Most plants can’t make use of anywhere near the full intensity of summer sunlight. Perhaps you knew that, but I didn’t. Depending on the plant and the conditions, photosynthesis maxes out at as little as one-seventh or -eighth of the intensity of noon-day sunlight (at my latitude). Anything beyond that is wasted.
(Note that some plants (e.g. tomato) show multiple different estimates above, so the measurements may (e.g.) depend on species and exact conditions and who knows what. The asterisk in the title, by contrast, denotes significant further uncertainty in my part of the process, which was to boil all those estimates down to a single scale of watts-per-square-meter. Plus, noon where, exactly? As I said, some significant uncertainty exists.)
Finally, I now grasp the logic of trellising plants in the back-yard garden.
Before I understood this, I never quite understood the mania for growing everything in the home garden vertically. The total amount of sun energy falling on a given plot of ground is fixed. Given that, I figured that growing plants on a tall structure was a straight-up case of robbing Peter to pay Paul. Plants on the trellis simply rob the sunlight from the shaded area behind the trellis. I thought the net effect had to be close to zero.
But that’s wrong. When the sun is high in the sky, the energy intensity of the resulting sunlight is in excess of what many plants can use. Spreading the crop leaf surface vertically reduces the intensity of sunlight per square foot of leaf area. This results in less light energy wasted by exceeding the plant’s photosynthesis energy-saturation threshold. The result is that much more of impinging solar energy is used for photosynthesis. If there are no other limitations on growth, this should materially improve yield per square foot of ground.
Restated: Trellising improves the efficiency with which your garden captures sun energy. Not necessarily by shading areas not used for growing (e.g., paths in your garden), though that can be true, but mainly because a vertical wall of green captures mid-day summer sun energy more efficiently than a flat bed of green. So that if a vertical trellis and a flat bed both intercept the same amount of solar energy (e.g., the trellis’ shadow exactly fills the underlying bed), the trellised plant converts more of the noon-ish sunlight into photosynthetic activity.
Conclusion, or final restatement: The upshot is that trellising isn’t simply a neater way to grow garden plants. It’s a significantly more efficient way to convert the energy in sunlight into food, relative to growing on flat ground. That, because for most plants of interest, photosynthesis maxes out at light intensities far lower than that of mid-day summer sunlight shining on flat ground. For a given amount of sunlight, trellising gives you more green area at or below the threshold, above which additional light intensity is wasted.
Details follow.
Green plants.
Source: American Chemical Society
They only have one job to do. Their task is to harness the energy in sunlight, thus making life on Earth possible.
And they suck at it. They harness only a tiny fraction of the available energy in sunlight. In the field, they typically manage to use a fraction of a percent of the energy in sunlight. This is vastly less efficient than, say, solar panels, where typically 15% of the energy in sunlight is converted to electrical energy.
Source: Wikipedia.
You have to wonder what God was thinking, making this the basis for all life. Does this mean it’s fundamentally OK to be a slacker?
But here’s my key point: It’s not that they are uniformly inefficient at all levels of light intensity. Instead, the typical garden plant’s photosynthesis processes can’t handle anything even close to the intensity of summer noon sunlight at my latitude. Noon-day summer sun (around 850 watts/square meter, in Northern Virginia) provides somewhere between three to seven times as much energy as plants can harness. Depending on the plant and the conditions, photosynthesis peaks at a light intensity as low as one-seventh of typical noon-day sun in my area. The rest of the energy is purposefully wasted by the plant. It converts anything beyond that to heat, to avoid overloading and damaging its photosynthetic organelles.
Source: Weebly.com. Annotations in red are mine.
In the experimental data graphed above, note that quadrupling the intensity of light beyond 127 watts/square meter resulted in no additional photosynthesis. So, that plant is able to use about one-seventh of the full strength of noon summer sun, at my latitude. And, while that fraction varies depending on the plant and conditions, you won’t see measured values that exceed one-quarter to one-third of the intensity of summer noon sunlight. Edit: See first table.
The key takeaway for the backyard gardener is this: A single layer of green leaves, arranged horizontally (like turf grass) is far less efficient at capturing the sun’s energy than the same green area, arranged vertically (like a tree).
That’s because, in the summer, at mid-latitudes, the average intensity of sunlight striking the vertical surface is far lower than that striking the horizontal surface. Below I show the results for the same set of solar panels, flat on the ground (0 degrees tilt) versus vertical (90 degrees tilt), from the Department of Energy PVWATTS calculator. At mid-summer, a one-square-meter surface receives about three times as much solar energy over the course of a day when oriented horizontally, compared to vertically.
Source: Department of Energy PVWATTS calculator.
Conclusion: Trellising is not simply a case of robbing Peter to pay Paul.
If you look up why you should trellis (e.g.) vines in your back-yard garden, rather than let them ramble on the ground, you’ll see every explanation except that. E.g., it’s neater, it keeps the fruit off the ground, it allows for air circulation, it “saves space” in the garden.
