Planning your choices around the three most familiar bulb types—crocuses for early spring, then daffodils and finally tulips—assures the longest period of bloom, but beyond these basic bulbs are many opportunities for attractive interplantings. For true naturalizing, though, you want to seek types and varieties that best suit the technique.
For early spring, small bulbs such as eranthis, or winter aconite, bloom with small, yellow, buttercup-like flowers that add a delicate, frilly accent to crocuses. They're also good naturalizers; they're hardy and can thrive in shade and under trees. One warning, though: Winter aconite tends to be invasive, so you'd better love it or leave it. A less aggressive accent choice for early spring is chionodoxa, or glory-of-the-snow, which yields dwarf flower spikes of 8 to 10 azure blossoms with a white heart. Chionodoxa luciliae is one popular variety.
Choices for daffodils are many, but the Tazetta type of narcissus is good for naturalizing. Not only is this the oldest type of daffodil in cultivation, but its vigor, height (16 to 18 inches), and adaptability to sun or shade assure a bold yield. These bulbs also are good choices for gardeners in the South because they're winter hardy only to USDA Plant Hardiness Zone 9. 'Scarlet Gem' is a vivid highly fragrant variety, with rounded primrose-yellow petals and a deep orange cup.
An even better choice for naturalizing in Far South gardens is the hybrid Narcissus cyclamineus. They are strong and hardy despite their delicate appearance, and they can be planted safely far in the South. Their very early spring-flowering bulbs yield nodding blooms with backward-turned petals resembling cyclamens. They're also low-growing and make an excellent foreground planting with Tazettas. 'February Gold', with its long golden-yellow trumpets, is considered the strongest and most enduring of the cyclamineus hybrids.
Other hybrid varieties, widely adaptable and good for naturalizing, include 'Thalia', a popular orchid like daffodil with pure white flowers, and 'Trevithian', a cluster-flowering type with pale yellow petals, deep yellow trumpets, and a heady fragrance.
For Floridians and other gardeners in the Far South, the choice of spring-flowering bulbs is limited and generally chancy. Tulips are out (because they need more chilling hours than they get in warm winter climates), but Narcissus tazetta is successfully cultivated in north to central Florida. An even more widely adaptable choice for Florida naturalizers is Iris xiphium, or Spanish iris, which grows to about 2.5 inches high, usually in blue-purple with an orange blotch. It easily can cover whole yards anywhere in the state.
Part of the art of naturalizing your bulbs is to balance the formal aspects of your garden—and extend it. Where strict borders of daffodils or tulips may produce a lovely, sculpted effect, a naturalized area nearby gives an aura of carefree grace, an invitation to interact more fully with your garden than merely to observe its floral display.
Tulip companions are a must, then, to give your naturalized plantings an air of casual charm; unmixed tulip groupings are simply too stiff and formal for the wild look. The hardy wood hyacinth Hyacinthoides hispanicus, or Spanish bluebells, makes an unusual yet worthy tulip companion. Its high stalks are laden with bell-shaped blossoms in blue, pink, or white. 'Excelsior' is a hardy variety, with a subtle blue color.
Then there is camass, hardy to Zone 2, fine in sun or partial shade, amenable to lots of water during flowering, yet resistant to drought after blooming. By the time your tulips bloom, camassia's 2.5-to 3-foot-high stems are hung with delicate thin-petaled flowers in baby blue with accenting yellow stamens. Their effect softens and harmonizes with even the most vivid tulips. For naturalizing, Camassia Cusickii is the variety of choice.
As a general rule, bulbs are planted at a depth of about three times their height. But research has found that planting tulips 8 rather than 6 inches deep tends to increase their life span significantly.
Generally, you need to plant bulbs, in shade or full sun, before your area's first hard frost—September, October, or November in most areas north of the Mason-Dixon line and below the line as far south as Georgia and Arizona and in the Pacific Northwest. Further south, plant in the shade in October, November or December. Again, well-drained soil is the crucial factor, along with choosing bulbs appropriate for your area and for naturalizing. Find sites for them that showcase their wild charms, and enjoy a spring-after-spring celebration of natural gardening at its colorful best.