Trees turn sunlight into food we can eat and drink. Our job as farmers is to do what we can to make that process go well.
Right now, late fall/early winter, we’re in pruning season. Rolling into this time of year, I spend a lot of time thinking about sunlight. This year it struck me that having a quantifiable way of tracking light distribution in the trees is a good idea.
You can tell with your eyes when part of a plant isn’t getting enough light. They look spindly and weak or “leggy”. Leggy is the first step before trees get what we call “Shade Disease”, where they start having branch dieback. What we want to come up with is a way to tell when the trees are struggling to get enough light inside the tree, before they get leggy. The picture below shows a leggy (top) branch next to a non-leggy (bottom) one, the space between the leaves is 9% longer on the leggy branch.
Figure 1: top leggy bottom not leggy, from the same tree
Legginess happens when plants don’t get enough light. On the plant physiology level, there isn’t enough light to break down the plant hormone that regulates cell elongation, called auxin. Lots of auxin makes for long cells; little auxin makes for compact cells. Auxin is the hormone that makes plants grow towards light.
What to do about it
When there’s too much shade, not only do parts of the tree get leggy, but those shaded parts don’t produce very many fruit buds. The fruit buds that are produced make small fruit that doesn’t taste very good. We want BIG fruit that tastes good. To make the leggy go away, we cut out the bits of tree that throw shade.
Four kinds of big wood cause us the most shade problems: heads, hangers, forks and posts.
Figure 2: Normal tree
Figure 3: Tree with head
Figure 4: Tree with hanger
Figure 5: Tree with fork
Figure 6: Tree with post
Not enough shade (the tree isn’t big enough) causes a different set of problems. Unprotected fruit gets sunburnt. There isn’t enough fruit, because there aren’t enough places for it to hang. To make trees bigger, you can force growth by doing things like adding extra fertilizer and making invigorating pruning cuts. Invigorating cuts look like this (but on the tree):
Figure 7: heading cut
Above is a heading cut. You cut the tip of an upright growing branch so that the stick is about 18 inches long. It’s pointless to try and invigorate a branch that is thinner than a pencil: any heading cuts will fail to drive strong growth because the wood is too weak. In the picture above, you’d either make the heading cut at 12 inches or cut back to older wood, cutting this branch out completely. Heading cuts are about managing auxin, Pen State has a good write up.
There is a guy named Padovan who said that the ratios 3:4 and 1:7 are approaching the limits of human perception.
By his logic, you can’t tell the difference between a 2 ½ inch leaf blade and a 2 ¾ inch leaf blade. Without effort, I think Padovan’s right. I have a counter strategy for these limits: a tape measure (and to make things even higher tech) a pencil and notebook. Take that J limits of human perception.
I measured leaves from two different Bartlett pear orchards. Orchard A has big freestanding trees that have some legginess in the centers of the trees. Field B has smaller trees where the fruit had some sunburn. I measured leaf blade lengths, along the mid vein, from leaves on both the inside and outside of the tree.
Figure 8: State of the art Measurement System
Figure 9: Leaves from the outside of the tree on left, inside on right'
Field A (leggy) Field B (too much sun) Difference
Inside leaves 3.05-inch average blade length 2.75-inch average blade length 17%
Outside leaves 2.65-inch average blade length 2.60-inch average blade length 4%
If 17% is too much of a difference and 4% is too little, 10% should be the happy middle. I measured the differences in percentage terms because the overall leaf size isn’t determined by only sunlight.
The University of California has a much fancier way to measure the same light interception issue we’re goofing around with. They have attached a light measuring bar to a UTV. I’m sure it works great and I’m sure it costs more than my tape measure.
Figure 10: the UC’s Mule type UTV with big light meter
The waiting game
We’ve made our light management decisions for this year. The hard part is always actually going out and doing it. The frustrating part is waiting for a year to see if what we have done helps anything. The meaningful part is that the trees are healthy and productive. But the most important part is that you have the opportunity to enjoy delicious fruit and, of course, the cider we make from that fruit.
While doing this I figured out that taking pictures of trees is hard. It is tough to take a picture where what you are trying to point out doesn’t get lost in the background of the rest of the tree, which looks just like what you are trying to point out, but slightly different. If anyone has any tricks, please let me know.