Tying the roots of a bonsai tree to a pot has several functions. It can help hold roots down that have a natural tendency pop up above the surface of the soil and can help to secure a top-heavy tree in a pot. In some cases, tying the roots down can help train the roots when creating an above-soil root effect. Above-soil root effects can give a relatively young tree an older look and feel.
Cover the holes in the bottom of the bonsai pot with bonsai mesh, window screen or other mesh. This will keep the soil from dropping through the holes to the bottom of the pot.
Put 2 to 3 cm of soil in the bottom of the pot. This layer of soil will help support the roots.
Slip the ends of a loop of bonsai wire through two holes in the bottom of the pot.
Trim the roots by either 1/2 or to a size that will fit in the pot with a pair of sharp bonsai pruners.
Place the bonsai tree in the pot on top of the thin layer of soil. Slip the roots under the wire loop.
Pull the wire loop taut from the bottom of the pot and twist the wires to hold the roots in place. Clip the ends of the wire.
Fill the bonsai pot with soil.