It's natural for limes (citrus spp.) to turn yellow and drop as they mature to full ripeness. The tangy fruit, which takes about four months from formation to harvest, is picked for best flavor when light green and slightly soft when squeezed. Perennials in U.S. Department of Agriculture plant hardiness zones 9 to 11, lime trees can also be grown in containers in- or outdoors. Move pots inside if temperatures fall below 50 degrees Fahrenheit.
Mostly Seen When Green
Sometimes mature limes are confused with lemons (citrus limon), also hardy in USDA zones 9 to 11. Thicker-skinned Persian limes (citrus latifolia), the type most often stocked in grocery stores, are typically harvested when green. Ditto for Mexican limes (citrus aurantifolia), known as key limes due to their association with the Florida Keys, which will undergo the same yellow transformation and drop off the tree if not harvested earlier in the ripening process.
Perpetual Harvest Cycle
Trees begin bearing fruit about three years after planting. The Mexican lime tree bears fruit year-round, with peak production approximately May to June and November to December. Persian limes, sometimes known as Tahiti limes, ripen throughout the year but are most prolific during the summer months, producing 20-30 pounds of fruit in year three, 60-90 in year four, 130-180 in year five and continuing to increase in yield as long as the tree remains healthy. If you aren't regularly harvesting you can expect to find dropped yellow limes around the base of your tree.
Falling Fruit Exam
Check any limes that have yellowed and dropped from the tree before their time for firmness, appearance and other clues that will help diagnose the problem. Fruit splitting, which leads to rot and yellowing, can be an issue after heavy rainfall. The flush of precipitation causes interior swelling but the peel doesn't expand and ultimately splits, causing premature fruit drop.
Avoiding Lime Disease
Limes, like other citrus trees, are vulnerable to a variety of diseases. Choose disease-resistant stock to avoid untreatable genetic disorders such as citrus blotch, which causes yellowing of the outer skin.
You can also keep lime trees in peak condition by planting in full sun in well-draining soil, keeping soil moist but not oversaturated. Manually check by digging a few inches into soil or insert a moisture meter. Fertilize three times yearly using one pound of 8-8-8 fertilizer per tree age up to a maximum 10 pounds. Scratch in well from the root area to outer branches.