How much light your houseplants lose in winter

Houseplants don’t read calendars, but they do count light. The measure that matters is the daily light integral (DLI): the total amount of usable light a leaf collects in a day, in mol/m²/day. Iowa State Extension sorts indoor plants into low light (3 to 6), medium light (6 to 10) and high light (12 to 16). Winter pushes many windows down a class or two.

Two things shrink in winter: hours and strength

The day gets shorter. FAO’s standard day-length table (mid-month values) gives:

Latitude	June	October	December	February
30°N	13.9 h	11.3 h	10.1 h	11.0 h
40°N	14.6 h	11.0 h	9.4 h	10.6 h
50°N	16.1 h	10.4 h	7.9 h	9.8 h

The sun is lower. Korczynski, Logan and Faust mapped 30 years of US solar data and found mean outdoor DLI of 5 to 10 mol/m²/day across the northern US in December, against 55 to 60 in the Southwest from May through July. For Columbia, Missouri, their monthly means run from 47 in June down to 13 in December. They attribute the winter pattern, with the big differences running between the northern and southern US from October through February, largely to day length and the sun’s elevation angle.

Outdoor numbers are far above what reaches a houseplant, but indoor light follows the same seasonal swing, and often a steeper one.

What that means at a real window

The chart below shows our planner’s month-by-month estimates for one common spot: an east-facing window, plant 2 ft (0.6 m) back, at 30°N, 40°N and 50°N (roughly Houston, Philadelphia and Winnipeg). It assumes a standard 3 x 4 ft window, double-pane clear glass, open sky and a “mixed” climate. Typical values from those estimates:

Latitude	June	October	December	February
30°N	9.5	6.0	3.5	5.5
40°N	9.9	4.8	2.0	4.2
50°N	10.3	3.5	1.05	2.8

(mol/m²/day, planner estimates)

A few things stand out:

• Summer is similar everywhere; winter is not. Long northern summer days make up for a lower sun, so June is about 10 at all three latitudes. December light is about 37% of June’s at 30°N, 20% at 40°N and 10% at 50°N.
• October to December is the steep part. At 40°N the estimate falls from 4.8 to 2.0, a drop of almost 60% in two months.
• Direct sun nearly disappears. At 40°N the planner gives this window about 5 hours of direct sun on a June day but only about 1.75 in October and almost none in December.
• A plant that grows all summer can stall all winter. At 40°N, this spot meets the medium-light growth level (6 mol/m²/day) from March to September and misses it from October to February.

What to do about it

Move plants closer to the glass

This is the single biggest, free change. For the same 40°N east window in December, the planner’s estimate roughly doubles from about 2.0 mol/m²/day at 2 ft to about 4.1 at 1 ft (0.3 m). Watch for cold, though: Wisconsin Extension warns that air currents around windows and doors can produce cold drafts some plants cannot tolerate.

Clean the glass and open the view

Maryland Extension lists the cleanliness and type of window, curtains and insect screens among the things that reduce indoor light. Glass type alone is a measurable factor: with low-E double glazing instead of clear double glazing, the planner’s December estimate at 2 ft drops from about 2.0 to about 1.75.

Rotate pots

Maryland Extension notes that plants lit from one direction develop a lean. A quarter turn every week or two keeps growth even, and in winter, when light is scarce, it means no side of the plant is left in the dark for months.

Water and feed less

Less light means slower growth, and slower growth means less water. Wisconsin Extension says actively growing plants need more water than in slow-growth periods such as winter, and advises watering only when the soil is dry to the touch about an inch down. Maryland Extension’s watering guide recommends checking soil about two inches down and never watering on a fixed schedule. Clemson notes that many plants with little or no artificial light enter a winter “resting stage” and should not be fertilized then; Wisconsin says most plants need feeding only from spring to fall, while winter-blooming plants are fed before they flower.

Add a grow light for the dark months

Iowa State Extension says nearly all indoor plants would benefit from supplemental light in winter. The useful question is how much. The Window Light Planner shows which months your spot falls short and by how much. For the 40°N east window above, the worst-month gap to medium light is about 4 mol/m²/day. Since DLI = PPFD × hours × 0.0036 (Iowa State’s formula), closing it over a 14-hour light period takes about 80 µmol/m²/s at leaf level. Our light converter does that math in both directions, and our guide on how far a grow light should be from plants covers placement.

One caution: Purdue Extension points out that a single light reading is a snapshot, and natural light changes all day. Window light is better judged over a whole day or from an estimate like the planner’s.

When light returns

The swing is fast in spring too. Korczynski and colleagues found outdoor DLI rising by more than 60% from February to April in many US locations. When growth restarts, go back to normal watering and feeding, and shade sun-sensitive plants that you moved right up to the glass: Clemson warns that changes in light should be gradual. If your plants live by a north window, see plants for a north-facing window.