This is the first of two posts on finding the sunniest spot in a yard that has shade trees on either side. This one uses geometry. The next one will use time-lapse photography on a sunny day.
With any luck, both approaches will tell me the same thing.
If your yard is bordered by shade trees, locate the beds so that due south (180 degrees) splits the compass bearing from your bed to each line of trees. This gives a surprising-looking result for my back yard. It’s not at all what you’d naively think, just looking at the trees and the yard.
Garden bed location 1: Wrong.
I started gardening seriously during the pandemic. Temporary raised beds were made from recycled campaign yard signs and bamboo. I placed those in seemingly-reasonable locations in my back yard. In part, they were filling in low spots on the lawn. But it seemed like they were located so as to get the best sun.
I’m now getting around to putting in something more permanent. This time, I’m not going to wing it, but instead want to know what spot in my back yard gets the most sunlight.
It’s not obvious. I have tall trees on either edge of my yard. And, interestingly enough, what appears to be the obvious solution — locate the garden beds in the middle of the yard, away from both tree lines — isn’t even close to being right.
So, eyeball a couple of birds’-eye views of my back yard, and see if you think I put the beds in roughly the right place:
Looks pretty good, doesn’t it? You might even say that the location doesn’t much matter, because you’re going to get the same number of hours of sunlight almost anywhere in that back yard, regardless. What’s shaded in the morning will be sunny in the afternoon, and vice-versa.
Problem is, an hour of sun is not an hour of sun. Sunlight is much stronger around solar noon, and is weaker the farther you are from noon. And, because the sun is due south at noon (in the Northern hemisphere), you have to know which direction is south, in order to judge what part of the yard gets the most solar energy.
Source: Curtonics.com
You need to figure out the locations in your yard that place due south directly between those lines of trees. Those locations get the greatest amount of high-intensity, near-noon sunlight.
To cut to the chase, you need to calculate where your potential garden site is, relative to the obstructing trees, and to due south. The sunniest locations in the yard will have these two properties.
- Due south (180 degrees) bisects the angle from your location to each side of obstructing trees. E.g., find a spot where the bearing to one set of trees is 150 degrees (180 – 30), and the bearing to the other set of trees is 210 degrees (180 + 30). That is, you get equal hours of morning and afternoon sun.
- The angle from your location, to the obstructing trees, is as wide as possible. For example, the location with a 60 degree spread above will get more total sunlight than a location with a 40 degree spread. That is, you get as many total hours of sun as possible.
So now, take a look at my back yard, oriented so that south is directly down. Do you want to change your prior answer? By the look of the shadows, this is about 11 AM solar time. Note that the left edge of the yard is already in sunlight.
Skirting a couple of pitfalls.
Let me take a brief break to mention a couple of pitfalls that can mess up your attempts to locate your garden in the sunniest spot on the yard.
Daylight savings time. Man I hate having to get up at 2 AM to turn all the clocks forward, as required by law. But the upshot is that solar noon occurs around 1 PM during daylight savings time. For example, on the hourly insolation graph above, peak insolation occurs around 13:00, or 1 PM. That’s not a mistake, that’s just daylight savings time. So if it’s summer, and you look to see where the shadows fall at noon, you’re screwing up. Because noon, daylight savings time, is actually 11 AM solar time.
Above: Compass set up for 10 degrees west magnetic declination
Magnetic declination. Declination is the extent to which magnetic north — where the compass needle points — deviates from true north. Because of magnetic declination, you can’t simply use the raw readings from a standard magnetic compass in order to locate your garden in the right spot.
If you have a compass made for use on land, and it’s anything but the most basic compass, chances are you can adjust the compass to account for declination.
You can find the magnetic declination for your locality at the US Geological Survey, among other places. Currently, magnetic declination at Vienna VA is about 10 degrees west. That means that the compass needle actually points to a heading of about 350 degrees, not 360 degrees (true north). That’s about 2.5 degrees further west than when I was a kid in the 1970s.
Magnetic declination is one of those incredibly simple topics that always manages to get an incredibly opaque explanation. But as long as you have a compass that can be set to account for your local declination, it’s really simple. The picture above shows a compass set up for 10 degrees west declination. Despite the fuzziness of the photo, I think it’s obvious that the compass body has been offset 10 degrees relative to the degree ring. When the needle points to 350 degrees (10 degrees west of true north), 360 or 0 on the degree ring shows you true north.
The sunniest spots in my back yard are directly next to the trees.
I can now take Google Earth, and start drawing in the angles between various backyard locations, and the ends of the lines of shading trees at the sides of the yard. It’s a little crude, but the conclusion is inescapable. I put the temporary beds too close to the middle of the yard. For the most solar energy possible, they ought to be almost under the trees at the side of the yard. Like so:
Which, to be honest, I would not have guessed, just eyeballing it.
Over the coming weekend, I’ll set up a stop-motion camera to film my back yard for one sunny day. With that, I should be able to validate that the area that gets the most solar energy is the one outlined. And I should be able to determine just how much energy I lose if I move away from that optimum spot.
