Answer: Leaves with a sticky substance on them usually indicate that aphid or scale insects are feeding on the undersides of leaves or stems. Inspect your tree for colonies of these pests and remove them by handpicking, or control them with insecticidal soap applications. Once you've gotten rid of the pests, you should have no more sticky residue on the leaves.
You can grow your citrus trees indoors during the cold months, and move them outside while the weather is frost-free. Indoors, citrus trees need average warmth, freedom from drafts, ample water, and well draining soil. You may need to supplement natural light with fluorescent or gro-lites especially made for plants. Turn the light on for 12 to 14 hours each day and turn it off at night. Citrus trees need ample moisture, so water often enough to keep the soil moist but not soggy, and feed in the spring and summer months with a diluted liquid fertilizer, especially if the leaves are yellowish instead of glossy deep green. Watch out for a resurgence of scale, aphids and mites, and use insecticidal soap or light horticultural oil according to label instructions if they do show up.
Be sure to give your trees a gradual transition from indoors to outdoors, and vice versa, so they have time to adjust to the changes in the environment.
Following the above recommendations should help your trees regain and maintain good health.
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