IN WASHINGTON, spring begins with the first purple, white and gold signs of the crocus, but these -- like the past week's sudden shows of forsythia and cherry blossoms -- are only the beginning.

Spring time is a rare treat in the nation's capital. There are more acres of parkland here than in any other major urban area. Open spaces lining the Potomac River and scattered throughout the city's neighborhoods are alive with pastels, rich scents and the fresh pale green of new leaves.

Although the crocuses and daffodils emerge during the first few days of the spring equinox, most blooms don't appear until mid or late April. While landscapers and gardeners can't accurately predict when certain plants will flower, they usually plant spring gardens to stagger the blossoms throughout the season. Most public gardens are designed to bloom all through the spring and summer, but the peak times are during April and May.

Although recently warm days are hastening the season, the following is a tentative schedule of the best weeks to view certain flowers: FIRST, SECOND WEEK OF APRIL

The capital's world-renowned cherry blossoms, as you certainly have heard, are out. Besides the fabulous displays at Hains Point and around the Tidal Basin, flowering cherries can be found at Scott Circle, Washington Circle and Meridian Hill Park as well as along the quiet avenues of the suburban Montgomery County communities of Kenwood and Somerset.

Crocuses and daffodils planted by the city are mostly found in natural settings. There are spectacular displays along Rock Creek Parkway, especially opposite P Street Beach below Rose Park in Georgetown; and along the George Washington Parkway on the Virginia side just above the 14th Street Bridge.

Witch hazel, winter jasmine, pussy willow and yellow shocks of forsythia are already brightening up the landscape. Check out forsythia banks at Dumbarton Oaks or near the Jefferson Memorial.

Cornelian cherries (actually a yellow-blooming dogwood) are complemented by the delicate purple trilians, a tiny wild flower on the grounds of the National Arboretum.

The large pink blossoms on the tulip trees also herald the spring. The displays on 17th Street in front of the National Geographic Society and in Rawlins Park (18th and Virginia Avenue NW.) are particularly attractive. THIRD WEEK OF APRIL

This is the week for flowering fruit trees: Crabapple, quince, pear, nectarine and all the late-blooming cherry trees should be opening their fragrant blossoms. The courtyard on the grounds of the National Geographic Society complex downtown has a gorgeous stand of flowering pear.

The flowering fruit trees of the colonial gardens at Mount Vernon are presented in a unique manner. Some are trained to grow, hedgelike, along brick walls, the limbs branching out in a fanlike manner. Others, in the Kitchen Garden, grow on trellises, creating a fragrant canopy overhead.

Azaleas, tulips and several varieties of wild flowers should also begin to show this week. FOURTH WEEK OF APRIL

Banks of red and white azaleas should be at their peak this week. The azalea gardens at the National Arboretum and scattered throughout Takoma Park are simply outstanding. In addition, several varieties of violets will be blooming and the first rhododendron blooms beginning to emerge.

If you're a tulip fancier, the display at Fountain No. 4 (near the parking lot of the Jefferson Memorial) is always attractive. The Tulip Library on the southeast side of the Tidal Basin features 96 varieties of this popular Dutch bulb. And don't forget the grounds of the embassy of The Netherlands, which are decorated with the country's national flower.

Bishop's Garden at the Washington Cathedral has a unique arrangement of a non-hybrid tulip, T. Cruseana. This small, unusual-shaped variety has purple blotched leaves, and blooms earlier than the hybrid varieties.

George Washington, also a tulip lover, imported bulbs from Holland. (A difficult feat in the 18th century, as most bulbs did not survive the long Atlantic crossing, either rotting or being eaten by mice.) One of Washington's favorite survivors, the Rembrandt tulip, is still cultivated on the grounds at Mount Vernon. END OF APRIL, EARLY MAY

The dogwood's cross-shaped blossom can be seen in both natural and cultivated settings. For the domestic varieties, check the displays at the National Arboretum, Gunston Hall Plantation and Dumbarton Oaks. Both pink and white dogwoods were favorites of George Washington; Mount Vernon has beautiful displays of flowering dogwood and red bud, another favorite tree of the first president. For wild dogwood, visit Roosevelt Island, Rock Creek Park and Great Falls Park in Virginia. SECOND WEEK OF MAY

Several varieties of lilacs, peonies, old roses, azaleas, Sweet Bay magnolias and locust trees should be in full bloom. Herbs are also beginning to flower. The herb garden at the National Arboretum is planted with several varieties of medicinal and culinary herbs. The Kitchen Garden at Mount Vernon has several kinds of lavendar, rosemary, thyme and santellina, as well as other herbs used during colonial times. BLOSSOMING POSSIBILITIES

DUMBARTON OAKS -- 32nd and R streets NW (338-8278). Gardens open to the public daily (except national holidays and in severe weather) from 2 to 6 p.m. Adults $2, children $1. Senior citizens admitted free on Wednesdays.

NATIONAL ARBORETUM -- 3501 New York Ave. NE. (475-4815). Open all year, Monday-Friday from 8 to 5, weekends from 10 to 5. Free.

GUNSTON HALL PLANTATION -- Lorton, Virginia (550-9220). An excellent example of English and Dutch 18th-century formal gardens. Open daily 9:30 to 5. Adults $3, students $1, senior citizens and group members, $2.50.

MOUNT VERNON -- 780-2000. Open daily, 9 to 5. Adults $4, children $3, senior citizens $3.50.

BISHOP'S GARDEN -- at the Washington Cathedral, Wisconsin and Cathedral avenues NW. Open daily until dusk.

MERIDIAN HILL PARK -- 16th and Euclid streets NW. Open until dusk.