Most horticultural experts try to explain the degree of sun or shade of a garden location by the number of hours of direct sunlight it receives.
That sounds great, but have you ever heard any one of these experts explain how to actually determine that magic number? Do you really need to sit in the spot with a stopwatch and count the minutes the sun shines, making a total at the end of the day? Because my experience is that most spots slide back and forth from full sun to sun filtered through overhanging branches (and how do you measure that?) to no sun at all several times a day.
And if you’re expected to measure the number of hours of sunlight spot by spot throughout your garden (which is apparently what you’d have to do, since sunshine varies enormously from one location to another), during which month of the year are you supposed to take the measurement? Because clearly day length (and thus the number of hours of sunlight) is going be vary vastly from December to June, even more strikingly so in northern latitudes.
I have never seen an answer to these questions. So, garden experts have been sending you on an impossible mission all these years. In actual fact, the number of hours of sun per day a spot in your garden receives is essentially impossible to determine!
I think the proof that these same experts don’t know what they are talking about is that they can’t even agree among themselves about the result. One says 6 hours or more of direct sunlight per day counts as full sun… and another says 4 hours. Yet another says that should be 8. Partial shade is theoretically between 2 and 4 hours of sun… or is that between 4 and 6 hours? And what about shade? Some say less than 4 hours of direct sunlight per day is shade, others claim less than 2 hours per day, and yet others say no sun at all. How confusing!
Using Your Own Eyes: the Empirical Method
If you can’t physically count the number of hours of sunshine in a given location, however, it is actually fairly easy to determine if a location is sunny, shady or partially shaded using the empirical method.
If the location seems to receive full sun most of the day, it’s sunny. If it appears to be in the shade most of the time, it probably is shady. And if it is between sunny or shady, it would logically be in part shade. At least 99% of gardeners use this method, usually with fairly good success.
But the empirical method isn’t perfect: sometimes you’ll find shade plants burning from too much sun in a spot you thought was partially shady or sun plants doing really well in a spot that seems only in partial sun to you. So you end up moving plants around, making endless adjustments.
Petunia Test to the Rescue
However, there is a much more concrete method: the petunia test.
Just plant petunias wherever you want to determine the whether a spot is sunny, partly shaded or shaded. Where petunias grow well and bloom beautifully, consider the spot sunny. Where they grow and flower, but with less vigor, that’s partial shade. And where they grow little and bloom less, you’ve got shade.
Simple, isn’t it!