Fragrant plants are an important element of any garden, and not just because they smell nice. The scent attracts the insects that distribute pollen and ensures the survival of the species. The bees and bugs that come to the garden are beneficial and help maintain the balance of nature.
A plant’s aromatic oil travels inside it in much the same way as blood travels through our bodies.  The plant “breathes”, too—taking in oxygen and exuding carbon dioxide  through minute openings in its leaves.  When these holes are open, scent from the oil escapes into the air.
For the most enjoyment, locate scented plants close to the house or a sitting area. As a bonus, the reflected heat from a wall or patio strengthens most scents.  On the other hand, wind can dilute or displace it, so try to provide a sheltered space.
Light-coloured  or white annual flowers tend to be the most fragrant.  Most plants release their scent during warm,  humid mornings or evenings.
Here are five popular annual varieties, all easy to grow. The can be started in pots or flats indoors two  months before the last frost, or sown directly outside after the threat of frost has passed.

Flowering Tobacco (Nicotiana sylvestris, N. alata, N. suaveolens), 90 to 120 cm, full sun to partial shade. Tubular white or pink flowers, sweetly fragrant primarily at night.  Good in containers.
Heliotrope (Heliotropium arborescens), 45 to 60 cm, full sun to partial shade. The large white or violet blossoms have a very appealing cherry-vanilla fragrance that attracts butterflies.
Night Phlox (Zaluzianskya ovata), 15 to 30 cm, full sun. As the name suggests, the small flowers of night phlox are strongly fragrant at night and have an unusual licorice or sweet almond scent.
Stock (Matthiola incana), 15 to 45 cm, full sun. These strongly clove-scented flowers are recommended for cut flowers and thrive in cooler weather. Night-blooming stock (M. bicornus) is an unremarkable plant during the day, with small, curled-up flowers and trailing foliage, but transforms itself at night as the flowers open and fill the air with spicy  perfume.
Sweet Alyssum (Lobularia maritime), 15 cm, full sun. The delightfully tiny white flowers have a honey-like fragrance that attracts beneficial insects. Often used as a ground cover.