Blueberry bush not producing berries: age, pruning, chill and pollination

Ask one question first: did the bush flower? If it did not, the cause is usually age, pruning, winter cold or shade. If it flowered and set no berries, look at pollination, a virus or birds. The set-up that prevents most of this is in the guide to growing blueberries in pots. If the problem is the foliage rather than the fruit, the yellow and red leaves page covers it.

No flowers at all

The plant is young

Oregon State Extension says a new northern highbush produces fruit for the first time one or two years after planting, and that the yield per plant in the ground climbs from 0.5 to 1.5 pounds in year two to 9 to 15 pounds at maturity. Maryland Extension, writing about pots, says a full crop takes about five years. A bush bought as a small plant last spring is on schedule if it has not fruited.

Some of the missing fruit is deliberate. Maryland Extension tells growers to pick the flowers off one-year-old plants in spring so the plant puts its energy into roots, and Oregon State warns that plants that put energy into fruit too young may not recover and keep yielding less than a well-pruned bush. If you or the nursery did this, there is nothing to fix.

The flower buds were pruned off

Oregon State Extension says blueberry flowers and fruit grow on one-year-old wood, and that in winter the flower buds are the fat buds at the tip of last year’s growth. Each one makes a flower cluster and no leaves. The small, scale-like buds lower on the shoot make leaves only.

So a tidy trim of the shoot tips in winter takes off the berries. Maine Extension, answering a grower whose mature bushes fruited poorly, points the other way as well: unpruned bushes become overgrown with old canes, and the fix is to remove the oldest canes at the base each year and keep the strongest shoots. Oregon State adds that pruning too severely lowers yield. The middle path is to cut out old and twiggy canes in winter, leave the good fruiting shoots alone, and look at the buds before you cut.

Repotting costs a crop too. Oregon State says to transplant blueberries of any age in late winter before bud break, and to remove another 25 to 30 percent of the flower buds to offset transplant shock. A plant moved into a bigger pot this year will fruit lightly.

The winter did not suit the variety

A dormant blueberry needs a stretch of cold to flower normally. Oregon State Extension gives the range as roughly 32 to 45 °F and puts the requirement at 200 to 300 hours for southern highbush and more than 800 hours for northern highbush. The University of Georgia gives about 800 to 1,000 chill units for northern highbush, 200 to 600 for southern highbush and 300 to 600 for rabbiteye. University of California Cooperative Extension warns that figures like these were mostly worked out in the eastern US and are only a rough guide for California, where some varieties do better than their numbers predict.

The mismatch runs both ways:

• A high-chill type in a mild climate. Northern highbush that falls short of its hours may flower poorly.
• A low-chill type in a cold climate. Oregon State says southern highbush blooms in late winter and its flowers are frequently damaged by frost, so it does not recommend them for the Pacific Northwest. Its home-garden guide says most or all of the flowers can be lost to frost in the Willamette Valley.

If you suspect frost took the buds, Oregon State suggests slicing flower buds lengthwise a few weeks after a cold event and looking for brown or dead tissue. The plant label or the nursery listing should say which type you have; if it does not, treat that as the first thing to find out.

Too little sun, or the wrong pH

Oregon State Extension says full sun is ideal and that blueberries tolerate partial shade, but yield and fruit quality may be lower. Maine Extension lists soil pH among the causes of poor fruit set, saying a pH that creeps too high will impact the plant’s health and productivity. Oregon State gives the range as 4.5 to 5.5. Pot mixes are covered in the container soil pH guide.

Flowers, but no berries

Bees did not reach the flowers

Maine Extension says blueberries are pollinated by bumblebees and honey bees, and that too few of them around the plants gives poor pollination. Its pointer for telling this apart: the same number of flowers as usual, but poor fruit set.

Weather matters. Michigan State University Extension says honey bees prefer warm, sunny conditions to forage, while bumble bees, especially native species, do not mind working under cloudy, cold or windy conditions. A cold, damp week at bloom can leave flowers unvisited. Oregon State says this does not commonly happen in Oregon, where many home gardens have enough bees, so it is worth weighing against your own bloom weather.

The pollinator guide for balconies covers what to plant to bring bees closer.

There is no partner variety

Most highbush types set some fruit alone, but not all, and partners improve it. Oregon State says northern highbush will fruit with self-pollination but cross-pollination improves berry size and quantity, and that half-high cultivars do not produce much fruit alone. University of Wisconsin Extension names St. Cloud and Polaris as the exceptions that do need another cultivar. Maryland Extension says the second variety can live in a separate container, and it should bloom at the same time. Oregon State adds that highbush cultivars ripen at different times but all bloom together, so any two will cross-pollinate.

Blueberry shock virus

Oregon State Extension describes blueberry shock as a pollen-borne virus, common in its region and carried from plant to plant by bees. An infected plant drops all its flowers the following season, and there is no fruit on the plant, or on the affected part of it, in the shock year. There is no control for the virus, but Oregon State says plants come back into full production after the shock year, and to prune hard the following winter because the plant will have produced a lot more growth than usual. A bush that bore well, dropped its flowers and then grew vigorously is a likely match, not a diagnosis.

Frost on open flowers

A frost after the flowers open can cost the crop, which is the risk Oregon State describes for early-blooming types. A pot can be moved on frost nights during bloom; the cold snap protection guide lists ways to do that.

Berries form, then vanish

Oregon State Extension says many species of birds feed on blueberries and can eat an entire crop if not controlled, either pecking at fruit before it is ripe or taking whole berries. Scare devices have limited effectiveness because birds adapt. The most effective control, it says, is light plastic overhead netting with a small mesh, on a frame, pinned to the soil so birds cannot find a way in. Maryland Extension suggests bird netting or floating row cover for containers. If you want to buy one, you can browse bird netting on Amazon.

Berries that die while still white are a different problem. Maine Extension points growers who see that to Penn State Extension’s page on mummy berry, a somewhat common blueberry disease.

What to check, in order

1. Did it flower? No: go to steps 2 to 5. Yes: go to step 6.
2. How old is it? Under two or three years from planting, or recently repotted, patience is the answer.
3. Look at last year’s shoots in winter. Fat tip buds are fruit. Do not cut them off.
4. Check the variety against your winters. The label should say northern highbush, southern highbush, half-high or rabbiteye.
5. Move the pot to full sun and test the mix pH (a soil pH test kit on Amazon works).
6. Check the bloom week. Cold and wet, no bees, a single plant, or a frost after flowers opened each have a different fix.
7. Net the ripening fruit.

Sources

• Growing blueberries in your home garden (EC 1304), Oregon State University Extension
• Blueberry cultivars for the Pacific Northwest (PNW 656), Oregon State University Extension
• Why are my mature highbush blueberry plants not producing berries?, University of Maine Cooperative Extension
• Blueberry production (Circular 1278), University of Georgia Extension
• Blueberries, University of California Cooperative Extension, Ventura County
• West central Michigan small fruit update, May 28, 2019, Michigan State University Extension
• Growing blueberries in containers, University of Maryland Extension
• Growing blueberries in containers, University of Wisconsin-Madison Division of Extension