One of the biggest issues gardeners can face is in the difference between how much light a plant needs, how much a plant wants, and how much it actually receives. Let’s start with looking at light in the outside world, where the sun doesn’t present problems of brand choice and wire installation. (on a future Lighting article, I’ll be diving into LED and other indoor lighting – currently testing out a few new bulbs)
Plants enjoy growing towards light (typically). This is a trait known as phototropism, and plants do not always display it equally. When a plant is grown in the right amount of light, it grows as thickly as it can. If light is coming from above, the plant will grow vertically, towards the light, with the leaves relatively close together. When the light is coming from one side (as in a window plant, or my southwest-facing balcony), the side facing the light grows less, while the shady side grows more (elongates) to curve the plant towards the light. To keep a plant evenly growing, you’ll need to turn it from time to time. However – not all plants are created equally. Sun-loving plants will display phototropism more heavily. Shade-tolerant plants display less phototropism (they’re used to having indirect or uneven light already), and sometimes no reaction at all.
In addition to a schedule of watering and fertilizing, you may need to add in rotating or walking your plant to get it the right amount of light in the ways it needs. If you’re used to moving your plants in from the cold, you’ve probably already strained your back doing this!
Be sure to check out my latest guided meditation!
Quantity and Direction
A good first step is to figure out where the sunlight will be coming from, and how long it might be there. When during the day isn’t necessarily important, unless you’re a very active gardener who moves their plants throughout the day to get them into the best positions – an activity only recommended for potted plants.
To start, we’ll take a look at suncalc.org, or at your garden throughout the year. We need to get a concrete idea of where the light will be coming from at different points of the year, or which areas are brighter and darker during which seasons. During different parts of the year, the sun will move through the sky at drastically different angles – closer or farther from the horizon. A handrail may block the light during winter, but offer no protection during summer.
Looking at the particular angles the sun moves through – as a number, and not just a rough guess – was incredibly revealing for me. I knew from experience that the sun would be higher in the sky during summer, and lower in winter. Looking at the actual angle, though, revealed how absolutely wide the difference was.
For example – I grow most of my (larger) plants on my apartment balcony. It faces Southwest, and hangs from the face of the building. Another balcony is directly above. Due to the placement of the handrails and my own building, light direction is critical to plan out. The low sun in winter provides more light towards the rear of the balcony (close to the doors), but is quite a bit darker towards the front. The high sun of summer shines more light directly down, illuminating the front of the balcony quite well, though the balcony above blocks some light towards the rear of my own. No matter what, I can assume that plants right up against the front handrail won’t get direct sun except in small amounts during the summer. I have more room of full sun towards the front of the balcony during summer, and should crowd plants closer to the apartment during winter (conveniently also keeping them a bit warmer, close to a big heat source). Everything works out.
Except – I want my little garden to work better. And so I need to understand where the light is coming from, and where it’s going to. You may want to take a peek at SunCalc and get ideas about the light in your neck of the woods. I’ll be walking you through how I did that for my balcony later on in this post. I pulled up way more information than I probably needed, but it helped me to understand that blazing ball of plant-food a little better.
Let’s start with some terms
When you’re looking for your solar info, there’s a few key terms to keep in mind:
Dawn vs Sunrise – Dawn is the start of morning twilight, when light and color first start appearing. Sunrise is when the actual disc of the sun begins peeking over the horizon.
Sunset vs Dusk – Much like the Dawn/Sunrise side of things, Sunset is when the disc of the sun sinks below the horizon. Dusk is the period of diminishing light from then until total darkness. For the purposes of gardening, I don’t care about any light after sunset or before sunrise.
Culmination – The highest point in the sky. Curiously, rarely ever high noon.
Altitude – The angle formed between the flat land, you, and the sun above.
Azimuth – The direction as described on a compass, in terms of an angle. (For my balcony, I need the sun to be at least at 145 degrees before it starts giving me light)
How much light is Full Sun?
In gardening terms, some plants are full sun while others are partial shade. Not many garden plants are full shade plants (where they may only get bright, indirect light). If you’re reading the light requirements on the back of a seed packet, you’ll probably come across these terms:
Full sun is generally regarded as sunlight for at least 6 hours in a day. These hours don’t need to be continuous – You could have two hours in the morning and four in the afternoon, with a tree’s long branch breaking up the middle of the day. However it adds up is fine.
Partial Sun, on the other hand, is between four and six hours per day.
Partial Shade is two to four hours per day.
Shade, the darkest of the general outside gardening light level hierarchy, means less than two hours.
