Orange juice has some components that are beneficial to house or garden plants, such as natural sugars and citric acid, which is especially good for acid-loving plants like roses, azaleas, and many others. However, it can also attract bugs with its sweetness, and too much acid can damage plants, so it's best used in diluted form, and infrequently. Adding a few spoonfuls to your watering can once a month is the best way to use orange juice in the garden. Watering plants with diluted orange juice and other common drinks is a popular student science project.
Mix water and orange juice in a watering can. Add no more than 1/2 cup of orange juice to a large outdoor watering can, and for a small indoor watering can, just add 1 tbsp. of orange juice, filling the rest of the cans with water. Stir well with a spoon.
Water the plants properly, without pouring the orange juice water on the leaves or stems of the plants, where the sugars will leave a sweet residue that attracts bugs. Pour it onto the soil around the outside of the plants, not letting it touch the plant base. Use just as much water as needed to moisten the soil; don't overwater.
Clean out the watering can when you're done. The orange juice can leave a stickiness inside the container. Use gentle dish soap and rinse the can out very well to avoid adding any soap residue to the next round of watering plants.
Fertilize your plants regularly. Unlike most other acids, citric acid does not provide added minerals or nutrients that plants need.