The peace lily that wilted every third day
The peace lily came with the apartment, more or less, left behind by the previous tenant in a plastic pot with no drainage holes, and for the first month it collapsed dramatically every third day. The leaves would go from upright to flat over the rim overnight, I would water it, and within four hours it would stand back up as if nothing had happened. It is a famously theatrical plant, but this seemed excessive, and I started keeping track.
Day one, water; day four, wilted; water; day seven, wilted; water. Three cycles like that. The soil, when I finally pushed a finger into it, was dry and dusty on top and, two inches down, bone dry and pulling away from the pot sides. Water poured on top was running straight down the gap and out of the bottom without wetting the root ball at all. The plant was wilting because it was not actually getting watered, however much I poured.
The fix was a long soak. I filled the sink with about four inches of lukewarm water and stood the pot in it for forty minutes, until bubbles stopped rising and the surface was dark. The pot weighed about twice as much when I lifted it out. I let it drain for an hour, then left it alone.
It did not wilt for nine days. When it did, I soaked it again rather than watering from the top, and that has been the pattern since: a bottom soak roughly every week to ten days, judged by the weight of the pot more than by the look of the leaves. I also repotted it in a terracotta pot one size up with proper drainage and fresh mix, which got rid of the pot-side gap entirely, and moved it from the dim hallway to the north window, where it put out two white flowers in August.
The general lesson, which has applied to several plants since, is that a plant that wilts right after watering is not thirsty, it is unwatered, and the two look identical from above. Dry peat-based mix is water-repellent once it has fully dried out, and a watering can is not enough to fix that. A sink is.