At a recent meeting of the Yew Dell Botanical Gardens board of directors, our board president closed the meeting as he often does, by citing an inspirational or thought-provoking, garden-related quote. For this particular instance he decided to go with an oft-cited Chinese proverb ‘All gardeners know better than other gardeners.”
Hard to argue with this one.
As a group, gardeners tend to have opinions that are more strongly held than sometimes the data might support. But to be fair, the acquisition of gardening knowledge is a bit of an osmotic process — a slow absorption of many tiny bits of information. Sometimes it can be hard to know when you actually know something.
And so it was perfect timing when a regular reader of this column emailed me with a question, or more accurately, a suggested topic. “I’ve always wondered about the unpredictability of trees putting out fruit.”
So with that lead in, I’ll tell you what I think I might possibly know about the topic.
Why do plants produce fruit?
Raylan Mann competed in the Watermelon Eating contest at the Bullitt County Fair Wednesday afternoon. June 18, 2025
All plants produce fruit of some kind. Might be a tiny, dried up little poppy seed, a wispy affair that floats on the breeze, or a giant, juicy, red watermelon. It’s how plants generally reproduce themselves. The melding of one plant’s pollen with the stigmatic apparatus of another provides the genetic variation that makes possible the magic of evolution through natural selection. It’s pretty important stuff so you’d think we’d have a better understanding of it.
And for some parts, we do have a pretty solid base of knowledge. But not all.
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There are many general factors that influence how much fruit/seed is produced by any particular plant or group of plants. But for this discussion we’ll gloss over things like overall plant health, insect and disease problems, and having appropriate pollinators around at the right time. We’ll just consider reasonably healthy plants in reasonably typical growing seasons and without the influence of inane social media memes.
What are alternate year bearing trees?
The dogwoods in bloom in Audubon Park. April 17, 2015
Some fruit trees are notorious for producing a big crop one year and a much smaller crop the next. This pattern is generally thought to be due to a variation in what us plant science dweebs refer to as the carbon/nitrogen balance. The general scenario is that a large fruit crop creates a major draw down of carbon reserves in the plant — carbon being the primary structural element in plant tissue. An apple tree with a seriously depleted carbon level (too low a C/N ratio) will produce fewer flowers and subsequently less fruit.
To deal with alternate bearing tendencies in some fruit trees, crops are sometimes thinned — either mechanically or chemically — to reduce fruit load in year one and leave more carbon for year two.
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And less you think this alternate year thing is restricted to traditional culinary fruit trees, the same can be seen in dogwoods. Our native flowering dogwood (Cornus florida) is a reliable flowerer/fruiter one year to the next whereas many plants of its Asian cousin, the kousa dogwood (Cornus kousa,) can show strong alternate bearing characteristics.
So far so good. At least we know something about that one . . .
Why do some trees drop immature, unripe fruit?
Some ornamental and commercially-grown fruit trees, such as this crabapple, exhibit alternate year bearing, one heavy crop followed by a much lighter crop the next year. This is caused by fluctuating levels of carbon and nitrogen in the plant tissue.
A slightly different case, this one occurs when trees set relatively consistent quantities of fruit but early in the crop’s development, some portion of that crop will spontaneously abort. If you take a late spring walk through a park with a good population of black walnuts (Juglans nigra), you’ll notice some percentage of the nut crop dropping on your head. No need to invest in a hard hat — these are not full-sized walnuts. They’re about one third the size of a fully formed walnut.
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In the case of immature fruit drop as in our black walnuts, trees have an internal mechanism that senses fruit load early in the season and adjusts that load based on internal nutrition balance. It’s an amazing bit of feedback loop biochemistry but essentially works just like the grower’s mechanical thinning in an apple orchard. It’s just that in the case of the black walnut, the decision on what and how much to reduce the fruit load is left up to the tree.
What is a mast year? How it happens
Production of hard mast crops (acorns, beech nuts, etc.) varies tremendously from one year to the next but the underlying causal factors are not well understood.
The term mast refers to tree crops of fruit that serve as a primary food source for wildlife. But in practice the term is most often used to describe what we call hard mast — the acorns, nuts, etc. produced by oaks, beeches and other related species.
Mast crop production has been a biological enigma for millennia. The size/volume of a mast crop from one year to the next is one of the great mysteries of tree science. None other than Leonardo DaVinci and Archimedes are said to have spent more than a lunch hour pondering and philosophizing about the underlying causes of annual variation in mast production.
Now, you can get online and find all sorts of hypotheses on what controls patterns of mast crop production. But the gold standard in a hypothesis is, can it be used to predict and describe what actually happens in real life. The answer to that one is a great big, nope! What we do know about mast production is that it is infuriatingly unpredictable. Break your ankle on a zillion acorns rolling around the driveway one fall only to have next to nothing another year.
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When it comes to knowing or not knowing, mast crop production pattern is one we definitely don’t know, unless of course, there’s another gardener out there who knows.
Paul Cappiello is the executive director at Yew Dell Botanical Gardens, 6220 Old Lagrange Road, yewdellgardens.org.
This article originally appeared on Louisville Courier Journal: Why are there no nuts on my tree? What to know about alternate bearing trees