# What is the level of sun on the north side of this railing?

I am planning some container plantings that will go on the north side of a deck railing very much like this one (in the Northern Hemisphere):

Without the railing, this location would receive unobstructed sun from essentially sunrise to sunset. I am trying to understand how the presence of the railing changes the equation.

I have done some Python simulations to calculate the size/shape of the shadows cast by the railing and pickets (and how those shadows change over the course of the day/year). Those simulations indicate that there is a band of points right next to the railing that receive 4-6 hours of direct sunlight per day. That band is pretty narrow in the summer, but wider in the winter.

My question is whether this is the right way to "count" sun from the plants' perspective. In particular, any point next to the railing gets a cumulative 4-6 hours of sun, but a plant next to the railing is partially in shadow and partially in sun at all times because of the shadows cast by the pickets. Would this change how the plant "perceives" how much light it is receiving (relative being fully in sun for 4-6 hours and in shadow the rest of the day)?

• Have you noticed that trees and other shrubs are casting shadows as well? This is just a railing, not a jungle roof. Jan 3, 2021 at 4:24
• do some math: youtube.com/watch?v=1ryTz3mA3qc Jan 3, 2021 at 4:47
• @Johannes_B: There are no other obstructions. The railing is all that's there.
– Matt
Jan 3, 2021 at 15:10
• @blackthumb: Thank you for your comment. I actually have done some math along these lines. I left it out of my original post because I was trying to simplify, but I've now edited the post to describe what I've done. In any case, the crux of my question is not really about the size of the shadows but rather about how to think about the fact that any plant next to the railing will be partially in sun and partially in shadow at all times and whether that introduces any special considerations.
– Matt
Jan 3, 2021 at 18:49

It depends on what you are growing; seasonal summer plantings will effectively get full sun (the railings are neither here nor there,they are relatively slim). But in winter, if it faces north, there will be no sun at all, and during spring and autumn, the amount of sunlight will be reduced, so that would not count as full sun.

If you wanted containers full of year round plants like shrubs, then choose those suited to partial shade.

The railings have a really negligible effect. It's much more important that it's oriented towards north. As you said it yourself: probably from early spring till mid autumn you'll receive full sun on the deck and should place there any summer plants accordingly. This means you shouldn't put there any plant that mustn't receive direct sunlight (as their leaves would be burned and they would die). If you'll bring out any plants from the house to keep them out there for the summer you should leave them in partial shade for a week or two for allowing them to adjust to direct sunlight. In the winter you'll probably barely have it illuminated, so only plants that don't need a lot of sunlight should be left there.

The railing will have a negligible effect. You are way overthinking this.

1. Even though the sun is directional, sunlight is more omni-directional. When you are outside and look at something flat, you cannot see any different in the level of sunlight across it.
2. It is diffused by water vapour, dust and clouds. And is reflected by all sorts of things.
3. The leaves and branches will be moving, so no part of it will get no sunlight.
4. Even if all the leaves were still, as the sun moves, almost all leaves will receive sunlight.
5. There are hundreds of leaves, it hardly matters if 10-30% (very rough guestimate) do not get any sun.