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Blueberries
 (Vaccinium sp.)


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Blueberry Cultivars

Liberty blueberries

The first and foremost thing the home grower needs to know about blueberries is that they have a literally vital need for a very specialized kind of soil: highly acidic soil, with a pH in the range of 4.0 to not over 5.5 (ideally from 4.5 to 4.8). That is a most unusual soil, and it is thoroughly unlikely that you will have it, or anything like it, naturally.

There are, of course, ways to alter soil pH, even fairly drastically; but it is our experience, supported by reports from elsewhere, that to so drastically modify any material amount of soil that is not somehow physically segregated from its environment is something from difficult to impossible in a reasonably practical sense. Does that mean that we give up on blueberries? Not at all.

Blueberries have very shallow root systems, and are thus ideally suited for container growing. But, if we are going to grow them—or any perennial plant—in containers outdoors, there are some things we need to realize and allow for. Blueberries are famously cold-tolerant—one often sees remarks about this or that variety overwintering through -20° or even -30° (or sometimes even -40°); that’s swell, except those aren’t normally plants in containers, they’re plants with their roots snuggled into good old Mother Earth, and soil has quite a temperature-buffering value.

When plants are grown in containers, their root systems tend to wind around the inside of the pot near the walls of it, and so are almost in contact with the outside air: university studies have shown that the root balls of outdoor containerized plants reach very nearly the outdoor air temperature. That is the chief thing we need to take into account when considering growable cultivars. One source suggested as a rule of thumb subtracting 2 from one’s USDA Zone; if one is in Zone 6, by that rule, one should be looking for cultivars that are described as hardy down to at least USDA Zone 4. On the other hand, if one’s summers, while short, are quite warm, one also needs cultivars that are known to fruit well in hot weather.

One other option worth mentioning is the “buried container” approach, which is pretty much what it sounds like: you containerize a blueberry bush, then bury the container with its rim flush to the soil-top level (or bury first then containerize the plant); you could even bury a rather large container—some use old children’s lawn pools—and do several bushes in it. We prefer to do individual containers and keep them above ground for periodic freshening of the soil, but keep this alternative in mind.

There’s yet more jugglery about climate effects, for we in our region are pinched a bit at both ends of the season: we don’t want early ripeness, lest late-spring frosts damage the crop; but we don’t want terribly late ripeness, lest the berries not ripen before autumn frosts. (As noted, blueberries are quite cold-hardy—but that’s the plant, not the berries proper.) The old shibboleth about selecting late-harvest types to avoid damaging spring frosts is an error: while harvest dates for different blueberry cultivars can vary substantially, most cultivars actually bloom within about a week of one another, and it’s the blooms that get damaged by late spring (or early summer) frosts. Our late spring frosts can hit even in early June, but that’s about the latest; our fall frosts can arrive from late August on, but that’s not so important.

As the TV hucksters put it, “But wait! There’s more!” Blueberries come in five fundamental types (representing three main species); but only three of these groups are suitable for northern gardens. In the South you’ll see “rabbiteye” and “southern highbush” cultivars, neither of which is hardy in the north (they simply cannot survive temperatures below about 0 to -10 degrees). Of the other three—“northern highbush”, “half-high”, and “lowbush”—the so-called lowbush type is common in New England both wild and domesticated, but is typically notably smaller in growth, with less time having been put into developing specialized cultivars of them (indeed, they are rarely offered, when offered at all, by cultivar, but rather as either just “low bush” or sometimes as “Maine” blueberries). We can thus also omit those from consideration.

That leaves just the two types, the northern highbush type (often abbreviated to NHB) and the half-high type. The latter are the results of crosses between highbush and lowbush types. Each has its own growth style and climatic needs. Most of the blueberry cultivars you will see in seedsmen’s and nurseries’ catalogues—and on discussion forums—are of the “high-bush” type, which—as the name suggests—typically make relatively large, high bushes; the half-high types are, of course, shorter in growth.

Possibly either type would work in our climate. But NHB varieties, if grown in outdoors in containers, may not be sufficiently cold-hardy in our climate, because (as we said) the root ball is, for all practical purposes, exposed to the ambient air temperature. Half-high varieties are very cold-hardy (most down to Zone 3), reportedly have better “real blueberry” taste than NHB varieties (meaning, we take it, that they are less likely to have been bred chiefly or solely for that quintessentially American culinary desideratum, sweetness), are fairly productive, and are easily containerized.

Rather obviously, we reckon half-high types are the way to go, but we also look at some NHB varieties farther below.

(We notice that many discussions of blueberries, by both amateurs and professionals, seem not to grasp the existence half-high as a distinct type, and they are quite often discussed as if they were NHB varieties. Patriot is especially often cited or described as a NHB, but we have seen many others also so mistakenly identified. Caveat lector.)

Finally, but arguably first in importance: no matter whether it’s half-highs or NHBs, to select your varieties you really, really, really should identify a reputable nursery in more or less your climate zone—ideally not far from you—and get their advice. Really.

