Daffodils, from the genus Narcissus, are harbingers of spring; their trumpet-shaped flowers in shades of yellow or white grow from bulbs. Daffodils are perennials that return year after year in the garden when given the proper culture. American gardeners grow daffodils across most of the temperate-climate states.
The key to a daffodil's life is the health of the underground bulb. The bulb is a modified stem that stores carbohydrates. From the bulb base, thin roots grow to absorb water and nutrients. Inside the bulb are layers of tissues, including cells that develop into flower buds and leaves. The bulb persists year after year. Cool soil temperatures in winter trigger hormones to form a preliminary flower bud. In late winter's warmth, the bud and leaves emerge from the soil. The blossom lasts one to three weeks, but the leaves endure into late spring. The foliage conducts photosynthesis to replenish the bulb with food so it can produce side bulbs and bloom again next spring.
Daffodils grow best when planted in a moist but well-drained soil. Avoid planting them in compacted or heavy soils that readily flood or remain wet for long periods after rain. Bulbs rot in oxygen-depleted ground. Ideally, when daffodil bulbs are dormant in summer, warm and dry soil is better than cool, wet conditions. Plant daffodils where the foliage basks in no less than 6 hours of sunlight daily. Without sufficient light, the leaves cannot replenish the energy reserves of the bulbs. Weak, malnourished bulbs fail to return another year.
In a broad context, daffodils grow well in U.S. Department of Agriculture plant hardiness zones 3b through 9a. However, not all species or cultivars of daffodils do well in regions with climatic extremes, such as winter cold in zones 3 and 4 or summer heat in zone 9. In these areas, only some daffodils grow as perennials. In Minnesota, in zones 3b and 4, only the hardiest daffodils survive the long, cold winters and cool, moist soils in summer. Conversely, the long, hot summer in northern and central Florida, zone 9, can harm some daffodils since the heat causes foliage to prematurely wither or summertime rains can lead to bulb rot.
Daffodils contain toxins that prevent rodents and deer from eating the bulbs, flowers and foliage. Therefore, the plants tend to grow without any predators. As long as leaves make food after flowering ends and naturally wither away by late spring, daffodils tend to naturalize. Naturalizing means that the bulbs reproduce and that seed occasionally forms. This allows plants to multiply in number and physically spread across a landscape. For example, if you plant 10 daffodil bulbs on a sunny hillside, you should expect to see three to five times as many bulbs and springtime flowers within five years.