions take a turn for the worse, these underchilled plants do not have the vigor to help them pull through and will suffer more than those adequately chilled.  Moreover, while UC day neutral strawberry plants chilled significantly less than the recommended 10-18 days can produce fruit earlier than others (probably owing to the earlier planting date), these plants quite likely will not perform optimally in terms of overall yield and quality along with showing a tendency to produce smaller fruit later on in the season.

The only case where one would want to go short on supplemental chilling time would be if the plant harvest was so late that a minimum chill time of 10 days would result in a planting date so late that it would compromise plant growth and establishment.

Other than that, it is still recommended to give the UC day neutral varieties 10-18 days of supplemental chill.

The cold weather we have been experiencing over the past few days has prompted a lot of talk and even articles in the popular press over what the effect of this cold would be to local berry growers.  Beyond the damage that very cold temperatures could cause tender plant parts such as flowers and emerging vegetative parts (of which we fortunately don’t have very many right now), the question worth exploring is what benefit this weather could be bringing to our berry crops.

Many of our cultivated fruiting plants originate from temperate regions, including many berry species and tree fruits, and as such go dormant in response to oncoming cold weather in the autumn.  This adaptation of dormancy protects the plant buds from injury when temperatures fall below freezing and the buds stay this way until enough cold has been accumulated over time.

This accumulation of cold over time, known as chilling requirement and measured in hours as chill units, is the minimum amount of cold after which many fruit trees, caneberries and strawberries need to be exposed to in order to grow properly in the following spring.  The total number of hours of chill needed to establish proper flowering and vegetative growth vary substantially for plant types and even between varieties of the same plant species.

If plants requiring a certain amount of chill hours do not receive it, they may end up blooming or leafing out late in the spring or in an spread out, uneven fashion. Additionally, they may subsequently experience reduced fruit production and quality.

Another complication of calculating chill units in California, as compared to much colder regions of the country, for example Wisconsin where temperatures can be below freezing for weeks at a time (go Badgers!), is that our region tends to have a cycling of warm and cold weather throughout the winter.  How then do we as agriculturalists in California calculate chill accumulation in this back and forth between cold and warm?

To calculate chill hours, there are three common models all based on the principle that plants accumulate chill between 45 degrees F and freezing (32 degrees F and not below).   One model ignores the below freezing threshold and simply calculates total number of hours under 45 degrees F, another calculates number of hours between 32 degrees and 45 degrees, and another, called the Utah model, is bounded by 34 degrees and 45 degrees but also accounts for negative chill accumulation, being the understanding that temperatures above 61 degrees detract from chill hours already accumulated.  It is worth pointing out that in the Utah model, temperatures under 34 degrees do not accumulate chill, nor do they detract from it.

Yet, the fluctuating temperatures of California still are a challenge to some degree for these models, and the University of California is engaged in research to get a better handle on these conditions, and is has been testing a “Dynamic Chill Model” and a “Chill Portion Model”.   Both of these are beyond the scope of this blog, but Central Coast agriculturalists seeking to further their understanding about chill and how to manage it, will find an excellent resource at :

http://fruitsandnuts.ucdavis.edu/Weather_Services/chilling_accumulation_models/

One of the first posts to this blog regarded the importance of chill, both field and supplemental, to the day neutral (ie Albion, San Andreas, Portola, Monterey among others) strawberry varieties.  Right now, field chill in MacDoel is in the area of 600 hours, which is plenty, even in the light of the very warm stretch of weather that took place in September.  Still, be reminded that this abundance of field chill should not be considered to be a replacement for supplemental chill taking place in the cooler after the transplant has been harvested.

However, this year because of the delay in transplant harvest growers really should be striving to strike a balance between getting adequate supplemental chill to obtain good vigor and planting sufficiently early to get sufficient plant growth here in the fall.  To accommodate this idea, every one of the day neutral varieties listed above can be well established with supplemental chill of 7 to 10 days, but no less.  More days of chill, up to 18 days, are of course in the printed recommendations, but this year a large delay in planting may not result in acceptable plant growth and establishment.

If one follows the suggestion given above, supplemental chill will end up being on the low side, and subsequently growers must be more vigilant than usual in planting practices.  Transplants must not be allowed to dry out in the field during planting, transplants should be properly placed the planting hole (no “J” rooting, and only portion of the crown above the soil line) and irrigation for establishment should keep the beds at field capacity for a few weeks.