Soil and drainage in atypical situation

Question

Do I need to drill holes in the sides of this metal lined 9foot x 3ft x 2ft deep hole in the asphalt that I am making into a rose and flower garden? How should the lower 3rd soil be amended for adequate drainage in an enclosed hole like this?

I took out 4 inches of asphalt, 4 inches of builders sand, and 10 inches of clay soil.

I put back in, @ 25% builders sand and 60% clay soil and 15% mixture of garden soil, compost, dead leaves, a bit of mulch, 10lb of gypsum, bucket of pumice pieces. I dug a hole and tested drainage. It was ok (the surrounding mixture was dry and loosened) but it produced mud for a few hours.

I’m worried that when we have the rains in Albuquerque (few times a year) the metal will prevent water from soaking into surrounding soil under the asphalt and the soil might get soggy and stay that way. I can’t ever dig it out again and can’t afford to lose plants so I want to set it up as perfectly as possible for preventing mud and rot deep in the bed. I will grow 1 or 2 roses and as many perennials and bulbs as possible.

Advice on the metal? Drill holes? (I expected it to only go down 2/3 of the way, but it did not turn out that way). Advice on the bottom 1/3 of the soil? Advice on the middle and top thirds of the soil?

I am grateful to anyone who will put thought into this. Please give your rationale for any advice because there are usually differences of opinion. I’ve spent more than I can afford already and don’t want it all to be ruined. During our dry and hot times in Albuquerque NM I think it will be good but I have lost plants in barrels due to drowning and rotting when it rained.

Thank you very much!

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Answer 1

This is a complicated question, and it requires some difficult answers as well.

First, it would be good to know what that metal is made of. Steel is ok, but aluminum and copper are to be avoided. The zinc would not usually be too bad. Unfortuntely, unless you knew otherwise, we will have to assume that it is aluminum or tin base.

This is not the ideal choice material for a raised bed, but may work if you get a liner that is able to be used for plants. I would drill holes in this, AND the metal to allow for proper drainage.

Now for your soil... I do not like to incorporate much clay at all, or even in some cases, any at all. If you do, make sure it is a very small amount. Raised beds are to be utilized as large containers, so the soil much be treated as such. This would be a lengthy narrative... So I am going to place a couple of articles that will give you some more information here:

https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/best-soil-raised-garden-beds.htm