Poinsettia leaves turning yellow

Iowa State University Extension says overwatering makes the lower leaves turn yellow and drop. It often happens when the plant stays in its foil sleeve and water collects at the bottom.

Take the plant out of the sleeve to water it, let it drain and empty the saucer. Water again only when the top 2.5 to 5 cm (1 to 2 in) of soil is dry.

Water standing in the sleeve encourages root rot. As the roots die, the plant turns yellow and drops leaves, says the Texas A&M Plant Clinic.

Slide the plant out of its pot and look at the roots. Healthy roots are white. Rotted roots are brown and mushy. If you find them, let the soil dry more between waterings and never let the pot stand in water.

A poinsettia that gets too cold can turn yellow and drop leaves quickly. A cold trip home, a draft from a door or leaves touching a cold window are the usual reasons.

Keep it at 15 to 21 °C (59 to 70 °F), with the leaves off cold glass and away from doors that open onto winter air.

The small yellow buttons in the center of the colored bracts are the true flowers. Over the weeks they open, shed pollen and fall off. That is normal aging.

A few leaves falling in late winter is normal too, as the plant slows down.

Feel the soil first. If it is wet, empty the sleeve and saucer and wait until the top 2.5 to 5 cm (1⁠–⁠2 in) is dry before you water again. If it is dry, water until it runs out of the bottom, then empty the saucer.

Then think about what changed in the last few days, such as a cold trip home or a new spot by a door or window, and fix that. Yellow leaves usually drop, so judge the plant by the leaves that stay green.