From the University of Art and Music:
"You sign up for the Test of the Formal Garden and learn to construct a Gazebo, the centerpiece of any formal garden. You also learn to build decorative Stellae, which may contain information on your plants. You are instructed that formal gardens must have between 7 and 100 flowers within a 100 foot radius. Formal gardens may be judged by any Initiate of Art and Music, and should be judged for beauty and variety of flora, not raw volume of plant matter.
Unlike other art Tests, Formal Gardens may be tended after the judging has opened."
Having more than 100 flowers within the 100 foot radius prevents you from opening your gazebo for judging!
Choosing a scenic location can be helpful, as making pretty terrain prettier can be short work. You can also use other kinds of buildings to enhance the area, but remember that flowers create quite a bit of lag - you might want to take this in consideration when you build. Also, too many things in one place means that it will be tricky to see it all because of the caps on object rendering...Sometimes less is more.
1 coordinate = 16 feet = about 5 meters. So 100 feet is roughly 6.25 coords (conversion source: Travel)
Some Roses of Ra and Sea Lilies can be acquired at Universities of Worship, but take care...Once you have received a rose from a university, you will never be able to get another rose from a university, and it is the same with lilies.
Orchids and Sand Blooms are both event flowers, meaning that they were introduced to us through means other than university distribution.
Probably the best method of getting flowers is via trade; there are many crossbreeders who have new strains of flowers for you to plant and/or cross. This is quite useful if you are interested in crossbreeding. Using hybrid bulbs that current breeders have developed means you can advance the progress of breeding rather than redoing the same basic crosses they've done. This means you can reach what would otherwise be difficult color/size combinations more quickly. So do a little homework, and contact breeders. =)
New strains of these flowers are made with Crossbreeding and/or Mutagenics (see below).