In addition, some plants are adapted to thrive on 10+ hours of summer sun – the kind of plants that might grow out in the desert or the middle of a meadow. Not quite the sort of plant I can manage on my balcony, unfortunately.
So where do these different lighting conditions occur in nature? Think of a forest: the plants in the middle of a meadow will have sun all day, with the trees only giving some shade at sunrise and sunset. Plants near the edges of the meadow will have light blocked for about half the day by tree leaves (which may provide dappled or completely shaded light). Plants a little deeper in the woods might get direct light for only an hour or two, with shade the rest of the time. And in the deepest parts of the woods, with the greatest canopy cover overhead, plants thrive that never see direct sun.
If you’re thinking of growing plants with the environmental conditions that they’ll have in the wild, try to look into where they commonly grow – in the open, under and around other plants (margin plants), or deep under canopy cover. This will give you an idea of the quantity and type of light that they’ll need to thrive.
What’s the difference between sun and shade?
And why is the shade cooler?
In photographic terms, shade is about 3 stops below full sun; 1/8th of the total light quantity. Functionally, though, plants are more adaptable than film and alter how they grow to try to accommodate the light.
The air temperature in the shade and the air temperature in sunlight are actually nearly the same. The key difference between the two is in getting rid of excess heat. With the sunlight on you, heat is being added to your system constantly and directly. Surfaces heat up. Your body (or the plant’s) will try to offload the heat – through transpiration or perspiration, depending on plants or animals. It can’t always keep up, and you overheat. Maybe also get a sunburn – a risk some plants share.
In shade, there’s less additional heat being added continually to the system. Heat is easier to shed. It feels cooler because it takes less work to be in the same air.
Quantity of light and temperature can be a delicate balance in summer!
To make things even more interesting: Full sun at the equator is higher intensity than full sun in the far north. Sunlight in the afternoon is typically hotter than sunlight in the morning (not because of the sun, but because the air’s been warming up all morning). A plant that thrives on full sun in Wisconsin may need a bit of shade growing in SoCal, especially in the afternoon.
How do leaves adjust to the light?
Leaves that have grown to survive in shade have a thin layer of photosynthetic cells, spread across a broad (and thin) leaf. This allows the meagre sunlight to pierce across a relatively large surface area, penetrating as deep as it can, to produce food for the plant. The plant won’t have to worry as much about the sunlight burning the thin leaves, transpiring out all its water trying to keep cool.
Meanwhile, leaves that have grown to thrive in the sun are usually shorter and thicker. The layers of photosynthetic cells stack on top of each other. Direct sunlight can penetrate the leaves quite deeply, hitting multiple layers of the palisade cells (the light-absorbing cells right under the cuticle). The leaf likewise stays shorter to keep the surface area (potential water loss) down in direct sun.
Plants may grow somewhat different leaves depending on where they are and what light they’re growing under. A plant that’s adapted to partial shade may have to drop its wilting leaves in direct sun – and regrow some sun-loving leaves right after. Some plants expect full sun in Spring, when trees still haven’t regrown their leaves from Winter. When the trees are back to providing canopy cover, those ground-level plants transition to a different stage of shady growth or go dormant altogether.
And you can replicate all that by placing potted plants in slightly different areas.
Follow along as I figure out the sunlight for my balcony
Useful if you also have a southwest-facing balcony
On the balcony where I grow the majority of my plants, I can’t get full sun the whole day. It’s rather tough, actually. The buildings, the handrails. The plants are all set in pots on the floor of the balcony. The balcony railings are 38” high. It’s 148” wide by 58” deep. (Goodness, I hadn’t actually measured it before now). That’s 60 square feet or so. That also means that when the sun is coming up at a 45 angle (as it does now in March or later in November), full sunlight reaches the middle of the balcony at the height of the plants. The planters actually lift the plants up from the ground about 8-10 inches, which keeps the potted plants up in the light. I keep most of my plants towards the back wall of the balcony to ensure the sun is more likely to reach them – but I’ll probably draw a series of lines across my balcony to show where the sun would reach during the main seasons.
Let’s start with the balcony’s placement. It faces fairly perfectly towards the southwest. That means that I can’t get sun – due to my own building – early in the morning. It just won’t ever happen. I have to wait for the sun to swing around to 145 degrees on the compass (Southeast) before it has a chance of striking my balcony on the side.
