A balcony door is usually a sheet of glass from waist height to the ceiling, or floor to ceiling. In light terms that makes the space beside it one of the best interior positions in the apartment, frequently better than any actual window in the same room.
It is also a door. Those two facts are in tension and the tension is the whole subject.
Why the position is so good
The University of Illinois Extension sets light by direction and distance: Bright is an east or west window directly in front or a south window up to 5 feet away, Medium is a north window directly in front, an east or west window a few feet away, or a south window 3 to 10 feet away.
A balcony door gives you that band over a taller span than a window does. A window starts at sill height. A door starts at the floor. So the bright zone beside a door extends all the way down, which means it will take a floor-standing plant, and floor-standing plants are the ones that need a good position most.
In a small apartment where the windowsills are narrow and the surfaces are taken, the floor beside a balcony door is often the only place a large plant can stand in real light. That is a genuinely valuable square foot and it is worth using deliberately.
If the door faces south, note that you are working near the top of the range; what a south aspect delivers covers that intensity.
The catch, which is that people walk through it
Every problem with this position comes from the same source.
Traffic. It is a route, and in summer it is a route used constantly. A plant with broad leaves standing beside it gets brushed every trip.
The door swings or slides. Either way there is an arc or a track that has to stay clear, and the clear zone is larger than people allow for.
Things get put down there. Shoes, bags, the thing you are taking outside. A plant in a doorway is competing with a natural drop zone.
It opens. Which is the interesting one.
What "it opens" actually does
When the door is open, the position beside it stops being an indoor position. Outdoor air, outdoor temperature and outdoor air movement arrive directly onto whatever is standing there.
In summer that is mostly fine and sometimes welcome. In winter it is the coldest interior position in the apartment, repeatedly, for as long as somebody stands there with the door open.
I am deliberately not giving you a temperature for that, because I have no verified figure and an invented one would be worse than none. What matters for placement is the pattern: the spot beside the door is not a stable position, it is a position that briefly becomes an outdoor one, several times a day, in the season when that is hardest.
The same problem in its steadier form is covered in what happens to plants beside a cold window in winter, and the door version is sharper because it is intermittent rather than constant.
So the position is seasonal
This is the practical conclusion and it is the one people miss.
In summer, the space beside an open balcony door is excellent. Bright, ventilated, and continuous with the balcony.
In winter, it is the position most exposed to cold air ingress in the whole apartment, while still being the brightest.
Which means the right plant for that spot in July is not necessarily the right plant for it in January, and the sensible arrangement is one you expect to change twice a year rather than one you set and forget.
Placing it properly
Beside the door, not in front of it. The bright zone extends sideways along the wall, and a plant standing to one side gets almost all the light with none of the traffic.
On the hinge side or the fixed-panel side. With a sliding door, the half that does not open is the half to use. With a hinged door, the side the door does not swing across.
Back from the threshold. A step or two into the room takes the plant out of the direct path of cold air without costing much light, per the distances above.
Not where it narrows the doorway. In a small apartment the balcony door is often the widest source of light and the only route out. Narrowing it makes the room feel smaller and gets the plant damaged.
Where you can still get out with your hands full, which is when things get knocked.
The plants that suit it
In habit terms rather than species, and the criteria follow directly from the above.
Firm foliage rather than soft, broad leaves, because it will be brushed.
Upright rather than spreading, because the footprint has to stay narrow.
Stable, because it is beside a route and a door that moves air.
Tolerant of the seasonal swing, or willing to be moved in autumn. The second is easier to arrange than the first.
The thing worth doing before you place anything
Stand in the doorway and open the door. Watch what the door actually does: how far it swings or slides, what it passes over, where you naturally put your feet, and where you would set something down.
That arc, plus a hand's width, is the zone that cannot hold a plant. What is left is usually a good deal narrower than it looked, and it is better to know before the plant arrives.
If you have pets
The balcony door is the route animals use to get outside, so a plant there is in the busiest, most reachable position in the apartment. Check any plant, by botanical name, against the ASPCA's toxic and non-toxic plants database before it goes there.
Frequently Asked Questions
Why is the spot beside a balcony door such a good plant position?
Because the glass runs to the floor. The University of Illinois Extension's Bright band is an east or west window directly in front or a south window up to 5 feet away, and a balcony door delivers that band over a taller span than a window does, since a window starts at sill height and a door starts at floor level. In a small apartment with narrow sills and taken surfaces, the floor beside a balcony door is often the only place a large plant can stand in real light.
What is the catch?
It is a doorway. It is a route, used constantly in summer, so a broad-leaved plant standing beside it gets brushed every trip. The door swings or slides, and the clear zone that arc needs is larger than people allow for. It attracts shoes, bags and whatever is being carried out. And it opens, which is the one that matters most.
What does opening the door do to a plant standing there?
It stops being an indoor position while the door is open. Outdoor air, temperature and air movement arrive directly onto whatever is standing there. In summer that is mostly fine and sometimes welcome. In winter it is the most exposed interior spot in the apartment, repeatedly, for as long as the door is open. It is not a stable position, it is one that briefly becomes an outdoor position several times a day in the season when that is hardest.
Where exactly should the plant go?
Beside the door rather than in front of it, since the bright zone runs sideways along the wall and a plant to one side gets almost all the light with none of the traffic. Use the hinge side, or with a sliding door the half that does not open. Set it back a step or two from the threshold, which takes it out of the direct path of cold air without costing much light. And never narrow the doorway, because that makes the room feel smaller and gets the plant damaged.
What kind of plant suits the position?
Judge by habit. Firm foliage rather than soft broad leaves, because it will be brushed. Upright rather than spreading, because the footprint has to stay narrow. Stable, because it sits beside a route and a door that moves air. And either tolerant of the seasonal swing or easy to move in autumn, with the second being much easier to arrange than the first.
The part worth remembering
The floor beside a balcony door is frequently the best plant position in a small apartment, because the bright band runs all the way down to floor level and nothing else in the room does that.
It is also a doorway that opens onto the weather. Place beside it rather than in front of it, back from the threshold, and expect to change the arrangement between seasons. If you have not yet mapped your positions generally, the room-by-room survey is where to start.