Sweet orange (Citrus sinensis) is a generous tree that gifts the garden with evergreen foliage, fragrant white flowers and juicy, vitamin C-rich fruits. Flower drop is a disappointing occurrence that may be the result of environmental factors such as frost or drought, or a lack of pollination. Improving the overall cultural conditions will help any treatment plan.
Sweet orange thrives in U.S. Department of Agriculture plant hardiness zones 9 to 11. In USDA zone 9, where temperatures fall below 24 degrees Fahrenheit, blossoms may drop due to frost damage. Protecting your tree can be as simple as covering the tree with a blanket. You can also build a frame of PVC pipe around the tree and cover it with plastic sheeting. Orange trees are best grown in a bright, full sun location, ideally on the south side of a building where they can receive additional shelter and warmth.
Though citrus trees are self-fertile, they can still struggle with pollination. This is especially true for indoor trees, which don't have access to pollinators. To prevent blossom drop on small trees, Citrus Trees Online recommends shaking the tree when the flowers are open, or using a cotton swap or small paint brush to move pollen from flower to flower. Avoid using broad spectrum insecticides outdoors, which kill many natural pollinators such as bees and butterflies.
If drought strikes while the tree is blooming, flowers often drop. It is crucial to water newly planted trees: at least every other day for the first couple weeks and twice a week for the next month or two after. Even once the tree is established, you may need to water as much as three times a week during the summer. Do not water so often that the soil becomes flooded however. If you stick your finger 2 to 3 inches in the soil and it feels wet, do not water.
You can help prevent problems such as flower drop by following proper cultural guidelines. Orange trees do best in a well draining or sandy loam. They suffer in poorly draining, salty or poorly draining soils. The University of California Davis recommends giving trees less than 3 years old 2 tablespoons of nitrogen, applied under the tree before irrigation three or four times a year. Mature trees can benefit from 1 to 3 pounds of nitrogen. Water after fertilizing to help the soil absorb nutrients.