You may not even need to do that, if the garden soil is well drained. If it's clay, you may need to build a mini sump under the pot. Dig a hole that is wider at the bottom than it is at the top. Put about a cm of silt in the bottom of the hole, and scratch it into the bottom. Put another 3-4 cm of silt on top of that. Then put a cm of sand on top of the silt, and scratch it in. Fill the hole with sand until you get to where the bottom of the pot will be. Set the pot on the sand, and fill in around it with garden soil. with the rim of the pot just sticking above ground. The scratched in zones will keep sudden changes of soil texture from slowing the drainage--the same principle as rapid infiltration beds.

Another thing that helps is planting in terra cotta pots, instead of plastic ones. water diffuses through the clay sides, as well as out the bottom.

Good luck!

Wouldn't it be easier to simply dig a hole much larger than the pot, part fill with ericaceous compost and then plant the Azalea into the ground? In a pot you are going to constrict the roots and, if it is plastic you run the risk of the plant getting waterlogged if there is a really wet period in spite of the sand and silt method TugB has suggested.

Thank you both for your 2 well informed answers.

Moon grower, The area of the garden i wanted to plant a couple of Azaleas is very wet clay even if i dig massive holes with ericaceous compost, what happens when the roots spread and hit the clay?

I might reconsider and plant something there that likes wet clay, Do you know any flowering evergreens that would be suitable for that area?

Thanks again.

Japon - send a PM to SpritzH she should have some good ideas. I've never grown on clay.

That said if the hole is big enough by the time the roots reach the clay they will work their way into it.

I've found out the hard way that a hole in clay soil filled with compost of any kind turns into an anaerobic compost heap, rapidly killing any plant stuck into it. Better to mix the compost with the native soil, make the holes square--or even star-shaped, since the roots escape better on the corners--and scratch in a cm of the mix before filling the hole.

Wise words TugB