Growing Blueberries Low and High, South & North
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"I suspected this was because I live in northern California (USDA zone 9 b) and blueberries are a cool-climate crop. But the University of California's "chilling hours database" reports that my local station recorded at least 800 chilling hours a year in each of the last 5 years. (Eight to 1400 hundred hours by the 'lower than 45 degrees Fahrenheit' criteria; and 800 to a thousand hours by the 'more than 32 but lower than 45' criteria.) This makes me hopeful that I might be able to grow lowbush New England-style blueberries. I'm crazy, right?"
---Andre in Santa Rosa
A. If anybody's crazy here, it's me for picking this question—but it's hard to find new topics after 19 years of doing this!
(Yeah, yeah; I hear you: "Cry me a river.")
OK—let's talk about "chilling hours".
A "chill hour" is exactly that; an hour when you would feel chilly. The classic definition pegs this at temps below 45 degrees Fahrenheit for a full hour. The other range Andre cites—between 32 and 45 degrees—is felt to be more informative, as it excludes temps below freezing, because those hours do NOT contribute to the fruit formation that 'chilling hours' are based on.
So, in short: plants with a chilling requirement (like apples, raspberries, peaches, blueberries…) need a certain number of hours that are cold but not freezing over winter to produce good fruit.
Now—let's say that Andre continues to get at least 800 hours of chill a year, which does not surprise me; I've spent a lot of time where he lives up in the wine country and you often need a jacket at night.
But USDA Growing Zone 9 b sure sounds warm, right? After all, Zone 10 is about as hot as it gets in the US. Ah, but the USDA Zones only indicate the maximum low temps an area tends to get over winter. For Zone 9 b, the most extreme cold you should ever expect is 25 to 30 degrees Fahrenheit, and if you follow Andre's math, he actually does get quite a few hours below freezing each winter.
But let's forget the theory here. Andre found the really important information in those records of actual recorded chilling hours near his home. If he continues to get 800 hours of temps between 32 and 45 a year, he can certainly grow regular blueberries, and he can try and get the little guys—the lowbush types—to do well, especially with a little bit of extra work. After all, we know that he can grow highbush varieties; and most experts limit their range to no higher than UDSA Zone 7!
"Rabbiteye" types are the blueberries recommended for his zone; they're bred to produce in warmer climes. So are what are called "Southern highbush" varieties; some of which can get by with as little as 200 hours of chilling. Now—if Andre bought his plants locally, they could well be Southern highbush types, which would rock with 800 hours of chilling. But if he's getting fruit from regular "Northern" highbush types, the little guys might also do well.
All right; enough teasing. Time to talk directly about lowbush berries….
Lowbush blueberries grow wild in the Northern States and Canada; a true native fruit that locals have enjoyed for centuries—and the plant that provided the breeding stock to make highbush varieties, which must have been quite a task as they are the opposite of high. Lowbush blueberries are a ground cover—the plants are only about a foot tall, and the berries are small, so they're not cultivated commercially, which is why we don't really know that much about their needs or limits; they're not a cash crop. They're a regional treat that people who can still get down on their hands and knees look forward to each season.
Now—although I generally do not endorse growing plants in pots over winter, its actually an advantage in Andre's situation. Because they're always above ground, the roots of potted plants are in the same temp as the ambient air, which actually lowers their growing conditions a full two USDA Zones down and places them in a technical Zone 7 as opposed to the Zone 9 b they would be if they were planted in the ground.
(The reverse is true in places with really cold winters, where subtracting that same two USDA Zones could mean death instead of chilling.)
But my suggested container here is not a pot, but a hard-plastic kiddie pool. Really! Unlike highbush varieties, lowbush berries spread by unground root systems and need room to wander. And you don't get many berries-per-square-foot from the little guys. So you want something that's a lot bigger than a pot or even a half-whiskey barrel, but it doesn't have to be deep.
Punch holes in the bottom of the pool for drainage, fill it with half peat moss for the acidity blueberries require and half compost for stability and nutrition. Put the pool up on a picnic table or sawhorses and you'll get a double bonus—being off the ground will keep the roots even cooler at night and bring the berries up close for easy picking.
Triple bonus: the leaves of all types of blueberries turn bright red in the fall and stay on the plants a long time; you'll bring the autumn color of New England to California!