Indoor citrus plants thrive on moisture and light
Walk into some greenhouses in the winter and your nose will pick up a sweet scent. If you follow your nose, you're likely to come upon a plant with miniature orange or yellow fruit and delicate white flowers - a citrus tree. With edible fruit and fragrant flowers for months on end, an indoor potted citrus tree is a delight.
If you hope to harvest fruit, choose a naturally acidic citrus, not a sweet orange or grapefruit. Examples of acidic varieties include 'Improved Meyer' and 'Ponderosa' lemons, calamondins (used to flavor food and drinks), and kumquats (good for jams). These are most likely to produce fruit indoors in winter. Other citrus varieties will grow and flower, but they are less likely to produce fruit.
Our homes in winter are darker and warmer, and have much drier air than outdoors. So anything you can do to provide additional light and extra humidity is beneficial. Keep your citrus near a sunny window and use a room humidifier, if possible. Cool, bright rooms, such as a partially heated sunroom, are best.
Choose a pot about the size of a 15-gallon nursery container. The ubiquitous half whiskey barrel is a good size, and plastic and faux clay pots in the 30- to 36-inch-diameter range work well, too. Whatever you choose, make sure it has good drainage; drill extra holes if you're in doubt. To prevent soil from washing out, cover drain holes with small sections of window screen, but don't cover the holes with stones. Use a premixed sterile potting soil designed for container plants.
Keep the soil moist by soaking the rootball thoroughly until water drains out the bottom into the saucer beneath. Water again when the top two to three inches of soil are dry. In some situations, water will drain out the bottom of the pot without soaking the rootball. This happens when the rootball dries and shrinks slightly, pulling away from the edges of the container. The water moves down the gap without rewetting the roots. To help rewet the dried rootball, place three or four drops of a mild dish soap on it. The soap will help the water soak in, so the rootball can expand to fill the container again.
Citrus need regular fertilization to promote flowering and fruiting. You can use a controlled-release fertilizer or a soluble liquid fertilizer. Liquid fertilizers generally provide more exacting control but also require more frequent applications, every other week or so. In either case, follow the directions on the label.
More than most plants, citrus are prone to deficiencies of the micronutrients iron, manganese and zinc. Inadequate amounts of any one of them will cause leaves to yellow while veins remain green. Look for the micronutrients in the chelated form, which makes the micronutrients more accessible to citrus roots. The best time to add them in is early spring, just as new leaves are beginning to emerge.
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