Species tulips and hybrids of Tulipa fosteriana, T. greigii, and T. kaufmanniana are ideal candidates for naturalizing, as they spread rapidly by seed, stolons, and bulblets. These wildflowers of the tulip family are less vulnerable to stormy spring weather, and their generally short stems don't bend in strong winds. Even without these hardy characteristics, their colorful blossoms are reason enough to grow them.
Naturalized plantings are easy to maintain. It takes a few years for the bulbs to multiply enough to fill in and make their full impact, but in the meantime you still get to enjoy the sight of these harbingers of spring.
Choose bulbs for your site and zone. In an open space, you need at least 100 bulbs per 12 square feet to make an impact. To fill smaller nooks or to accent a rock garden requires fewer bulbs.
Choose varieties that are suited to your growing conditions. If you live in the North (USDA Climate Hardiness Zones 3 to 7), the varieties have to be cold-hardy; in warmer zones, heat tolerance is more crucial if bulbs are to thrive and multiply.
General requirements are full sun in the spring and well-drained soil. You can plant tulips under deciduous trees that will not fully leaf out until after the bulb foliage has faded. If the areas to be naturalized have poor drainage, work fully composted pine or fir bark or other organic amendment into the soil to lighten it up.
When to plant. Ideally, wait until the soil temperature is below 60oF. As a general guide, in zones 4 or 5, plant in late September through early October; zone 6, mid-October; zones 7 and 8, early November; zone9, early December; zone 10, mid-December (if you're not growing the mild-winter tulips mentioned in the Tips section below, refrigerate the bulbs for 8 to 10 weeks).
Lay out the planting area. For naturalistic plantings, lay bulbs out in informal masses with curved borders and asymmetrical shapes. Lay a hose or piece of rope on the ground to mark the boundary of your planting area, and plant within it.
Within the marked area, spread odd numbers of bulbs (three, five, or seven in a group), since even numbers are more formal-looking. Make spacing between groups random, too. If you're planting a large area, use stakes so you can keep track of where you've planted and where you have yet to plant. After you've planted a group, pull up the marking stake and lay it flat over the planted area so you don't dig there again.
Set the bulbs in a planting bed or in separate planting holes with their roots or basal plates downward. Plant bulbs 4 to 6 inches below the surface, or at a depth three times their widest diameter. In sandy soil, plant deeper, and in clay soils, shallower. Space the bulbs of most species tulips according to the supplier's instructions, usually 2 to 6 inches apart, or three times their width. Place a few stakes around the area when you're done, and water so moisture penetrates a couple of inches. In mild-winter areas, mulch after planting to help keep soil cool; in cold-winter areas, mulch after soil freezes.
Maintaining bulbs. In spring, water the growing plants if the garden doesn't receive about 1/2-inch of rain weekly. Species tulips are dormant in the summer and prefer dry soil then.
Feed the tulips a few times with half-strength liquid fertilizer while they're actively growing: once the leaves have fully emerged, once after flowering is complete, and again two weeks later if the leaves are still green. Choose a fertilizer that is lower in nitrogen and higher in phosphorus and potassium (such as a 9-9-6 analysis).
To help maintain soil health in the area, sprinkle 1/2-inch or so of aged manure or compost over the area in the early spring. Allow bulb foliage to remain in place until it fades completely, ensuring that the bulbs will have energy to multiply and add more volume to the display each year.
Mow the naturalized bed a few times over the summer and fall to remove competing plants, and to keep the area open.
Encourage these tulips' multiplying tendencies by leaving faded flower heads attached so that seeds can mature and spread.
If you live in the South or in mild-winter areas of the West, plant tulips that thrive in zones 8 through 10: lady tulip (T. clusiana), Candia tulip (T. saxatilis), and Florentine tulip (T. sylvestris). These tulips do not need chilling before planting in these regions. In areas such as Tallahassee, Houston, or San Diego, buy these types in the fall, plant them in a cool, shaded location, and forget them. They'll flower in spring and likely for many springs to come.
Naturalizing bulbs is a great group activity: share mass bulb planting tasks with your gardening neighbors.