If you’ve struggled with growing plants indoors before – especially the edible kind – then there’s a good chance you had a problem with your lighting. At the very least, this is a sensible place to start troubleshooting.
In this blog we are going to show you how to use a) a free light-metering app on your phone and b) a free online calculator we’ve built to make a ‘light map’ of your growing space. This lightmap will show you how much natural light you have inside your apartment, which you can then use to determine what is viable in terms of plant growth.
Natural Light For Growing Plants Indoors
Now, the best source of light you’re ever going to get for growing plants is natural light. It’s free, and it’s incredibly high quality.
The problem with it is that it can be inconsistent. The light you get in the morning is not the same as in the afternoon. In addition, cloud cover and seasonality have a huge impact on the availability of natural light.
To understand the ‘theory’ behind this exercise, and for an explanation of DLI we recommend you take a look at ‘Grow Lights For Indoor Plants – How To Measure It, and Understanding Watts, PPF, PPFD and DLI’.
Step 1 – Download A FREE Light Metering App
We use one called ‘Light Meter’ for iPhone (there is both a ‘lite’ and a paid version), but there are many others so don’t worry if you can’t find this exact app.
Step 2 – Check The Weather Forecast
Check the weather forecast in your area. We recommend doing this on an ‘average’ day for your location in terms of cloud cover. If you live somewhere that’s always sunny – make sure you do it on a sunny day, and vice versa. As we discussed in ‘Do You Have Enough Natural Light For Plants?’ cloud cover can reduce natural light intensity by as much as 75%.
Step 3 – Determine Your Light Measuring Points
Next, you need to determine the points you wish to create a lightmap for. We recommend when doing this that you:
- Select locations in your home or apartment that are good candidates for locating your plants. If you don’t want plants on your coffee table or in front of the TV, then don’t bother assessing these locations.
- Select locations that are a fair way apart from one another. If you have a large square room, then measuring all four corners is likely to teach you more than using four measurement locations close together.
Step 4 – Get Started Early
As we said, this project is a great candidate for when you are working from home one day, or on the weekends. Ideally, you will want to get started within an hour of sunrise and keep going until no earlier than an hour before sunset.
Step 5 – Start Measuring Your Light
Open up your light measuring app, and set the unit of measurement to ‘lux’. Make sure you know whether you are measuring from the front or back camera, and if necessary adjust (we recommend the back one).
Now walk around your space taking measurements at each of your pre-determined locations. As you are going, add these lux measurements into our spreadsheet. Finally, record the time in the final column.
Step 6 – Keep Going Until The Evening
Set an alarm or count-down timer to remind yourself to come back and take more measurements throughout the day. Each time you take a measurement, add a new row to the spreadsheet as you did in Step 5 above.
If your plants are going to be relying on 100% artificial light then we really only need to do Step 5 once, since the light intensity will be consistent throughout the day.
Step 7 – Select Your Light Source
Although the camera on your phone is going to a decent job of measuring light for you, the truth is that the type of light it measures is not quite the same as the type of light that matters to your plants. We’ve attempted to take this into account by adding a few different lighting factors or adjustments, including natural light as well as a handful of artificial options.
Step 8 – Behold! You Now Have Some DLI Measurements and A Light Map For Your Plants
Congratulations! You have now completed estimating the DLI (or daily light interval) in your home, and you have calculated a ‘light map’ for your home. The values on the bottom row of the table are your DLI values, and the chart next to them shows the same information graphically.
The final step here is to compare your measured DLI values with the light requirements that we have summarized for you below.
| Plant | Typical DLI Requirement |
| Tomato | 15-40 mol/m²/d |
| Sweet Basil | 12-17 mol/m²/d |
| Rosemary | 12-17 mol/m²/d |
| Thyme | 12-17 mol/m²/d |
| Oregano | 12-17 mol/m²/d |
| Bell Pepper | 15-40 mol/m²/d |
| Dill | 12-17 mol/m²/d |
| Chives | 12-17 mol/m²/d |
| Strawberry | Minimum DLI of 17 mol/m²/d |
| Parsley | 12-17 mol/m²/d |
| Cilantro | 12-17 mol/m²/d |
| Mint | 12-17 mol/m²/d |
| Cucumber | 15-40 mol/m²/d |
| Squash | 15-40 mol/m²/d |
| Zinnia | 15-40 mol/m²/d |
These ranges are deliberately broad. Plants are remarkably adaptable organisms, and over time can adjust to a range of circumstances. What’s more, within a certain type of plant (eg tomatoes) there can exist hundreds and sometimes even thousands of cultivars – many of which will have very different light requirements. The point is that you should treat this table as a guide. If you find your natural light is close to, but not quite within the ranges, we provide here don’t stress. Try your plants out there anyway – just keep an eye on them and adjust if necessary.
We hope you’ve found this practical guide to measuring the light intensity in your home and building a lightmap useful. If you’d like to learn more about how to grow your own food indoors, check out our indoor edible gardening library for practical guides that will take you all the way from sprout to harvest.
Thank you for such a beautiful write up. Do you recommend T5 or T8 for indoor herbs that get pretty much no light.