Recently, I was on the phone with a mentor and we were discussing hedgerows (my new pet project, aside from brewing all sorts of alcohol). With some of the species I mentioned, I was told that livestock would eat them down to nothing and render the hedgerow useless. After having a few tree species rejected, I frustratingly asked: “What if I planted my hedgerows with invasives like multi-flora rose, then?!”

Without any hesitation, my mentor said: “Invasives like multi-flora rose are very delicious to many animals, like my goats.  You might be suggesting invasive plants for your hedgerow because they are vigorous and seem to outcompete everything else, but try to think about vigor from another perspective. If plants with high vigor are also the most sought after by animals, don’t you think that vigor might be an evolutionary trait to survive browse?”

This is the first time I’ve heard this perspective on invasives and I’ve really enjoyed thinking about it. After some minimal research, I found out that the subject is still debated today by ecologists as the “plant vigor hypothesis.” Generally speaking, vigorous plants have higher nutrient densities than non-vigorous plants, so herbivores are more prone to eat them. However! If the very vigorous cultivars are able to put on a bunch of girth, many herbivores aren’t able to eat the whole thing because of their jaw size.

This, of course, has got me thinking about apples. Here’s why.

In many essays on this blog, I’ve talked about how I consider many cider and heirloom cultivars to be very vigorous as compared to most of the grocery store cultivars. Vigorous cultivars are harder to prune, occupy more space (so less trees per acre), have issues with vegetative vs fruit bud proportions, etc. In general, they are harder to grow.  After reading more about this “plant vigor hypothesis,” I wonder if there is a connection between vigor and nutrient density in apples cultivars?

From an evolutionary standpoint, a correlation between vigor and nutrient density makes sense to me. Many wild crab apples in the US have much higher tannins (aka polyphenols, which =nutrition density) than cultivated varieties. This is from the many lifetimes spent co-evolving with insects and herbivores who are trying to eat them. From observing crabapples in the “wild” and planted in landscapes, it seems as if many trees have low vigor and perhaps this is because they have evolved to have an unpalatable deterrence for animals and humans alike?

In hard cider, many of the wild crabs are too much for our palates to handle and though very nutritious, they will cause a harsh and likely negative consumer experience. So what have we done? Over time, cider drinkers/makers/apple growers have selected cultivars to grow which are palatable to the consumer, but also contain enough tannins (or polyphenols, or natural defense) to give the cider some substance.  Could it be that in selecting not-so-astringent apple cultivars for eating/drinking, we’re unknowingly selecting for more tree vigor? If the apple cultivar hasn’t evolved enough to deter herbivores through astringent taste, then do genetics dictate that it must rely on vigor to survive? 

These sorts of questions make me excited and I’ll keep learning about these processes in order to try and uncover different management ideas that don’t involve regulating vigor through the use of dwarfing rootstocks, black magic hormonal potions like Apogee (which converts vegetative buds into fruiting buds), and planting in light soils. All of those management aspects, I suspect, are making the vigorous cultivars less vigorous/more fibrous/less nutrient dense.

Thoughts to be continued, but in the meantime here are a few off the top of my head:

Thought 1: Pruning extremely vigorous varieties like an herbivore in order to get faster fruit set?

Thought 2: Continuing to fruit explore to find mixes of wild x cultivated which hit high nutrient densities, palatability, and lower vigor.  (I’m writing a fruit exploring book about how to do this at the moment)

Thought 3: Making crabapples a significant part of my home breeding program.