All of which is true. But none of which is really key.
For the backyard gardener, the key point is that trellising — growing plants vertically — dilutes mid-day sunlight down to a strength closer to what green plants can handle. And so improves the efficiency with which you use the sunlight falling on your garden plot.
Source: My garden. The bamboo towers above are for squash plants (tromboncini and Guinea Bean) to climb on.
The low saturation point for photosynthesis has further implications. Mainly, the intensity of sunlight, hour-by-hour, is NOT a good proxy for the amount of photosynthesis occurring at those times. The amount of photosynthesis, for plants covering the (horizontal) ground surface, is highly truncated, like so:
Where, exactly, that horizontal orange bar falls will depend on the plant and the conditions (e.g., temperature, soil moisture, C02 concentration in the air). But it falls way, way below the the blue peak of summer sunlight intensity. Above, everything between the orange and blue lines amounts to wasted sunlight.
By contrast, the graph just above shows photosynthesis, for the same garden area, for the same plant, but located on an isolated trellis (not casting shadow on other plants). The area under the orange curve represents the amount of photosynthesis over the course of a day. If the plants being trellised can handle no more than 150 watts/square meter, growing them on a trellis whose height is 4x the width of the garden bed triples the amount of photosynthesis occurring over the course of a day, compared to growing them flat on the ground.
Three times the photosynthesis occurring. On the same plot of land. Yield per square foot has to go up, I think. Tempered by the understanding that this trellised square foot of land makes more intensive use of water and nutrients. So, land aside, other costs (e.g., water costs) rise..
Is there an optimal trellis height? E.g., some multiple of existing bed width? Might be, but I know too little to do that. If nothing else, plants vary in that saturation point for photosynthesis, the point beyond which additional light intensity is (purposefully) wasted by the plant.
My takeaway is that I should be more aggressive at trellising plants in my back-yard garden. Vines, at least. Despite the effort that entails. Best guess, growing vertically isn’t just neater, or better for keeping the fruit clean. It’s a fundamental tool for increasing yield per square foot of garden space.
Addendum: The argument via a little numerical example.
Situation 1: You have 100 square feet of garden, in the form of a two-foot-wide horizontal row covered with sweet potato leaves.
Situation 2: You have 100 square feet of garden. It’s the same row as above, but with a 6′ trellis on it, supporting 300 square feet of sweet potato leaves.
(Necessary simplifying assumptions: The sun is standing still AND the shade of the trellis exactly matches the outline of the underlying garden bed.)
My argument: Look, see, you’ve got 3 times the leaf area by trellising. I should get 3 times the yield, per square foot.
The counter-argument: Oh, hold on there. Sure, you have three times the leaf area. But it’s the same amount of sunlight. The sun’s illumination of that vertical trellis area (energy per square foot) is a third that of the horizontal bed that you replaced.
The insight: True. But here’s what I hadn’t realized: Photosynthesis wastes almost all the mid-day solar intensity that falls on a flat plot of ground. (Which still seems weird to me, but I swear is true.) So, arranging a larger leaf area, vertically (and so less strongly illuminated than mid-summer horizontal ground) captures more of the solar energy (that would otherwise impinge on that flat ground on which the trellis sits).
How much more? In this hypothetical case of a plant with a plausible (?) 150 watts/square meter solar saturation point (call it one-fifth the energy supplied by noonday summer sunshine falling on flat ground), the two-foot-wide-row-with-six-foot-tall-trellis generates about 2.5 times the photosynthesis as the original, flat, two-foot-wide-row. This, despite being impinged with (by assumption here) the same amount of solar energy.
(Note that the assumed 150 watt/square meter saturation point isn’t all that critical. In the example above, if photosynthesis for the plant in question saturated at twice that sunlight intensity, trellising would still generate about twice as much photosynthesis as growing directly on the ground, versus the 2.5x using a 150 watts/square meter photosynthesis saturation point.)
There are some irreducible bits of loosey-goosey in this analysis, despite the outline of it being, I believe, generally correct. One major bit of looseness comes from the mis-match between “land supplying nutrients to the trellised plants” (i.e., the garden row or garden bed) versus “land shaded by the trellised plants”. The solar energy argument is all about “land shaded by the trellis”, but that’s such a squidgy thing that I’m not even going there. Same as not trying to find an optimal trellis height. So the argument is kinda-sorta right. But it’s basically correct.
I still have a lot of fundamental questions as to why seemingly successful plants can only use a fraction of mid-day sun. (That is, of the colors of light that photosynthesis can use, at all.) How did such inefficiency come to dominate this fundamental process of life on earth?
I suspect that the answer boils down to some sort of hard biochemical constraint on the process of photosynthesis. That is, somehow, photosynthesis must be a relatively delicate process. And we don’t see some super-efficient photosynthesizer taking over the world because you literally can’t design one. That is, I suspect that you can’t make a practical photosynthesis process that’s robust enough to handle that high an energy input. At least, that’s my guess for now.
