The poinsettia is one of the most reliably misread houseplants there is, and the misreading almost always goes in the same direction. Leaves start yellowing, the owner assumes something is wrong, and the instinct is to water more. In the majority of cases that instinct is exactly the wrong response, and following it is what turns a recoverable situation into a lost plant. Before you reach for the watering can, it is worth understanding what a poinsettia is actually telling you when its leaves go yellow, because the cause determines the cure and the two most common causes require opposite responses.

The first and most frequently misdiagnosed cause is natural and entirely benign. A poinsettia that has been growing well since December, sitting in good light and cared for consistently, will eventually start dropping its lower leaves as the plant gets taller and the upper canopy fills out. The leaves lower down simply stop receiving enough light. They yellow and fall. This is not disease, not distress and not a watering problem. It is what the plant does as it grows. The new growth at the top looks healthy, the coloured bracts are holding, and the plant as a whole is performing well. If this is what you are looking at, the answer is not to change what you are doing. The answer is to pot it on.

A poinsettia that has been in the same small nursery pot since it was purchased and those pots are almost always far too small, typically around ten to eleven centimetres in diameter, will run out of root space long before it runs out of growing season. Once the roots are congested and have nowhere to go, the plant cannot take up water or nutrients efficiently regardless of how much you give it, and the lower leaves are the first to show the stress. Moving it into a pot roughly a third larger, something in the seventeen to nineteen centimetre range, with a light, very free-draining compost, gives the root system room to expand and almost always stabilises the plant quickly. Put it back in the same position it has been in, go back to exactly the same watering frequency you were using before, and let it settle. The potting on is the intervention. More water on top of a congested root system is not.

The second cause of yellowing leaves is overwatering, and this one does require action, though not the action most people take. Overwatering does not mean you have watered too much on one occasion. It means the plant has been watered too frequently over a period of time, keeping the compost consistently damp when the plant needs it to partially dry out between waterings. The roots in a constantly wet compost cannot breathe, they begin to fail, and the plant cannot draw up what it needs even when nutrients are present. The symptom, again, is yellowing leaves, but in this case the yellowing tends to be more general across the plant rather than confined to the lowest growth, and the compost will feel wet or even waterlogged when you check it. The fix is to ease off entirely. Allow the compost to become noticeably drier before you water again, and when you do water, do so thoroughly and then leave it alone until the top of the compost feels dry to the touch. The brown tips that have already appeared will not recover, but new growth coming through should be clean if the watering is corrected.

Draughts are worth checking, but only if the plant has been moved or if there is an obvious source of cold air nearby. A poinsettia that has sat in the same spot all winter without issue and then starts dropping leaves in spring is almost certainly responding to pot congestion or watering rather than a temperature problem. Draughts tend to cause sudden, dramatic leaf drop rather than gradual yellowing. The position that has worked since December will continue to work. Do not move the plant trying to solve a problem that has a different cause entirely. Plants under stress from the wrong growing conditions are also more vulnerable to other problems, which is why getting the basics right, pot size, watering frequency, light, matters more than any treatment you might apply afterwards.

The broader point, and one that applies well beyond the poinsettia, is that the soil or compost a plant is growing in determines almost everything else about how it performs. A light, well-structured, free-draining compost that allows roots to breathe and holds moisture without becoming waterlogged gives a plant the foundation it needs to deal with whatever else comes at it. Incorporating biochar into potting compost improves that balance significantly, holding nutrients and moisture in a way that the roots can access them without the compost ever sitting wet. It is the same principle that applies when replanting bulbs in pots after they have flowered the compost the plant goes back into matters as much as anything else you do. Get that right and most other problems become much easier to manage.