The seasons of 2021, with the sun on Los Angeles: I’ll go ahead and get the numbers for the first of each month. The angles and times will stay fairly consistent month to month, so I’ll just fudge the actual value for any particular day if I need to know it.
|Season||Astrological Start Date||Meteorological Start Date||Sun’s highest altitude & time||Average max sun time (on my balcony)||AVG begin and end times|
|Spring||Mar 20||Mar 1||49° Mar 1, 12:05|
61° Apr 1, 12:57
72° May 1, 12:50
|7.3 hrs Mar |
7.4 hrs Apr
7.5 hrs May
|10:30 – 17:50|
11:40 – 19:15
12:00 – 19:37
|Summer||Jun 20||Jun 1||78° Jun 1, 12:51|
79° Jul 1, 12:57
74° Aug 1, 12:59
|7.6 hrs Jun|
7.7 hrs Jul
7.6 hrs Aug
|12:18 – 19:59|
12:26 – 20:08
12:16 – 19:53
|Autumn||Sep 22||Sep 1||64° Sep 1, 12:53|
53° Oct 1, 12:43
41° Nov 1, 12:37
|7.5 hrs Sep|
7.4 hrs Oct
7.3 hrs Nov
|11:47 – 19:18|
11:11 – 18:37
10:40 – 18:01
|Winter||Dec 21||Dec 1||34° Dec 1, 11:42|
33° Jan 1, 11:57
39° Feb 1, 12:07
|7.3 hrs Dec|
7.2 hrs Jan
7.3 hrs Feb
|9:28 – 16:45|
9:40 – 16:56
10:05 – 17:25
You’ll have to look up the specific times for your location – if you’d like, copy the table and delete the exact measurements. Fill in with your own info.
First realizations: I’ve been checking for when the sun is at 145° azimuth, for constancy on the start of light. In reality, I’ll likely have slightly less because of the balcony higher up and to the side blocking the first few minutes of actual light. No matter what, I’ll never have 8 hours of light. Some months I’ll only have 7 hours of effective light. I’m averaging more or less 7.5 hours around the year.
March 14th – November 7th will fall under the auspices of Daylight Savings Time. If your region doesn’t use this time-jumping system… I’m envious.
It looks like, during June, the day starts out with high-angle full intensity noonday sun for my plants. They’ll get blasted right off the bat, without a gentle ‘wake up’ phase in the morning. Something to keep an eye on. During the dimmer months, the sun is coming in at a pretty low angle – the front of the balcony will block light further back towards the wall. Darker all around. During summer, though, the light is coming nearly overhead. The balcony above me will block some of the light towards the back of the balcony. Now I know – during fall and winter, move the plants towards the rear wall. During summer, move the plants closer to the front wall. And I’ll have to restrict any fall and winter crops to ones that can survive on a little less light – though to be fair, I’ve had succulents against the back of the balcony, fairly low to the floor, for over a year with no issues. They even filled their previous pots with roots and had to be moved up.
Because of the handrails to the left and right, I’m fudging the light amounts there. There’s slats on the sides of the balcony that allow strips of light through when the sun is relatively low to the ground. With the balcony and building to the South East, I can really only get this extra splash of sun during sunset. My afternoons are worth more than my mornings, so the Left (East) side of the balcony is for plants that need less sun, and the West side is for plants that need just a dash more (the rest of the day should even out left-right). The wings of the balcony will get less than the middle by far, though I’ve grown full sun plants up against the sides. As plants gain in height, they’ll reach towards upwards and unlock longer stretches of full sun on higher portions of the plant. However, because the direct sun will generally come from one side or the other from the edge plants’ perspective, the plants will tend towards the light when further towards the sides.
The window blinds are pure white and provide a little bounce from behind. Even if the sun is fairly low, as it is in winter, the blinds pop a little extra light onto the plants from a direction they’re not used to (the building itself). In moments like this, it’s actually sometimes better to move a few of the plants away from the wall to allow more room for that reflection. I suppose if I wanted to go all the way, I could put tinfoil up. It would wreak havoc on the indoors plants, though. I have to balance this with the plants inside: more bounce means less light indoors.
What changes have I made since this research?
I’ll be moving my plants farther forward that I traditionally would during summer. I’ll have to bunch them up towards the middle length of the balcony, but I think I can squeeze the taller ones in back and make it work.
Aside from that, I might get some apple crates to put the shorter pots atop. This will increase their light time during winter.
And… I need to rotate the plants more.
Also, totally aside, I moved the bloodleaf all the way diagonally across the balcony, to a corner that gets partial shade (2-4 hours of sun). It’s been blasted with the strongest corner of light for most of its life thus far, and I’d like to see how it responds to being treated like a margin plant. So far it’s been quite happy, though that could be the repotting.
Thanks for stopping by! I hope you had a pleasant time checking out the plants. If you’re in the mood for more nature, please stay in touch!