My Meyer lemon tree has had leaves turning yellow, and eventually dropping, since I brought it indoors in the fall.
It sits in a South facing window, and I supplement the light with plant bulbs. I water it about every 10 days. I replaced part of the soil with new potting soil.
Thank you for your inquiry. Citrus trees grown in pots require a balance of bright light, consistent moisture and good drainage. It is not unusual for these plants to drop their leaves when they are moved indoors since these plants are particularly sensitive to this shift in light intensity, humidity, and water.
Lemon trees require 8 hours of direct sunlight per day; however, they will drop their leaves if it is too hot especially around the roots. Check the soil temperature with a thermometer, the soil temperature should be between 18C- 21C. The root zone temperature will always be 5-10 degrees lower than the room temperature due to evaporation from the soil surface. By removing the supplemental lighting and keeping the plant in the south facing window, you may reduce the temperature of the tree’s foliage and thus reduce the root-foliage differential.
Excessive fertilization can also lead to leaf drop. Young trees require light fertilization during the first year; about 1 tablespoon per month of 8-8-8 fertilizer during spring and summer. After the first year, fertilize trees once every four to six weeks during active growth.
Certain insect pests can cause leaf drop. Lemon trees are susceptible to scale which can cause yellowing of leaves and leaf drop. These armoured pests are most active in late summer and early fall. Scale appear as bumps on the plant stems and leaves.These pests suck plant sap and can kill plants if present in large numbers. Look for sticky, honey-like sap droppings, this will be a clue that scale may be present. Remove and destroy badly infested stems. Plants can also be sprayed with insecticidal soap that is available from your local nursery.
You may also wish to refer to another question on leaf curl in a Meyer Lemon tree which is posted on our Master Gardener Website: Growing Meyer Lemon Tree Indoors
Good Luck .