Measuring and acclimating

You don't have to guess. Three tools, from free to precise, will tell you what a spot delivers, and one habit will let you move plants into brighter light without bleaching them.

The shadow test (free, instant)

The shadow a plant or object casts tells you what kind of light a spot gets. North Carolina's extension service puts it this way: when the sun comes in the window, a plant casts a crisp shadow; in bright indirect light it might cast a fuzzy, soft shadow; and in low light the shadow is faint (Watkins, 2026). Our habit is to hold a hand about a foot (30 centimeters) above the spot around midday and look at its shadow. It takes two seconds and costs nothing, and checking it against a meter once or twice will tell you how far to trust your eye.

Fig. 12. The shadow test. A crisp shadow means direct sun, a soft, fuzzy shadow suggests bright indirect light, and a shadow you can barely see means low light.

The light-meter app (cheap, useful)

A light-meter app on your phone gives you a number. Some read foot-candles and some read lux, and as Lesson 4 explained, both measure light the way the human eye sees it. Iowa State's extension says smartphone apps are accurate enough for most home gardeners, and that you should always measure at the level of the foliage (Steil, 2023b). So hold the phone where the leaves are, not at the glass.

To turn a reading into a light level, use the table in Lesson 4, which follows the University of Florida's midday bands (UF/IFAS Gardening Solutions, n.d.). Under grow lights, use Lesson 4's second table and set the app for the kind of light you're measuring.

Other guides draw the lines differently. Mississippi State's, for instance, calls 50 to 500 foot-candles low, 500 to 1,000 medium, and 1,000 or more high (DelPrince, 2021). So when a care tag or book gives a light level, check which scale it's using.

DLI (the fuller picture)

A single reading tells you how bright a spot is at that moment. The daily light integral from Lesson 4 tells you how much light it gets over a whole day (Steil, 2023b). Iowa State's extension gives these rough targets, in moles per square meter per day (Steil, 2023b):

• Foliage houseplants: 3 to 6 (low light) or 6 to 10 (medium light)
• Flowering houseplants: 6 to 10 or 12 to 16, depending on the plant
• Herbs and vegetables grown for their leaves: 12 to 16, or 18 to 30
• Fruiting plants and other herbs and vegetables: 18 to 30

Some foliage plants can get by on far less for a while. In the growth-chamber study from Lesson 6, pothos kept growing for six months at 0.22 (Sugano et al., 2024). Outdoors there's much more to work with. Faust and Logan mapped outdoor DLI across the United States month by month (Faust & Logan, 2018), and Iowa State's extension notes that even the brightest indoor spots get only a fraction of the light of any shady spot outside (Steil, n.d.). That's why placement near the glass matters so much indoors.

Acclimate when you increase light

Whenever you move a plant into much brighter light, do it gradually (Shaughnessy & Pertuit, 1999). For houseplants going outdoors for the summer, Iowa State's extension advises starting them in full shade and introducing more light over 10 to 14 days, and never moving them from indoors straight into full sun (Steil, n.d.). We use a similar pace for a big move indoors, such as from a dim corner to a south window: start the plant short of its final spot and bring it closer over a week or two. The same goes for a new plant arriving from a dim shop or a shipping box. In the maple study from Lesson 6, existing leaves adjusted to more light only partly (Wyka et al., 2022), so in our view a gradual move gives leaves time to adjust and gives the plant time to grow new ones suited to the brighter spot.

Moving a plant into less light carries less risk, but expect some leaf drop. Shade leaves break even at lower light than sun leaves, so a plant grown in bright light often sheds some leaves after it's brought indoors. A few dropped leaves usually mean it's adjusting, and doing the move gradually helps avoid heavy leaf drop (Trinklein, 2016). Weeping fig and tropical hibiscus are especially prone to it, and new leaves usually emerge within a couple of weeks (Steil, n.d.).

One more small habit, for plants lit mostly from one side, as at a window. Plants that get their light mainly from one direction tend to lean toward it, and a quarter turn of the pot now and then keeps growth even (Mays et al., 2022). Guides differ on how often: the New York Botanical Garden suggests every few weeks (LuEsther T. Mertz Library, n.d.), and North Carolina's extension handbook every three months (Mays et al., 2022). Under a grow light directly overhead the light isn't coming from one side, so in our view there's nothing to correct. One exception to turning: Iowa State advises not moving a Christmas cactus while its flower buds are developing, because the buds may drop (Iowa State University Extension and Outreach, 2024), so we'd leave it be until the flowers open.