Leaf:fruit balance in blueberry

by Becky Sideman, UNH Extension Professor & Specialist, Vegetable & Berry Crops

Every year around this time, one of the most common questions I get is about blueberry fruit load. It is very typical for some branches, or in some cases, entire bushes, to have too many fruit and not enough leaves.

Too few leaves! After bloom, when fruits have set and the new growth is taking off, it usually becomes apparent that some branches do not have very many leaves. It is not that the leaves fell off – they never formed to start with. In some cases, the entire bush may be affected, but it’s usually only select canes. Canes or bushes that have very few leaves tend to have abnormally heavy fruit set, and those with lots of leaves have comparatively few fruits.

bbryimbalance2

This bush has a very heavy fruit load for the amount of leaves that it has produced.

What’s going on? During the growing season, the blueberry bush forms buds for the next year – in spring and early summer, it puts out new shoots and on these shoots it produces leaf buds; later in summer it produces fruit buds at the tips of those new shoots. Heavy fruit set this year means that the bush had sufficient energy to produce a lot of fruit buds last year. This may have been because yields were light last year (at least on that branch) so they weren’t spending energy ripening fruit. Many plants, including blueberry bushes, preferentially pour their energy into ripening fruit rather than leaves. This is why we often see branches that have either lots of fruit OR lots of leaves, but not both.

One of the goals of winter pruning is to bring fruit and leaf buds into balance on the bush. Heavier pruning reduces fruit but increases leaf canopy the following year. Therefore, bushes that were pruned more heavily do not generally show this problem as much as those that weren’t pruned or that were pruned lightly. The goal is therefore to balance removing enough wood to stimulate new growth (future yields) with keeping fruit buds (current yields).

 Another complicating factor is that winter injury can weaken canes slightly without actually killing buds. Weaker canes have less energy overall, so this problem may be more apparent on exposed canes or older canes that were more susceptible to winter injury. The same goes for canes plagued by other problems – diseases, insect pressure, etc.

Why is this a problem? A cane or bush without leaves cannot produce the energy it needs in order to ripen the berries. So even though there are many berries, the berries will likely be undersized and undersweet, if they ripen at all. Further, the bush will not have excess energy to pour into either 1) next years’ fruit buds or 2) the root system, strengthening it for the coming winter. This will stress the plant, or at least the cane in question.

What to do? The objective is to ripen and harvest as much of the current fruit set as possible. A couple of suggestions that may help:

 Fertilization: A soil application before July 1 of 10-15 lbs actual nitrogen per acre may help to stimulate leaf growth. Bill Lord, UNH fruit specialist emeritus, says that he has had success stimulating leaf cover with two foliar sprays, 7-10 days apart, of low-biuret (feed-grade) urea at a rate of 3 lbs/100 gallons of water. Caution: Foliar sprays should not be applied the day after a long period of rain or if air temperatures are over 80F, to avoid burning plants. Research shows that soil fertilization is more efficient, but rain or irrigation is required to get the fertilizer in the root zone. Avoid fertilizing after mid-July, since it will stimulate late-season growth that can make the plant more susceptible to winterkill.

 Fruit removal: Depending on the severity of the problem, you may want to manually strip fruit or do some selective pruning to reduce the fruit load. This should be done ASAP, prior to mid-July. This is more important for young bushes (<3 years), where stress may limit future plant growth. Fruit removal will increase the quality of the berries you do have. It will also reduce stress on the plant, and increase potential yields next year. Judging how much is ‘too heavy’ is subjective, but it’s probably too heavy if you are already seeing that berries are smaller than you would expect (or that are smaller than those on other canes). You will have to balance the labor required to remove fruit with potential benefits in yield, both this year and next. Even though you are cutting off fruit, these fruits will not likely ripen properly, and so you will not be losing profits. If not removed now, these canes should certainly be removed in the next pruning cycle. Caution: IF you decide to prune now, make sure to sanitize shears frequently (i.e. between bushes) to prevent transmission of viruses or other diseases.

A caveat: Having too few leaves is a sign that a bush is experiencing any number of stresses. A couple of such stresses are winter injury or very heavy fruit load, but others include nutritional problems, insects, diseases, improper pH, etc. Trying to determine the source of the underlying stress is important to help manage it in future years.

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