Oleanders are evergreen, flowering shrubs that thrive in warm climates. They are hardy only in U.S. Department of Agriculture plant hardiness zones 8 or 9, depending on the variety, but grow prolifically in the right setting. When determining planting space, consider the variety as well as your intended use.
Oleander cultivars vary widely in their mature height and width. Read the plant label and space plants according to their size. For example, Petite Salmon grows only 4 feet high and 4 feet wide. Space these plants at least four feet apart. Some tall varieties, such as Sister Agnes and Hardy Pink, grow 25 feet high and are best grown as specimen plants by themselves.
Oleanders have a natural rounded form. For a layered, open hedge, plant them to allow enough space so the mature plants barely touch. For example, plant oleander plants that grow 8 feet wide when mature no closer than seven or eight feet apart. For a tighter, heavily pruned hedge, space the plants two feet closer. To train oleanders to grow as trees, remove lower branches to one central leader. Oleander trees can be spaced more closely together.
Although oleanders may look sparse when first planted, don't be tempted to plant them more closely. Oleanders grow rapidly, gaining 12 to 18 inches per year. They'll quickly fill in, making a lush hedge. Planting them too closely together may contribute to diseases and insect problems.
For best growth, plant oleanders in full sun in well-drained soil. Oleanders tolerate sea salt and a variety of soils, as long as it drains well. Some oleanders are more winter hardy than others. Select a variety well adapted to your climate. All parts of the oleander plant are highly toxic. Do not plant oleander in a backyard with young children or pets.