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Water regularly during the summer. Apply compost each spring. Add mulch. Prune lightly after the main bloom period. Do extensive pruning in the spring. Be aware of possible pest and disease problems.
During its active growing season, oleanders need 1 inch (2.5 cm) of water per week. You may not need to water the plants often during a wet summer, but you should water them during a dry summer.  Even though established oleanders can resist drought conditions, they thrive when they receive an occasional deep watering. If the leaves begin to yellow, however, the plant is receiving too much water. Remove any leaves that have completely yellowed and allow the oleander to remain dry for a longer period of time. While harsh fertilizers are usually unnecessary, you can fertilize the oleander by spreading compost out from the base of the main stem to the space under the outermost branches. You could use a mild fertilizer with balanced amounts of phosphorus, potassium, and nitrogen in the spring if the soil is especially nutrient-deprived, but only do so within the first few years. After the plant establishes itself, switch to a light compost instead of an actual fertilizer. Apply a 2-inch (5-cm) layer of mulch each spring. In the late fall, remove this layer and replace it with another 2-inch (5-cm) layer.  Mulch added in the spring protects the plant against weeds and allows the roots to remain moist. Mulch added in the fall can help insulate the root system and protect the plant against cold weather. Use an organic mulch, like wood chips or grass clippings. Once a flower cluster dies, cut it off to encourage a longer bloom period overall. After flowers are spent, you should also cut off the stem tips to encourage better branching. Do not wait too late into the fall to do this, however. New growth needs several weeks to harden itself before the frost comes. In the summer, oleanders bloom on new growth, so it is best to do most of your heavy pruning in the early spring before the plant enters its active growing period.  You must remove wood that has been damaged by the cold, pests, or disease. Also remove overgrown and intertwined wood. During the spring, you should also remove suckers from the bottom of the plant since they can drain the plant's resources and reduce its flowering ability. Trim down the plant to its desired size, cutting it back to a place just above a node on the stem. The leaf node is a section where three leaves come out from the branch. Making the cut here will force new branches to form at the node, and you will get three new branches when the plant grows back. Avoid pruning more than one-third of the plant when possible. Oleanders can recover if you prune more, but severely heavy pruning can weaken it some. Usually, oleanders grow rapidly, extending themselves by another 1 to 2 feet (0.30 to 0.61 m) (30.5 to 61 cm) per year. If left unattended, oleanders can reach heights between 8 and 12 feet (2.4 and 3.7 m), and they can grow as wide as they are tall. Some oleander plants have even been known to reach 20 feet (6.1 m) (6.1 m). Dwarf varieties, however, usually stay between 3 and 5 feet (0.9 and 1.5 m) tall. Oleanders rarely face problems with pests and disease, but they are not completely immune. Treat the plant only after you spot a problem, though.  The most common disease faced by the oleander plant is botryosphaeria dieback, and it is most likely to happened if the plant has undergone stress from a drought or freeze. The branches and shoots will die and become a blackish brown. Completely remove the infected branches and dispose of them away from the rest of your garden. Oleander caterpillars are the most common pest. They feed in groups and can demolish young shoots. These pests are about 2 inches (5 cm) long with orange-red bodies and black tufts of hair. Quickly apply a pesticide as soon as you spot multiple caterpillars on your plant. Aphids, mealybugs, and scales can also cause problems, but are rare. Apply an appropriate pesticide as needed.