One of the first signs that spring is on its way is the sight of daffodils nodding their heads in the breeze. In mid-spring, tulips provide some of the most vibrant flower shades, with hues ranging from yellow to pink, red or violet. As the season changes to summer, pompom-headed alliums, hemerocallis, other lilies and stately multicolored gladioli provide beautiful accents in my garden. In the fall, dahlias add a wide variety of color, texture and design to the pallet. A carpet of cyclamens with their vivid colors brightens up a tired, lackluster winter garden when other plants are in their dormant phase. Marin gardeners can choose from a huge and colorful array of bulbs for every season of the year.

Bulbs exist for different reasons and each form has its own flowering and resting period. A bulb may be defined as the resting stage of certain perennials. Bulbs consist of a relatively large, usually globe-shaped, underground bud with fleshy overlapping leaves arising from a short stem. These leaves function as food reserves that enable a plant to lie dormant during winter or drought and to resume active growth when appropriate conditions prevail.

There are two main types of bulbs. One, typified by the onion, has a thin papery covering protecting its fleshy leaves. The other, the scaly bulb, as in true lilies, has naked storage leaves, with no papery covering, making the bulb appear to consist of angular scales.

Bulbs enable many ornamentals, such as the narcissus, tulip and hyacinth, to flower rapidly as growing conditions become favorable. While many people refer to similar structures in crocuses, gladiolus, dahlias and irises as "bulbs," technically, they're not. Instead, they're corms (crocuses and gladiolus), tubers (dahlias) and rhizomes (irises), the differences mostly being which plant parts store the nutrients. Since for our purposes they all act in a similar manner, we'll refer to them as "bulbs" to keep from having to repeatedly list them all.

The best time to shop for bulbs is as soon as they are available in garden centers, usually from late summer onward. From late winter to early summer, bulb catalogs appear; orders are sent out in three cycles: the first in late summer and very early autumn for autumn-flowering plants, the second from early to late autumn for spring-bloomers and the third in spring for summer bulbs.

Size is important as undersized bulbs will not flower in the first year. A simple rule of thumb is that the bigger the bulb, the bigger the flower. Gently squeeze a bulb to check that it is firm - an indication that it is healthy. The outer skin should be intact and clean as this is the bulb's defense against desiccation. Healthy bulbs exhibit an even color and are without stripes or blotches and/or any premature leaf or root growth. Reject bulbs that show signs of rot, fungus or mildew. Bulbs that naturalize well in Marin include bluebells, callas, daffodils, freesias, grape hyacinth, bearded and Dutch iris, and Asian and Oriental lilies. Avoid tiger lilies, which carry diseases that affect other bulbs.

For best results, get bulbs into the ground soon after you buy them or receive them by mail. The vast majority of bulbs come from soils that dry out to some degree during their resting period. To prevent the bulbs from getting too wet in Marin's Mediterranean climate, aerate the soil deeply and incorporate compost. Well-drained soil that is rich in organic material, e.g., compost, is ideal. If the soil is compacted or heavy clay, dig in organic material to a depth of 1 to 11Ú2 feet. Before planting in the ground or a container, gently stir a balanced organic fertilizer into the soil.

Generally, bulbs should be planted in holes about 5 to 8 inches deep. Space bulbs two to three bulb-widths apart, firm the site and water the new plantings. Check the planting instructions for each type of bulb, as guidelines vary. Most bulbs prefer full exposure to the sun, but a few, such as English bluebells, prefer a shady site. No matter what you do, including planting a bulb upside down, your efforts will bring desirable results.

Once bulbs are planted, they provide years of pleasure, requiring little aftercare. In summer, when the foliage has died down, it's best to remove the leaves and rake over the ground to stop insects from accessing the bulbs. After a few years, even if the bulbs are doing well, any dense clumps may need thinning out. Lift from the soil during the resting season, separate out and replant.

To overwinter bulbs, brush off excess soil and store in a cardboard box or paper bag between layers of peat moss. Label and place the box/bag in a dark, cool well-ventilated place. Examine monthly and discard any that are rotting.

By planting a range of bulbs, different species and various cultivars, it's possible to have flowers during the entire growing season. Planting bulbs is an almost effortless and foolproof way to guarantee a magnificent garden year-round.

The University of California Marin Master Gardeners are sponsored by UC Cooperative Extension. For questions about gardening, plant pests or diseases, call 499-4204 from 9 a.m. to noon, and 1 to 4 p.m. weekdays, or bring in samples or pictures to 1682 Novato Blvd., Suite 150B, Novato