Drainage for huge planters
We moved into a home with some enormous plant boxes built into the design of the front steps. I'm talking about 3 to 4 foot deep containers made of pressure treated lumber for taxus trees. After 12 years, we started noticing serious rot and water inundation of the wood. It's clear we should have drilled some drainage holes, but we are not gardening types. Fortunately, the sides of the containers are accessible, so we could drill some drainage holes. What kind of drainage should we do?
Outside United States
Drainage holes will help the situation but the soil moisture will ultimately rot most of the container. I suggest you have another container system constructed with a non-rotting material or coat the bottom/sides of the container with a material/"paint" that is impervious to water. The bottom of the container could be constructed of a grid (large spaces) to maximize drainage using a material that will not rust or rot.