Right on a south sill, a string of hearts scores 99 out of 100. The clear-day estimate there, 13,000 lux, reads as just right for it, and at latitude 40 the sill stays a great fit all year. Further back, the date counts: 1.5 metres in, it scores 98 in March and 66 in June.
Read on 22 October 2026, the answer box at the top of Google’s results calls south light intense, good for a full top and purple undersides but strong enough at midday to “scorch or crisp” the leaves. It puts the pot two to three feet back or a few feet to one side, with a sheer at peak summer hours. Three Reddit threads, seen only as snippets, share the first page with Plant Addicts, Greenery Unlimited and Wisconsin’s extension service.
Our numbers read the calendar the other way round. Seen facing the window, the light through south glass is weakest in June, when the high sun strikes the pane at a steep slant, and strongest in the cool months, when the low sun shines almost straight in. The sill barely notices; a hook further back does.
Great fit
This plant should be happy in this spot.
Just the right amount of light
Comfortable temperature
Humidity suits it
Airflow suits it
Is a south-facing window too much sun for a string of hearts?
No, going by the light. The card marks the bare sill just right at 13,000 lux, and in no month does our estimate push this plant’s light reading as far as a bit bright. Warmth, humidity and airflow sit in their bands too, which is how the sill gets to 99.
When leaves crisp at south glass, a sudden move from somewhere dim is the usual suspect, and a score of the spot has no way to see where the plant spent the last few months. If you’re bringing one to this sill, early summer is the gentle time to do it, because that’s when the window lets in the least light of the year. The box’s other promise, deeper purple undersides, is about colour, and the score doesn’t weigh colour.
Other sites publish these figures for it, each copied in its own unit:
| Who publishes it | What it says a string of hearts needs | What the figure is |
|---|---|---|
| getgrowli.app | 4,000–8,000 lux | Its Bright Indirect tier, a band the site prints for several plants, not one set for this species |
| getgrowli.app | 400–800 fc | The same Bright Indirect tier, given in foot-candles |
| foliage-factory.com | approx. 20,000–40,000 lux | Product page light line: very bright, some direct sun |
| theplantmanual.com | 1,500-3,000 foot-candles (fc) | The range it calls ideal (bright indirect) |
| theplantmanual.com | 500 foot-candles | Its floor: under this, it says, the vines get severely leggy |
| deepgreenpermaculture.com | 250 to 1000 lux | Its medium-light tier, which it calls the optimal range |
Foliage Factory, the one source here that mentions direct sun, starts its range above anything our estimate gives a south window. Growli’s tier tops out below the sill. The sill’s 13,000 lux sits between the two, and our scoring calls it just right.
Can it take full sun, then? Behind south glass, on our numbers, yes: even the brightest sill of the year still reads just right for it. A pot outdoors in the open is a separate question, and not one this page scores.
How far back from a south window can a string of hearts hang?
Leave the date out and it’s a little over two metres, which puts the answer box’s whole range well inside the great fit. Here is one pot walked back from the glass, with three feet as the second row:
| Distance from the glass | Estimated light | What the light is doing | Verdict |
|---|---|---|---|
| 0.3 m, against the glass | 13,000 lux | Just the right amount of light | Great fit, 99/100 |
| 0.9 m back | 10,473 lux | Just the right amount of light | Great fit, 99/100 |
| 1.5 m back | 7,937 lux | Just the right amount of light | Great fit, 98/100 |
| 2.1 m back | 6,015 lux | Just the right amount of light | Great fit, 97/100 |
| 2.2 m back | 5,743 lux | A little dim, but workable | Worth a try, 68/100 |
| 3 m back | 3,969 lux | A little dim, but workable | Worth a try, 65/100 |
| 4.5 m back | 1,984 lux | Too dim here | Not the best fit right now, 47/100 |
The great fit holds to 2.1 metres, where the estimate is 6,015 lux. Ten centimetres further it’s 5,743, the light tips into a little dim, and the score falls from 97 to 68. Worth a try then runs until just short of 4.5 metres, where the light reads too dim and the score is 47. A hoya at the same window changes verdict between the same two rows; our care data gives it the same light band.
Our lux figures are clear-day estimates built from the window’s direction and the pot’s distance, not meter readings. This table also has no date in it, and at a south window the date moves that 2.1 m line by more than a metre over a year.
Should a string of hearts move away from a south window in summer?
No. If anything, it should come closer. Here is the 1.5 m row from that table, followed through the year at latitude 40, the line through Ankara and Salt Lake City:
| On the 21st of | Estimated light | What the light is doing | Verdict |
|---|---|---|---|
| March | 7,979 lux | Just the right amount of light | Great fit, 98/100 |
| April | 6,358 lux | Just the right amount of light | Great fit, 97/100 |
| May | 4,984 lux | A little dim, but workable | Worth a try, 67/100 |
| June | 4,468 lux | A little dim, but workable | Worth a try, 66/100 |
| July | 4,935 lux | A little dim, but workable | Worth a try, 67/100 |
| August | 6,330 lux | Just the right amount of light | Great fit, 97/100 |
| December | 7,509 lux | Just the right amount of light | Great fit, 98/100 |
March scores 98 and April 97. By 21 May the estimate is down to 4,984 lux and the score to 67, June bottoms out at 4,468 and 66, and July is back at 67. August returns to 97. December, at 7,509 lux, gives this spot more light than April. Day by day, the 1.5 m hook is worth a try from 28 April to 14 August.
The further back the hook, the longer the dip. Three feet out it lasts about four weeks, 7 June to 5 July; a metre out, late May to mid-July. Within about 85 centimetres the plant stays a great fit every day of the year.
So the box’s two feet holds all year and its three feet falls about a month short. Its summer sheer takes light away in the months this window has least to give. Losing a tenth of the 1.5 m hook’s 6,358 lux in April would tip it past the distance table’s line.
What else can mark a string of hearts down on a south sill?
Air temperature. With the light held at the sill’s 13,000 lux, the plant stays a great fit from 18 to 27 °C, slips to worth a try just above 27 and to not the best fit above 30, the same warm edge the west-window page finds. Pulling the pot back helps only if the heat is a pocket by the glass.
Cold is the other end of that scale, and the cold months are this window’s best. Between 20 October and 20 February no other sill in the house keeps this plant at a great fit: an east sill drops a band from October and a west one by late October, while a north window never gets there. Keep this one above 18 °C.
What could change these numbers?
An overhang, mostly. A deep eave or balcony above cuts the high summer sun first, which makes the summer dip deeper, and neither is an input here. The south window guide covers the plants that want distance from this glass; the free spot checker scores your own spot with local weather.
Published October 22, 2026
