Looked at from a cultural perspective, Germany has five seasons - the usual four that the rest of the world has, and then a fifth one, the ''festival'' season. During a period extending from April to October, medieval castles, churches, and city squares become the atmospheric settings for special concerts and festivities celebrating the great composers of Europe, particularly Wagner, Beethoven, Mozart, and Bach.
This year, with the addition of two nonmusical special events, the festival season in Germany promises to be particularly lively. One of them is the International Horticultural Show in Munich, an extravaganza of gardens and artwork from around the world which will adorn a 180-acre park built in the center of the city for just that purpose. The other is the observance of the 500 th anniversary of Martin Luther's birthday, an event that will be the focus of special museum exhibits and other activities in the south central German cities of Nuremberg, Augsburg, Coburg, and Worms. (See map on preceding page.)
The International Horticultural Show, an exhibition that takes place just once every 10 years and is happening in Munich for the first time, will be in progress from April 28 to Oct. 9. The show is drawing upon the skills of gardeners, landscape architects, and flower breeders from 25 nations who will display the latest developments in horticulture as well as some of the most ancient. Gardens typical of those in China, Thailand, and Nepal, where horticulture is a centuries-old art, will share space with exhibits on how landscape can be used to improve the quality of life in an increasingly crowded and polluted world.
Some of the exhibits will be in specially built halls, but others will be in natural settings spread along meadows, forests, and wetlands in the park. About 15 miles of walkways, some of them accessible by a mini-railway, will connect all the outdoor exhibits.
For those interested in improving their own gardens, the Horticultural Exhibition will offer courses, lectures, and symposiums on subjects ranging from vegetable gardening to the care of houseplants. Nongardeners should find plenty of interest as well, because the busy schedule also includes folk-art festivals and musical, theatrical, and dance events. For children there will be play areas throughout the park, as well as troupes of puppeteers, clowns, and strolling musicians. Admission is $4 for adults, $2 for children, or $8 for a family.
Germany has designated 1983 as the Luther Commemorative Year, a time to reflect upon and learn about the life and times of the complex Protestant reformer Martin Luther, who was born in 1493. Because the cities of Nuremberg, Augsburg, Coburg, and Worms played pivotal roles in the life of Luther and the Reformation, they are the setting for a number of special museum exhibits, concerts, conferences, and tours.
In Nuremberg, where the city government became the first to enact the reformation in 1521, there will be an exhibition called ''Martin Luther and the Reformation in Germany'' at the German National Museum from June 25 to Sept. 25, a show including 600 paintings, sculptures, prints, broadsheets, and other items from the period. Another exhibition, this one called ''Luther in America'' and taking place at Amerika Haus from June 25 to Sept. 25, will include lectures by American scholars on Luther and the Reformation. And at Nuremberg's International Organ Festival, which takes place from June 23 to July 3 at various churches in the city, the theme will be music associated with Luther.
Augsburg is the city where Luther engaged in some of his most serious debates with representatives from the Church of Rome. This year the beautiful medieval city will commemorate those debates with a series of lectures called ''Luther's meeting with Cajetan in 1518,'' held Oct. 7-20 at the Augustanahaus. Other Luther-related events in Augsburg will be a staging in the Municipal Theater of August Strindberg's play ''Luther - the Nightingale From Wittenberg'' during June and an exhibition called ''Luther in Augsburg'' at the Zeughaus from June 3 to July 3.
Coburg is where Martin Luther found refuge during 1530, living at the Veste Coburg fortress while he and his doctrines faced censure from Rome. From April to October the art collection of the Veste Coburg will be on exhibit, a show containing important 16th- and 17th-century broadsheets, documents from the period of the Reformation and subsequent years of religious strife.
It was before the Diet of Worms in 1521, the city's governing body, that Luther refused to recant his writings, declaring that ''to act contrary to your conscience is neither safe nor right.'' From June until November Worms will be the scene of two exhibitions, ''A mighty fortress is our God . . .'' at the Church of St. Andrew and ''Art and Culture During the Reformation and Counter-Reformation'' at the Kunsthaus Helshof. Among other events will be a symposium called ''Luther and the World of Politics'' at Castle Herrnsheim Oct. 27-29.
Among the traditional musical events in Germany this year, perhaps the best known is the Richard Wagner Festival in Bayreuth, which dates back to 1876. From July 24 to Aug. 27, eight of the composer's operas will be staged at the Festival Theater.
Castles will be the delightful locations for many German music festivals, some of which date from the same eras as the works presented in them. The thoroughly appropriate setting for International Bach Week in Ansbach, from July 29 to Aug. 7, will be the Baroque-style Margrave's Palace on the edge of town. Another fine example of Baroque architecture, the Imperial Hall of the Residence Palace in Wurzburg, will be the scene of the Mozart Festival, June 9-27. Music of various composers from the Baroque period will be the theme of the Schweitzingen Castle Festival near Heidelberg. The castle theater, built between 1746 and 1752, will offer a program of opera and ballet performances and symphonic and chamber music concerts.
Some of Germany's most popular festivals are mixtures of both dramatic and musical performances. One such is the Herrenhausen Music and Drama Festival, near Hannover. During weeks extending from May through September the festival of plays and concerts will be given in the gardens of the Baroque palace of Herrenhausen. Another celebrated combination of drama and music will take place during the Berlin Festival Weeks, Sept. 3-25. Theater- and operagoers will be treated to numerous new productions and first nights at many of the city's theaters.
Further information (including how to secure tickets) on these and many other cultural events scheduled in Germany this year are available from the German National Tourist Office, 747 Third Avenue, New York, N.Y. 10017.