Question: Last fall, I brought my zonal geraniums indoors and also took cuttings. My plants are quite beautiful and green, but the bottom leaves turn yellow. I’m not sure if I’m watering them too much or not enough or if the container is too small. What’s the cause of those yellow leaves?
A. Gage, Maine
Answer: It’s actually quite natural for the lower leaves of the “zonal geranium” (in fact, the term zonal pelargonium would be more appropriate, because this plant is not a true geranium), that is, Pelargonium x hortorum, to turn yellow as they age.
When the plants are outdoors in the garden over the summer, older leaves do yellow, but this is less visible, because the plants produce a large number of younger leaves that hide the old ones as they die back. Indoors, where conditions are not as good as outside, pelargoniums produce fewer and less dense leaves and also lose them more quickly, so yellowing is more obvious.
To help reduce yellowing, increase the light intensity if you can (full sun is best) and start fertilizing as soon as spring arrives (but not in winter, as fertilizing at that season tends to cause etiolation).
Pelargoniums grow best when they are watered moderately: more lower leaves turn yellow when they are overwatered … but then, lower leaves also turn yellow (in fact, massively so: see the photo at the beginning of this article) when they’re underwatered. To know when to water, use the “finger test”: insert your finger into the potting mix. When the soil is dry to the touch, it’s time to give the root ball a thorough soaking; if not, test again in a few days.
If you discover that the potting mix dries out very quickly, in less than a week, it would be wise to repot the plant into a somewhat larger pot: with more room for the plant’s roots, the soil won’t dry out so quickly.
Now, it is also possible that the yellowing is caused by a leaf disease of some sort (verticillium wilt, rust, mildew, bacterial wilt, etc.), but this is most common under conditions of high atmospheric humidity, like inside a sealed greenhouse, than in the average home where the air is, in fact, usually very dry. So, I don’t think that’s a consideration in your case.
Finally, even if you do nothing at all, expect your plants to return to their usual dense, green appearance once you place them outside for the summer. Simply put, better conditions give better results!
Through the yellowing leaves you see presently, your plants are trying to tell you: “We’re tired of being indoors; we want to enjoy direct sun in the open air again.” And that’s a service that you’ll surely offer them once all risk of frost has left your area.