Half-High Blueberries

We started with a University of Minnesota list of the generally available half-high varieties, then dropped off those not hardy to Zone 3, those with relatively low productivity, those described as having a “mild” flavor, and those with disease susceptibilities (notably to “mummyberry”); what was left was these:

Reckoning that if variety A can pollinate variety B and variety B can pollinate variety C, then variety A can pollinate variety C (and always, of course, vice-versa), then each of those should be able to pollinate any of the others. Note that the “ripens” data above refers to fruit set: blooming, which is when pollination takes place, is at nearly the same time for all half-high blueberry plants, meaning cross-pollnation is easy.

There remain other half-high types, especially if your preferred nursery doesn’t have all of the types cited above. Included in this lot are Patriot and Bluetta (only hardy to Zone 5), Northsky and Northcountry (low productivity), and Northland (susceptible to mummyberry). There may be others yet, but that should be enough to be going on with. (Note that, as mentioned farther above, variety Patriot is often erroneously listed as a NHB.)

Note about those bush sizes and productivities: blueberries are slow-growing relative to other fruit plants, and will not reach mature size or maximum productivity for eight to ten years, even under optimal growing conditions. Of course, you usually start with stock that is anywhere from one to three years old. (But mature blueberry bushes can last 50 to 100 years.) So reckon by the smaller numbers for size and productivity in the early years.

High-Bush Blueberries

For the benefit of those who are in a more favorable climate than ours, here are a few notes on regular high-bush blueberry types.

There are many, many NHB varieties, and each has its partisans. We cannot see that there are any standout runaway favorites, so we will just name a few that seem to get mentioned often as great taste and good productivity. Note again that most NHBs are not as cold-hardy as half-high types, commonly being rated to Zone 4 or Zone 5—indeed, sometimes only to Zone 6. We remain convinced that half-highs are a better choice for northern gardeners, but it’s your garden….


Blueberry Culture

This discussion is focussed on container growing, but the applicability to direct in-ground or buried-container growing is simple enough. And remember again that virtually all blueberry types will be more productive if cross-fertilized, so grow at least two varieties.

Blueberry bushes are best initially planted out during their dormant season (which usually means March to April in our climate). You can get stock that is anywhere from 2 years old (usually bareroot) and on up (usually containerized). If you buy 2-year-old bareroot stock to start with, don’t let it bear fruit for the first couple of seasons, else the plant will go puny on you; 3-year-old and up container stock can be allowed to (and often will) set fruit in its first season. It costs a little more, but we think it’s a wiser investment to start out with the older, larger, healthier stock. And, needless to add, be sure to buy from a reputable specialist, preferably one fairly close by (to avoid shipping damage).

Soil

Use large pots, at least 15-inches in diameter—more if you’re doing highbush varieties. Those half-barrels one finds at hardware stores and the like make good containers. (Remember that blueberries are rather shallow-rooted.) They’re no longer really cheap, but they are sturdy and quite nice-looking.

Fill your containers with an appropriate planting medium for blueberries. The simplest thing is to use 100% peat moss; 3-cubic-foot bags are generally available at $10 to $15, and a good-size container will be filled with two bags with some left over. Peat moss has a natural pH of around 5.0, right in the ballpark for blueberries.

(A container 2' across and 1½' deep would require about 4.7 cubic feet to fill to the top—π x r2 is the surface area, and 3.14 square feet x 1½ feet depth is 4.7 cubic feet—so 6 cubic feet is more than enough—indeed, 3 bags is probably enough for two half-barrels.)

After filling the containers but before planting your bushes (one per container), water the peat very thoroughly, so that all of it is soaked through. Carefully plant the bush in the peat.

Growing

It is quite important to check the pH of your blueberry plants’ soil fairly frequently and reasonably accurately; one of those little probe-in-the-soil pH meters is a wise investment. If the pH gets up over 5 or so, it’s time to take action. (Blueberries can supposedly tolerate a pH up to 5.5, but we repeat that 4.5 to 4.8 is the optimum range.) One easy soil amendment is soil sulfur, but be aware that it is notoriously slow-acting (some say literally years). More useful, perhaps, is cottonseed meal. You can also try iron sulfate, which can be a bit pricey if you’re doing a lot of area, but should be no major wallet-smack if you’re just fine-tuning the soil for a few plants. Ammonium sulfate is not advised, despite some available recommendations of it (it turns toxic too easily).

Note that the acidifying effect of the original peat moss will gradually wear off after some years (typically 6 to 10 years); you may then need to re-pot the bush, or take to making heavier pH amendments, as circumstances suggest.

Remember again that blueberries are very shallow-rooted. Water frequently and heavily, but be sure the soil drains well so you have no standing water. (You did think to drill drainage holes in the bottom of the container, yes?) Figure on supplying 1 to 2 inches of water a week. The plants’ need for water is greatest from the time of fruit expansion through harvest. It is best to supply the water as steadily as possible—ideally daily—so that the fruit does not crack from uneven growth. Drip irrigation, if conveniently feasible, is a good approach.

In winter time, be sure to protect the blueberry bushes against wind damage and drying; one simple and effective method is wrapping them in burlap.

