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Understanding Natural Light and Seasonality

Getting your lighting right is one of the most important steps in growing your own food indoors. Most of the existing information you’ll find online classifies light, and plants’ requirements, as “low”, “medium” or “bright” light. But what do these terms mean? And do you actually have this available in your home? 

The focus of this article is on natural light. We look at how its intensity varies based on your location and time of year, and connect natural light availability with the appropriate selection of edible plants that you can (potentially!) grow at home. 

 

How Seasonality and the Time of Year Impacts Natural Light Availability for Your Plants

As the earth moves through its seasons from winter to summer, its axial tilt varies. There are two major implications of the earth’s tilt on natural light availability.

 

Duration of Daylight Varies

As the earth tilts away from the sun, days get shorter. Shorter days obviously mean less time for plants to soak up light. Here in New York, where Urban Leaf is based, day length varies from 9:15 hrs (winter solstice) up to 15:06 (summer solstice) – an increase of nearly 50%. If you’d like to see how much day-length varies throughout the year in your location, this website is pretty handy. 

 

Angle of Sun Varies

During winter months not only is the duration of daylight shorter, but its angle of incidence also increases. As the diagram below illustrates, this essentially means that the same amount of inbound light gets spread over a larger surface area. As a result, its intensity reduces. In the example above, with a 45 degree angle of incidence, the lower angle of the sun alone means that the intensity of light is 1 – 1/1.41 = 29% lower. 

In addition to the natural light being spread thinner due to simple geometry (as above), a large angle of incidence also means that the light has a thicker layer of ozone to pass through. This results in both more reflection and diffusion of the sun’s rays off earth’s ozone layer – so less of it actually penetrates all the way through to us, at ground level.

How Your Location Impacts Natural Light Availability for Your Plants

Natural Light Degrades As You Move Further Away from the Equator

Moving away from the equator, either North or South, has essentially the same effect on light intensity as seasonality does (see above) – with locations a long way North (or South) experiencing longer days in summer and shorter days in winter relative to locations that are closer to the equator. The diagram below shows how your location relative to the equator (from 40 degrees south to 40 degrees north shown here) impacts day length throughout the year. 

(source)

Natural Light Degrades With Cloud Cover

Cloud cover can also have a big impact on the amount of natural light that makes its way to the surface of the earth. On a clear and cloudless day, roughly 75% of extraterrestrial radiation makes its way to earth’s surface. On a cloudy day this might drop to around 25%. So clearly if you live somewhere like London or Vancouver – both cities known for being pretty cloudy – natural light availability for your plants is going to be far less than a different location at the same latitude but with less clouds. 

 

Putting It All Together: Daily Light Integral (DLI)

We’ve discussed the impact of seasonality, location (latitude) and cloud cover. Now it’s time to pull all of these pieces together, so we can show you what the combination of these factors means to natural light availability (measured as DLI) and what this all means in terms of what we could potentially grow. 

Below we’ve constructed DLI charts for New York, Seattle, Los Angeles and Miami in an effort to show you what the 4 corners of the US look like in terms of natural light availability.

The blue lines on these charts show the monthly expected average DLI, and you can evaluate these lines relative to the below list of typical DLI requirements for various plants:

  • Leafy Greens & Herbs: 16-19 mol / m2 / day
  • Strawberries: 20-24 mol / m2 / day
  • Tomatoes, Peppers & Cucumbers: 24-28 mol / m2 / day

At the risk of being the party pooper, we do need to remind you that everything we have discussed this far assumes that we are measuring light (or using it) out in the middle of a paddock – i.e. with no trees, buildings or other objects obstructing the sunlight or creating shadows. Clearly, for urban and indoor gardeners, this is not the case. So what impact does an urban setting have on natural light availability?

 

Additional Lighting Considerations for Indoor and Urban Gardeners

Urban and indoor gardeners rarely have access to natural light that is completely free from obstructions. It is impossible for us to consider every possible lighting scenario that might be encountered in such an environment (let alone quantify it), but some general rules of thumb related to lighting in an indoor or urban setting that you should keep in mind are: 

  • The absolute best-case lighting environment you’re going to encounter in an urban environment would be a rooftop that is never in the shadow of an adjacent building. If you’re lucky enough to have a space like this, then you’re effectively in the ‘paddock’ type scenario we talk about above, and the charts/values shown on the graphs would be fairly representative for you.
  • For edible plants, it’s really only direct light that matters. “Direct” means that if you sat next to the plant and looked up you could see the sun (although we don’t recommend this, especially without sunglasses). If you are in any sort of shadow – created by another building, the roof of your own apartment or house, the neighbors fence, a tree – then your DLI is likely <10% of what it would be in an ‘unobstructed’ scenario. In other words, it really doesn’t count.
  • Windows that face away from the equator (north in the northern hemisphere or south in the southern hemisphere) will always be facing away from the sun, meaning that you will only ever have indirect light, and you can assume that your actual DLI is <10% of the values shown on the charts above. 
  • Direct sunlight received during the middle of the day is more valuable (from a plant’s perspective) than direct light received during either the morning or afternoon. The reason for this is due to the angle of incidence, as we discussed earlier, although in this case it is longitude related and not latitude related.
  • If you have a window sill that faces either due west or due east, then you are essentially getting ~half of the natural light that would be available in our ‘paddock’ or ‘rooftop’ scenario. You could therefore halve the values shown on the graphs to get an estimate of your DLI availability. 

We realize that you might still have questions about natural light, but at the very least we hope you now understand enough about the fundamentals to be asking the right questions. 

If you’d like to learn more about lighting for indoor edible gardening, our indoor gardening library offers plenty more resources. A good next step from this article is our blog about How to Create a ‘Light Map’ of Your Apartment (coming soon).

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