Tuesday, April 26, 2016

Growing Bur Oak Trees (Quercus macrocarpa)

Oak trees (Quercus spp.) have a long history of being used as a food source. Acorns are produced in large amounts by older trees, usually in alternating years. This fruiting strategy, called "masting", helps to prevent the population of seed predators from overwhelming the tree and killing every acorn every year.

1. Bur Oak (Q. macrocarpa)
acorn cap.
I've made an occasional habit of tasting acorns over the years. They routinely have the strongly bitter taste of tannic acid, but occasionally I find one that doesn't taste so bad. When I was in Texas for my oldest brother's funeral (November 2014), I found a nice Bur Oak (Q. macrocarpa) tree on a walk. The tree had a few acorns (in huge caps) on its branches, but the ground beneath was littered with them in various conditions. I broke open one intact looking acorn with my shoe. I noted the nut-meat looked in moist and in good shape, so I picked up one of the pieces and tasted it. The juice tasted distinctly sweet, with no bitter aftertaste.

I was astounded by this and collected a large handful of the best looking acorns I could find. I then continued with my walk. Back at my parents' house, I put the acorns in a resealable plastic bag and put them with my things. When I returned home to Minnesota, I added some moisture to the bag and put it into a basement micro-fridge I've dedicated for botanical specimen storage.

2. Bur Oak acorn; partially
dissected and sprouting.
(USA quarter for scale.)
A year and some four months later (March 2016), I decided to try growing the acorns. Winter here was wrapping up and I was already growing a Chestnut (Castanea spp.) tree in my basement. I took the acorns out of the fridge to examine them. I had previously extracted one nut-meat from its shell and found it had begun to mold. I discarded this one and opened up a few others. Each acorn with an intact shell survived storage without molding.

3. Germinating acorn.
I left the acorns (shelled, partially shelled, and intact) in a dark, but room-temperature spot and waited. By late in March, I noticed a small swelling forming on one acorn (images 2 & 3). At first I wasn't sure if I was actually seeing a change in the acorn, but once I was sure that I was, I removed the remaining membrane layer from around the germinating acorn embryo to get a better view of it.

4. Germinating naked acorns.
I had fully extracted two of the acorn nut-meats during my examination for mold. Once I noted the one acorn germinating, I took a look at these two. Both were obviously germinating too. One (image 4, left) had split in two, revealing the anatomy of the embryonic oak tree and its two cotyledons. To stabilize the baby tree, and partly mimic the acorn shell I had removed, I secured the cotyledons together with some small cable-ties. I secured the other naked acorn in the same way.

5. Stem growth.
Three of the germinating acorns are planted into air-pruning planters (just like the Chestnut trees). The acorns produced a stout taproot which extended to the bottom of the planter well before the stem had grown an inch or unfolded its first leaf. After stalling for a while at less than an inch tall, the stem started elongating rapidly.

6. Air-pruning in process.
The taproots were too thick to pass through the screen material, so the either terminated on impact or grew sideways for a bit before catching on the screen and terminating (or as in image 6, growing through the screen anyway). Many side roots then grew, soon after poking through the bottom of the planter.

As long as the large and tasty nut remains attached to the seedling, it will be a strong incentive to squirrel predation. I suspect I'll have to keep the baby trees in protective custody until at least summer of 2017. One tree will probably go into the back corner of my yard. (There are some very large Buckthorn specimens there to clear out first.) The others will probably go to my wife's family farm, joining a pair of the chestnut trees to form the start of a nut orchard.

7. Bur Oak range, source, and me.
This species of Oak is native to a large swath of the central part of this country (figure 7). The specimens at the extreme southern part of the range tend to produce larger acorns than those at more northern locations. It seems that this gradient is in part due to the history of glaciation on this continent and the improved ability of individuals with smaller acorns to travel north into areas where it was previously extirpated, though the details have not been fully worked out.

If these very southern trees manage to survive the cooler climate here in Minnesota, they should start producing their first acorns in somewhere from 10 to 35 years (depending on who you talk to). Hopefully I'll still be around (at somewhere between 48 and the ripe old age of 73) to collect some acorns and make a few meals of them. For the purposes of the ongoing domestication of the tree as a food source, I hope I'll also be able to convince the young'uns around at that time to plant some acorns from the best of my trees so as to grow their own.