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Cold temperatures with cold soils and overwatering can result in yellowing leaves and root diseases.

Courtesy Meredith Seaver, USU Extension

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Small greenhouse-style windows can be very cold in the winter. Also, depending on the orientation of the window, light levels can be low in the winter, affecting the health and vigor of indoor plants.

Courtesy Meredith Seaver, USU Extension

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We have a little lemon tree that is sick. Yellowing leaves. We keep it in a little greenhouse window. Too cold, maybe? I was watering every day, pouring in a large glass. Water was coming out the bottom, so I know it was draining. When it started going yellow, I guessed the problem might be water and I backed off to water only every 2-3 days. But I’m not sure how to heal the roots.

Only time will tell whether your little lemon tree can be saved. The roots most likely have developed some root rot because of frequent watering. The tree may or may not recover, depending on how extensive the damage is. Plant roots need oxygen, and they only get it when air enters the soil. If soil stays wet, there isn’t room for air. Without oxygen, the roots won’t be vigorous and can even suffocate.

Wet soils also make root and crown rot more likely. Deep, less frequent watering makes it easier for air to enter the soil. I can see in your photos that you’ve used a potting mix that should give you good drainage in your container, but even the best mix is no match for daily watering of indoor plants.

Your potting soil needs to dry a bit between waterings. Watering every other or every third day is too often for most indoor plants, especially in the winter when light levels are lower and temperatures may be cooler. For a container the size that you’re using, you can wait between waterings until the soil is dry an inch or two down into the soil before watering again. Water gently over the soil surface, instead of simply pouring water onto one spot, with enough water so that some comes out the bottom of the container. Then, wait until the soil dries down an inch or two again.

The temperature around your pot can make a difference. Normally, citrus trees can tolerate cooler temperatures, but those low temperatures affect how quickly the tree can move water out of the soil, making chronic wet soil conditions more likely. Lower soil temperatures also affect the availability of plant nutrients. I don’t know what the temperature range is in your little greenhouse window on cold winter nights, but my best guess is that you can safely go with your own impression on this. Try setting your tree on a counter away from the window at night and return it back to the window after breakfast for the next several weeks.

Frequent watering can leach nutrients from the soil, causing nutrient deficiencies. The yellow leaves on your lemon tree may tempt you to “help” your tree by giving it some fertilizer to green it up. Nitrogen fertilizer can push plants to put on green growth, but sick plants don’t need that extra stress. Your tree may not have the root system it needs to support that growth, either. Delay fertilizer applications for a while to avoid this risk.

With a change in watering, slightly warmer conditions and a little patience, you may see some improvement. If your tree does improve, you can apply some fertilizer in early to mid-April before you start hardening it off for a summer outdoors again.