Every year when February seems like the longest month and the lion stage of March seems interminable, I take a day trip to where I know I can get a stronger breath of spring than the forced forsythia on the windowsill.

Longwood Gardens, more than a thousand acres of blossoming wonders, is open every day of the year, but its special joy is the way it steals a march on April and May with conservatories filled with azaleas, acacias, camellias, rhododendronds and every spring flower you know.

Longwood, just over the Delaware line in Kennett Square, Pa., is a product of one of the du Pont fortunes. Piere Samuel du Pont acquired the original 200 acres in 1906 and over the years he and his wife expanded the acreage and built the conservatories, fountains and waterfalls, chimes tower, open-air theater, arboretums and example gardens. The estate was first opened to the public in 1921. In 1937 Mrs. du Pont set up the Longwood Foundation, which operated the gardens until 1970, when, under the terms of Mr. du Pont's will, it became Longwood Gardens Inc.

The hightlight of a Longwood visit this time of year is likely to be the new Azalea House with its lamella arch of clear Lucite (a DuPont product, naturally). The example gardens along one side of this section are idea-provoking for gardeners of all persuasions, traditional to ultra-modern. Containers for plants in this area could include old tea kettles, tennis shoes and boots, dilapidated step-ladders. A bit farther along, espaliered trees and dwarf orientals.

A new feature at Longwood this year is the series of "balcony gardens," at first glance a small house built inside one of the conservatories.But it's five rooms with balconies all connected by gentle ramps. The first, featuring Lucite panels that can be dismantled in warm weather, has a display showing how to propagate plants from cuttings.

The second balcony features hanging plants and is set up as an "al fresco" dining area. The third displays hardy plants "for all seasons" with planters for annuals and bulbs and the fourth featues a travertine marble "living sculpture" with euphorbia plants. The fifth is set up to resemble the veranda of Victorian house with hanging baskets of abutilon and myosotis.

Also in the new exhibit are examples of window shelf, box and sill gardening.

As arresting as the displays are, many visitors find most interesting visits to the adjoining greenhouses for previews of future events and glimpses of experiments under way - a basket of pansies with three-inch-wide blooms, a pot of lily of the valley, a hanging planter dripping fuschia-like with chrysanthemums. One pathway always has sweet peas from floor to ceiling on string trellises.

There's a deseert garden complete with bleached bones, several tropical sections with bananas and orchids, always a rose display so fascinating you have to take a break on one of the benches to take it all in.If the day is mild enough, there are dozens of worthwhile paths to investigate - to see the vegetables areas and herb gardens so neatly laid out, and the woodland walks, formal rose gardens, Italian water gardens, the lily pond complete with ducks. The chimes tower marks the quarter-hour and you realize how swiftly time has flown.

But there should be time to stop in the town of Kennett Square so sample its claim to fame. It calls itself the mushrooms capital the world, and the claim is justified. The main street (Route 1) is lined with shops selling the finest you've ever seen. I always stock up.