Last but by no means least, if you don’t protect your blueberries from pests, most especially birds, you simply won’t get any. Refer to the protection notes on the main “fruits” page of this site for details.

Fertilization

First warning: never use any fertlilizer containing nitrate on blueberries—it will probably kill them. Be sure to use only a fertilizer specific for high-acid plants, something like Camellia/Azalea/Rhodedendron food; check the label to assure that it indeed is specific to high-acid plants, and explicitly mentions blueberries somewhere, and that it has no nitrate in it.

Don’t fertilize your blueberries right away after initial transplanting. Instead, fertilize them lightly 4 to 6 weeks later. Use an appropriate high-acid-plants food at a rate of about one ounce per year of plant age, up to a maximum of 8 ounces per plant per year for mature plants. Divide the fertilizer into three portions and apply one portion around Valentines’ Day, one around Memorial Day, and the last around Labor Day.

Do not, though, apply water after early September unless the soil gets really dry: blueberries need to start “slowing down” to prepare for winter dormancy, and if you try to keep them going too long, they won’t be fully prepared and can suffer freeze damage.

Keep in mind that the water you use on blueberries may have an effect on the soil’s pH. If you find that your water is alkaline (test it—inexpensive pH test kits are readily available) and so could raise the soil pH, add some white vinegar to the water till its pH is a neutral 7; test to see how what proportion of vinegar you need to add, and invariably add it before watering the berry bushes. (Pure water is that neutral pH 7, but both municipal and well water can contain alkaline salts.)

Because blueberries need moisture close to the soil surface, it is important to mulch them well. Use 2 to 4 inches’ depth of bark mulch, acid compost, sawdust, grass clippings, or the like, and refresh the mulch covering every year or two.

Pruning

Like all bushes and trees, blueberry bushes are optimally productive when properly pruned for optimum growth. Since blueberries are slow-developing, it’s important that they be well established before one begins trying to maximize their fruit. That in turn means pruning vigorously when they are young to assure that they do not over-fruit, which would stunt their long-term growth.

How to prune blueberries.

Pruning should be an annual task, undertaken during the bush’s dormancy (hereabouts, usually March or April is best). Prune lightly the first two years to remove low branches, overlapping branches, and flower buds (so there will be no fruit). From the third year on, remove old canes and weak spindly shoots. Prune out overlapping canes and branches and head back very vigorous upright shoots to force branching at a lower level. Reduce very heavily branched canes by one-third.

On mature plants, remove older stems lacking in vigour. Remove thin, twiggy stems as well as any damaged or diseased shoots, crossing or horizontal shoots (marked a in the diagram at left) or stems close to the ground (b in the diagram). Cut back some branches to the base (c in the diagram) and others to strong upright shoots. Stems that fruited the previous year should be pruned to a low, strong-growing upward-facing bud or shoot. By the time you’re finished pruning an established bush, you should have cut out roughly 15 percent of the old growth.


Harvesting

Do not assume that when a blueberry colors up it is ripe and ready to pick. Blueberries need to “hang” for a bit after they color up, so that they can convert their starches to sugars (supermarket blueberries are not optimally sweet because they are picked too early). Picked berries do not continue to ripen off the plant.

As blueberries ripen on the bush, their flavor goes from tastless to bitter to tasteless tart to tart blueberry flavor to sweet blueberry flavor. You want to pick plump, full blueberries with a light gray-blue color—a berry with any hint of red is not yet fully ripened.

Blueberries hang on their bushes in bunches (rather like grapes). To easily harvest, hold your bucket or other container under a bunch, cup the bunch in your hand, then gently rub the bunch. If you are careful with the rubbing, the truly ripe berries will detach and fall into your bucket, while the yet-unripe ones will stay on the bush.

Here is some more detailed picking advice from the North Carolina Blueberries site. And here’s some more advice, from the “Farm Flavor” site.

Blueberries can be kept in a refrigerator for a week, possibly two weeks; but the wise move is to either consume them very soon after picking, or (better) freeze them for later use. Do not wash blueberries before refrigerating or freezing them, as that will make them mushy; clean them only immediately before eating or cooking them.


More

Relevant Links

Besides any links presented above on this page, the following ought to be especially helpful:

(And don’t forget that we have listings of nurseries on our suppliers page.)


Odds and Ends

https://agsci.colostate.edu/specialtycrops/blueberries-in-pots/

Biology

All the species whose common names in English include “blueberry” are in section Cyanococcus of the genus Vaccinium. Other sections in the genus include other wild shrubs producing edible berries such as cranberries, bilberries, and cowberries, many of which have blue berries that are similar in use and taste to blueberries. Moreover, their names in languages other than English often translate as, literally, “blueberry”, giving considerable scope for confusion of species. As it happens, what North Americans usually think of as “blueberries” are actually native only to North America; in other areas, our “blueberries” are usually called “bilberries”. The species of blueberies include:

History

Blueberries arose in North America, and were well known to native Americans as a valuable food. In modern times, the native species have been crossed and their growth habits modified and selected for, giving a great variety for the grower to choose from.

Envoi

Blueberries have lately been found to be jam-packed with health benefits, so many that it would be almost comic to list them.


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