"Bonsai"
(Set of 5 stamps, issued January 23, 2012).

The first stamp depicts a Sierra juniper in semi-cascade style, in which the tip projects over the pot rim but does not extend below the base.

Second is a trident maple in informal upright style, in which the trunk bends slightly to the left or right.

Third is a black pine in formal upright style, with the trunk straight and tapering evenly, with symmetrical branches, from base to apex.

Fourth is an azalea plant in multiple-trunk style, with several trunks emerging from one root system.

The fifth and final stamp shows a banyan in cascade style, in which the trunk evokes a stream flowing down a mountainside, with the tip extending below the pot's base.

Art director and stamp designer Ethel Kessler worked with Hilo, Hawaii Artist John D. Dawson.
These were issued as Forever® stamps, which are always equal in value to the current First-Class Mail one-ounce rate.
Images per http://www.beyondtheperf.com/stamp-releases/bonsai.  See this video for additional background on this set.