A windowsill is the best light in the room and one of the busiest surfaces in the apartment. Most sill arrangements fail on the second half rather than the first.
What you are working with
The University of Illinois Extension puts Direct Light at a south window directly in front, High or Bright Light at an east or west window directly in front or a south window up to 5 feet away, and Medium Light at a north window directly in front.
Directly in front means the sill. So whatever your window's aspect, the sill is the top of that window's range, and everything you do on it is spending the best light you have.
That is the argument for taking the arrangement seriously rather than using the sill as the place things end up.
Front to back is a real distance
A sill is shallow and it still has a gradient. The front edge, against the glass, is a different position from the back edge against the room.
Put whatever needs the most light at the glass. On a narrow sill this is a matter of inches and it still matters, because at the window the light falls off fastest.
Use the back edge for things that need less, or for the objects that are not plants.
Side to side is about the window, not the plants
The useful question along the sill is not composition, it is what each position can physically do.
The ends are calmer. The middle of a sill is where a window is opened from, where a blind lands, and where you reach through. The ends are quieter and things survive there.
Height belongs at an end. A tall plant in the middle of a sill blocks the view out and gets caught by anything that closes over the window. At an end it frames rather than obstructs.
Leave a working gap. If the window opens, the part it opens over cannot hold a plant, and the part you reach through to open it cannot either. Work that out before you arrange, not after.
The two seasons that undo sill arrangements
This is the part people find out about rather than plan for.
In summer, the sill is the most intense position in the room, and in a south or west window it can be more than some plants want. Iowa State University Extension's signal is "burning or discoloration of the foliage", which means the light is too intense, and on a bright sill it can appear quickly.
In winter, the same sill is the coldest surface in the room at night, while remaining the brightest by day. That is a genuine trade rather than a fault, and what happens beside a cold window in winter covers it.
The practical conclusion: a sill display is a seasonal arrangement. Expect to change it twice a year, and it will keep working. Set it once and it will be wrong for half of every year.
The other signal
The opposite of burning, and on a north sill it is the one you will see: a plant that "starts to lean or stretch towards the light" should move to a brighter spot.
On a sill there is nowhere brighter in that room, so a plant stretching on a sill is telling you something useful: this window is not enough for it. That is a purchase decision rather than an arrangement one.
Things that are not plants
Worth saying, because a sill full of plants is not automatically better than a sill with three.
Leave some sill visible. An arrangement with space around it reads as arranged. A sill packed end to end reads as storage and makes a small room feel smaller.
Anything you put there in front of a plant is taking its light. Ornaments, jars, a stack of post. On a sill, clutter has a cost that it does not have on a table.
Stability, because this is a sill
Nothing top-heavy. Sills get knocked by curtains, blinds, opening windows and sleeves.
Nothing overhanging the inside edge where it will be caught by somebody walking past.
Nothing on the outside edge of an opening window. This one is worth being blunt about: on an upper floor, a pot that can be nudged off a sill is a pot that can fall into a public space below.
The kitchen sill is its own case
Kitchens add heat, steam and traffic to everything above, and the sill above a sink is the most used one in the home. What actually lasts on a kitchen windowsill covers that specifically.
If you have pets
A sill is a cat's preferred position in any room, and a sill display is directly in the path of an animal that wants to sit there. Check any plant, by botanical name, against the ASPCA's toxic and non-toxic plants database, and expect the arrangement to be rearranged by somebody else.
Frequently Asked Questions
Which plant goes at the front of the sill and which at the back?
Whatever needs the most light goes at the glass. Even on a shallow sill there is a real gradient between the front edge and the back edge, and at the window the light falls off fastest, so a matter of inches still matters. The back edge suits the plants that need less, and the objects that are not plants.
How should a windowsill display be arranged along its length?
By what each position can physically do rather than by composition. The ends are calmer, because the middle is where a window is opened from, where a blind lands, and where you reach through. Height belongs at an end, since a tall plant in the middle blocks the view and gets caught by anything closing over the window. And leave a working gap wherever the window opens, worked out before you arrange rather than after.
Why does my sill arrangement stop working after a few months?
Because a sill is a seasonal position. In summer it is the most intense spot in the room, and Iowa State University Extension's signal for light that is too intense is burning or discoloration of the foliage, which on a bright sill can appear quickly. In winter the same sill is the coldest surface in the room at night while staying the brightest by day. Expect to change the arrangement twice a year.
What does it mean when a plant on the sill leans or stretches?
Iowa State's other signal is that a plant which starts to lean or stretch towards the light should move to a brighter spot. On a sill there is nowhere brighter in that room, so a plant stretching there is telling you the window itself is not enough for it. That makes it a purchase decision rather than an arrangement one.
Should I fill the whole sill with plants?
No. Leave some sill visible, because an arrangement with space around it reads as arranged while a sill packed end to end reads as storage and makes a small room feel smaller. Anything placed in front of a plant is also taking its light, so on a sill clutter carries a cost it does not carry on a table. Keep nothing top-heavy, nothing overhanging the inside edge, and nothing on the outside edge of an opening window.
The part worth remembering
The sill is the top of your window's light range and a working surface at the same time.
Highest requirement at the glass, height at the ends, a gap where the window opens, and a rearrangement twice a year. If you are still working out which of your windows is worth the effort, the room-by-room survey answers that in about ten minutes.