Blueberries and Chilling Hours

One of the last things one might consider when choosing a blueberry cultivar is chilling requirement.  A chill hour can vary depending on the model used, but the most common model in this region defines it as the number of hours below 45 F.  This is a requirement for the plant to satisfy its dormancy and thus to grow and fruit normally the following year.  In regions where cold temperatures are more common, plants can remain in a quiescent phase even after their chilling hour requirement has been met.  However, in warmer climates this may not be the case, so early flowering is a problem in low-chill cultivars.  Today, I have an example of that with a Florida-based, Southern Highbush cultivar, Springhigh.  As defined by the Florida Foundation Seed Producers, Inc., “‘Springhigh’ is a very vigorous, upright bush with excellent survival in the field.  It ripens about 9 days earlier than Star.  Berries are very large and medium dark, with good scar and flavor and medium firmness.  Sets fruit well even when pollination conditions are poor.”

Springhigh blueberry

‘Springhigh’ is reported to have a chilling requirement of around 200 hours, which is considered to be low-chill.  Today is December 18, 2012 and the place is Poplarville, MS.  We have gotten past the 200 chill hour mark for this year, although we have not gotten very cold at all (barely below 32F).  Let’s take a look at the state of  ‘Spring High’ in mid-December.

Buds are popping in mid-December.

Flowers are showing in mid-December.

Fruit set in mid-December.

Large, developed fruit in mid-December.

I was surprised at how much fruit this plant had on it.  As you can see it has all stages of reproductive growth on it right now.  Now, it is not all over the plant, many buds have not pushed yet, but if the weather continues as it has….who knows?  If we get a hard freeze then the fruit would certainly be a loss, but in the absence of that the productivity could be poor because no one is prepared to harvest plants so early in the season.  There is a reason we don’t really recommend very low chill types here.  Even along the coast they could get zapped.

So, chilling requirement is an important consideration in your growing region.  There are many cultivars out there that “fit”, so finding one is not a problem — it is knowing the chilling requirement before planting and pairing the cultivar with the site that is important.