There was a row of four little pots on my kitchen windowsill in February and by the end of March two of them were sticks. That was the point at which I stopped assuming that indoors was the safe option and outdoors was the risky one. I had it backwards.
The appeal of the windowsill is obvious. It is warm, nothing eats the plants, and the herbs are two steps from the pan. I wanted it to work. I spent a season deliberately growing the same herbs in both places to find out where the honest line is.
The kitchen window test
My kitchen window faces roughly east. It gets direct sun in the morning and then nothing but grey light for the rest of the day. That is a normal kitchen window and it is also, I now know, the entire problem.
I put basil, parsley, thyme, and chives on the sill in matching pots in late winter. Outside, in a bed and a couple of larger containers, I grew the same four, starting them later once the weather allowed. Same source plants where I could manage it. Same watering habits, roughly, though the indoor ones dried out faster than I expected because of the radiator underneath.
By midsummer the outdoor versions were three or four times the size of the indoor ones and I was picking from them constantly. The indoor ones were alive, technically. I picked from them maybe twice.
Light is the whole argument
Everything else is a detail. A herb on a windowsill is getting a fraction of the light it would get in an open bed, and the plants respond by stretching. The stems get long and pale and the space between leaves grows, and you end up with a tall thin thing that looks like it is reaching for something, which it is.
The clearest example was the basil. Outdoors it built a compact bush with short gaps between leaf pairs. Indoors it made a leggy stem with a few sad leaves at the top and then leaned so far toward the glass that it fell over. I turned the pot every few days, which just meant it fell over in a different direction.
The other thing I underestimated was airflow. Indoors, still air plus a bit of overwatering gave me a white fuzz on the compost surface more than once and some soft brown patches at the base of stems. Outside, even a mild breeze seems to sort that out on its own.
What genuinely works indoors
Chives were the surprise. They did not thrive indoors, but they did fine, and they stayed usable through the coldest part of the year when the outdoor clump was doing nothing at all. Snipping green onions into eggs in February is worth having even from a mediocre plant.
Mint is the other one that tolerates the sill. It grows more slowly and with weaker flavour than the outdoor pot, but it does not collapse, and it is forgiving about irregular watering. If your goal is a few leaves for tea rather than a bowlful for a salad, a windowsill mint delivers that.
And there is a category I would call short term rather than growing. A supermarket pot of parsley on the sill, treated as something you use up over a few weeks, works perfectly well. It is not a plant you are keeping. It is a bunch of herbs with roots attached, and judged that way it is good value.
What sulks indoors no matter what you do
Basil, for me, is the headline failure. It wants heat and it wants strong direct light and a kitchen window only gives it the first one. Everyone I know who claims to grow good basil indoors turns out to have a south facing window with nothing in front of it or a grow light, and at that point we are having a different conversation.
Rosemary and thyme also disappointed me indoors. Both are plants that want sun and dry gritty conditions, and a warm kitchen with regular watering is close to the opposite of that. My indoor thyme went soft and floppy and tasted of very little. The one out by the path, in poor soil and full sun, was intensely aromatic by comparison.
Coriander bolted indoors just as fast as it does outdoors, so at least it was consistent.
The outdoor trade
Outdoors is not free. Slugs got into my parsley twice in one spring. Something, I suspect birds, shredded a young basil. A hot week without me watering nearly finished the container herbs while I was away for a weekend. The failures outdoors are dramatic and sudden.
Indoors the failures are slow and quiet, which is somehow worse. Nothing dies in a single night. The plant just gets a little worse every week until you notice, in June, that you have not picked from it since March.
Outdoors also gives you scale. An outdoor parsley plant will supply a household. A windowsill parsley plant will supply a garnish. If the goal is cooking rather than decoration, that difference decides it.
How I split them now
Everything I actually cook with lives outside from spring to autumn, in the ground or in big containers by the back door, close enough for the trip in socks. The windowsill gets two things over winter, chives and mint, plus whatever supermarket pot I am working through that week.
I stopped trying to keep a full herb garden indoors and the relief was immediate. The sill now holds two pots instead of six, both of them plants I know can cope, and I am not staring at four struggling stems every time I do the washing up.