By Larry Hodgson
Question: I started a home orchard 12 years ago and one thing puzzles me. Most of my apple trees produce a good crop every year, but some, especially ‘Honeycrisp’, produce a massive amount of fruit one year, but practically nothing the following year. They keep up this two-year pattern over time, even when weather conditions vary. What’s going on?
Answer: This trait is called “biennial bearing” or “alternate bearing” and is well known in pome fruits (apples and pears) and also tropical fruits like mangoes, coffee plants and avocados and subtropical ones like olives. It can also occur in other fruits, like apricots, plums, cranberries, blueberries and citrus, but to a lesser extent. It’s most common in wild trees: it’s the norm for many species. However, many popular cultivars will be annual bearers when conditions are good. Indeed, the fact that they tend to produce a good crop every year is probably one of the reasons they became popular! However, there are still quite a few cultivars that tend to be biennial bearing.
Other factors to consider in biennial bearing is that an overly heavy crop in the on-year can cause branches to break off under the weight of the fruit. Also, the fruit may be smaller during a heavy year, although not all biennial bearers seem to have that problem.
What Causes It
The cause of biennial bearing is believed to be mostly hormonal. During a year of heavy bearing, plant hormones (mostly gibberellins) are produced in large quantities by the seeds in the developping fruits. This excess causes the tree to produce fewer flowers on those branches the following year. Another theory is that it is caused when a heavy crop uses up the tree’s carbohydrate reserves, leaving it weakened until it rebuilds its reserves during the off year.
Even trees that are normally annual bearers can be “shocked into” a biennial habit when something happens to their crop. When a hard spring frost or a severe drought kills off most of the fruit in one year, that can lead to massive blooming and fruiting the following year. And the year following the huge crop can be a poor one, setting off a biennial bearing habit where there wasn’t one before.
Also, trees growing in poor soil are more likely to be biennial bearing than those growing in rich ones. They seem to need two years of “mineral gathering” before they can produce a decent crop.
What to Do?
First, you can just accept biennial bearing as being normal for that variety and expect a heavy crop one year out of two. Since you have multiple trees and they’re unlikely to all have the same on-year, that often balances out nicely.
You can also try to avoid cultivars reputed for their biennial bearing. In apples, besides ‘Honeycrisp’, ‘Belle de Boskoop’, ‘Cox’s Orange’, ‘Fuji’, ‘Golden Delicious’, ‘Macou’, ‘Mutsu’, ‘Pacific Rose’, ‘Reine des Reinettes’, ‘Starkrimson’ and ‘York Imperial’ are considered to be among the most serious biennial bearers.
Also, maintain good soil fertility. Fertilize annually with a slow-release organic fertilizer, following the guidelines on the label. (Don’t overfertilize, that just reduces fruit production!) Also, cover the soil around the trees with 3 inches (7.5 cm) of mulch rather than letting grasses and weeds take over and “steal” their minerals.
And do water thoroughly during periods of severe drought.
Professional orchardists often spray their trees with hormone sprays like Fruitone and Ethrel that reduce the number of fruits that year, resulting in better fruiting the following one. These are not necessarily available to home gardeners and must be applied carefully, as poorly timed applications or excessive ones can cause the entire crop to abort. Always read the label for recommendations.
For the home gardener, hand thinning is probably the best solution. You can remove half or even three quarters of the flower buds, flowers or young fruits in a heavy year, resulting in a more moderate harvest that season and plentiful bloom the next one. Some gardeners mark half the branches with tape and remove all the buds or blooms from them one year, then do the same to the unmarked branches the next.
For truly biennial bearing varieties that you want to “annualize”, hand thinning will be something you may simply have to get used to doing every year!
Top photo: tristana, depositphotos