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02/14/2009: "Blueberry Pruning"

Yesterday I was privileged to spend the day with my friend Andrea at her farm learning about pruning blueberries. Andrea and her partner run a thriving farm that includes 400 blueberry bushes on a half acre, and she grows fantastic blueberries! Pruning is of course an essential skill for managing the berries for optimum production, and I was eager to learn how she does it.

The first thing you have to know about blueberry pruning is timing. It is best to do the pruning at a specific time, shortly after the buds have emerged and become visible. In the Willamette Valley the buds usually emerge sometime in February; early-ripening varieties emerge sooner than later-ripening varieties. The reason that the buds are important is that bud size and quantity are part of how you evaluate each candidate branch for pruning.

Andrea explained that blueberry branches (or, more accurately, blueberry canes) can be grouped into roughly three categories. The first category is new growth, pictured above in the lead-in photo. New growth is characterized by smooth orange bark and a lack of lateral branches. Generally speaking, you always leave new growth alone and don't prune it.



The next category of canes is a mature, producing branch like the one pictured immediately above. Note that this cane still has orange bark, but now there are laterals that look healthy and vigorous. These canes, too, should be kept and not cut; these will produce this year's crop of fat, large blueberries! One thing that Andrea told me that surprised me is that she has found that it is best to not thin a plant, even if it has a riot of these producing canes all bunched together; it just means that you will get a lot of berries from this plant. This is in contrast to the pruning wisdom for fruit trees, in which you prune even producing branches so as to let in light.



Finally, there are canes that are past their prime. One typical example is pictured above; an older cane may be identified by its irregular, orange-grey bark, lack of buds (or smaller buds relative to the younger branches), frost damage on the tips of the laterals, and general twiggy-ness. Canes like these are prime candidates for pruning; they may produce some berries if you leave them, but the berries will be small. It is better to direct the plant's growth and production into the younger canes, which will produce better berries.

I wanted to highlight the importance of relative bud sizes, since that is a big clue to vigor. Every plant will be at slightly different stages in its growth, and so you can't give an absolute standard for what constitutes a vigorous bud. However, on a single plant you can make decisions based on relative bud size, and that will be a big clue to how well a cane will produce in the coming year. A productive cane will have many consistently-sized buds, and they will occur on every lateral and twig. An older cane will have missing buds and tiny buds relative to the rest of the bush.

Andrea says that she will prune up to about one-fifth of the main canes emerging from the ground. You're always walking a fine line when pruning: you want to stimulate new growth by removing older canes, but you also want to get berries from a mature bush! Note that you nearly always end up cutting one or two old canes emerging from the root mass; what happens is that you'll notice a cane past its prime, so you mentally mark it for cutting while you look at the other branches connected to that same stem. If the other branches are marginal (and they often are), then you might as well prune down to the ground on that cane. You should also evaluate the vigor of canes emerging from the root ball; if a new cane looks small and weak (more like a thin twig than a pencil-sized shoot), cut it back to the ground. Cutting a whole cane like this helps encourage the plant to grow an entirely new strong shoot from the root mass, which will eventually grow into a whole new producing cane.

The procedure that Andrea says works well for her is to first go down a row of bushes with long-handled loppers; the goal here is to identify the obvious candidates for cutting down at ground level. Then she will go back with hand pruners to look for smaller branches that need pruning. When cutting back with the hand pruners it often happens that there is new growth further away from the end (closer to the roots); in this case you cut the cane back to the first strong new shoot, leaving a quarter inch. If you have two people working it also helps to have one person review the other person's initial cutting; one can use the loppers, and then the second person using the hand pruners might notice something that the first person missed.

Note that the foregoing discussion applies to mature blueberry plants that have been in the ground at least 4 or 5 years. The procedure for new plants and younger plants is somewhat different. The first year you should remove fruiting buds shortly after planting to help the plant develop a good root system (you can wait till you see actual flowers before doing this). The second year you select the biggest and best 4 or 5 canes and prune out the rest. The third year you still might need to prune out some of the smaller new canes and again just leave the best new canes. After that, prune as needed using the method originally outlined above.

Andrea sells fresh blueberries in season at the Corvallis Farmer's Market, along with a wide selection of scrumptious jams and jellies (she told me that lavender jelly is a local favorite! who knew?). Be sure to look for her Kings Valley Gardens booth when you're at the market this summer!