"Most of the natives of Japan take great delight in their gardens, and cultivate with much care, many kinds of flowers, and plant flowering-shrubs before their houses, and also form hedges of shrubs about their forms, on account of their beautiful blossoms.  Nothing can exceed the beauty of the hedges of the maples indigenous to this country.
       "Like their neighbours the Chinese, the Japanese are very fond of double-flowers, and have an endless variety of double-blossomed peach, cherry, plum, and many other varieties.  They also plant dwarf trees in flower-pots, often with pumice or other porous stones, instead of earth.  It would be endless to enumerate their favourite plants; of the Azalea and Chrysanthemum, they have numerous beautiful varieties." 1

1     The Saturday Magazine (London: John William Parker), Vol. 6, No. 182, May 2, 1835, pg. 175.  "Under the Direction of the Committee of General Literature and Education Appointed by the Society for Promoting Christian Knowledge."  The price for this periodical was one penny.  The original article in Paxton's from which this was adapted has not yet been tracked down by RJB.

The above article was reproduced verbatim in The Book of Entertainment, or Curiosities and Wonder in Nature, Art, and Mind (New York: C.S. Francis & Co.; 1848. Second series), pg. 258.

What is interesting is that all of the earliest Western reports specify that the Japanese dwarf trees were grown on or with pumice or other porous stone:
Mundy, Meister, and Kaempfer.