Drainage screen is used to prevent the soil from washing out of the bonsai pot. If it were to do so, you would get air gaps under your tree and the roots would dry out quickly. The screen is secured in the holes using bonsai wire formed into the shape of a staple.
First off, note that the drainage screen will be placed
inside the pot. I actually bought a bonsai at auction where the screen was outside the pot. We'll do our measuring and references from the outside, but that's just because it is easier to illustrate here.
First begin by measuring the drainage holes in your pot, referring to the figure below.
1. Cut a square of
drainage screen that is about twice as wide as the diameter of the hole. You can use real bonsai drainage cloth, or even the needlecraft material from the craft store, but be aware that some of that stuff can get pretty brittle with age. Some even use a hardware cloth, but be aware that roots tend to grow into and through this stuff and the fewer hidden metal objects in the soil come repotting time, the better on your tools!
2. You will need to secure the drainage screen to the inside of the pot. This is done by creating a wire staple from
bonsai wire. You can use aluminum or copper wire for this purpose. Be sure to use a fairly stiff wire gauge. For aluminum I typically use a 2.0mm which fits easily through the drainage holes.
3. Cut the wire to a length about equal to the diagonal of the drainage cloth The actual length depends to some degree on the thickness of the bonsai pot.
4. Preform the staple using a pair of
bonsai pliers. Form it into a sharp-edged U shape. Measure right against the drainage hole as you do this. If you make the U a little too wide to fit into the hole you can bend the base of the U a little to bring the edges a touch closer together.
5. Now, from the inside of the pot, drop the screen over the drainage hole, drop the U shaped staple through the screen and from the underside of the pot, bend the legs over flat and outward so they are even against the bottom of the pot
6. When completed it will look something like this in cross section above, in the bottom right picture.
7. Repeat for the remaining holes. If you have some very small holes, like the ones used for the tie wires, you might just drop a small piece of drainage cloth over the holes when you place the wire in the pot. If you have very large holes, such as the rectangular ones you sometimes see in larger, high-quality pots, you might use two staples to secure it.
Variations: some folks shape their staples a bit differently, folding the wire back on itself to make it a bit more secure.
the resulting shape looks like this:
TT instead of an inverted
U, but this is all up to you. Personally, I have found that if you use a good stiff wire, this is all unnecessary.
One final note is to be sure that you want the screen to remain as flat as possible. Do not make the staple so tight that it pulls up on the edges as shown in the picture below.
If you have a problem keeping the screen flat, go ahead and use the TT shaped staple.
Regards,
Matt